Diabetes I - Fasting vs ToleranceIn a study by Basu et al. 7 lean nondiabetic, 13 obese nondiabetic, and 14 obese diabetics were administered somatostatin intravenously which inhibited their endogenous production of insulin and glucagon, then using computer controlled release of insulin and glucose to match diabetic and nondiabetic patterns, each individual was subjected to these patterns and the effects of insulin secretion and insulin action were looked at. This is what they found:

As you can see even in the lean individuals a diabetic profile of insulin secretion resulted in a hyperglycemic peak (also with an associated increase in the AUC) however the levels quickly return to baseline, with the return quicker then the obese nondiabetic. There are two aspects of glucose levels at work, the appearance and clearance, and with clearance there is insulin sensitivity and glucose effectiveness. The latter is glucose's ability to facilitate its own uptake (thus higher levels lead to increased uptake). Since glucose effectiveness doesn't seem to differ between nondiabetic and diabetic individuals, the difference in >1hr glucose levels seems to be due to insulin resistance.

There are 3 major diagnostic criterion that is used to measure glucose homeostasis for diabetes, fasting glucose, glucose tolerance, glycated hemoglobin:

If you do not reach the threshold for diabetes you are classified as pre-diabetic (preDM). Currently preDM is only considered a risk factor for disease, not an actual clinical diagnosis, however there are pathological changes in metabolism associated with preDM. HbA1c is a measurement of both OGTT (oral glucose tolerance test) and fasting sugar, however not everyone with impaired fasting glucose (IFG) has impaired glucose tolerance (IGT) and vice versa. In a study screened for IFG only 50% have IGT (2). Both IFG and IGT increases risk of DM, and the presence of both increases it even more. In a great review looking at the difference between IGT and IGF the author's of (3) came out with this chart:

Clearly a difference is seen between the two states. In the abstract they summarize both pathogenesis and etiology:

PathophysiologyImpaired Fasting Glucose (IFG) Impaired Glucose Tolerance (IGT) reduced hepatic insulin sensitivity,

stationary beta cell dysfunction and/or chronic low beta cell mass,

altered glucagon-like peptide-1 secretion (GLP) and

 inappropriately elevated glucagon secretion.

reduced peripheral insulin sensitivity,

near-normal hepatic insulin sensitivity,

progressive loss of beta cell function,

 reduced secretion of glucose-dependent insulinotropic polypeptide (GIP) and

inappropriately elevated glucagon secretion.

Etiology genetic factors,

smoking and

male sex

physical inactivity,

unhealthy diet and

short stature

The short-stature is interesting and may account for the prevalence of IGT in the Western Pacific Region (4):

My running hypothesis is that due to undernutrition as compared to western countries, Asian countries have babies that not only do not receive the optimal amount of protein (and other nutrients), thus amino acids are spared for the brain (at the expense of lean tissue) (5), and the resultant catch-up growth due to access to empty calories (6), result in IGT then DM.

The difference between these two states are important because in discussions of treatment in terms of both pharmacology and lifestyle, it has different implications for different individuals. A lot of focus over the years has been on insulin resistance, when in reality

insulin resistance plays one part (insulin resistance also occurs in different tissues), while the secretory defect plays another. However the secretory defect should be something that is focused upon because this is what ultimately determines whether you become truly diabetic. From a Lilly Lecture of 1988, the difference between obese diabetic and nondiabetic is not the degree of insulin resistance, but the beta-cells capability to pump out insulin to overcome this resistance. Even though you and I may be perfectly healthy if you destroyed our beta-cells we would all become diabetic.

In a more recent paper by the same author as the 1988 paper, DeFronzo, the theme of beta-cell protection is brought forward (8).

When comparing diabetes prone strains to diabetes resistant strains the authors said this in the discussion (16):Our investigation also examined differences between B6 and 129 mice in multiple organs, allowing us to conclude that inflammation in adipose tissue, and to a lesser extent in liver, but not in skeletal muscle or spleen, is associated with the predisposition to insulin resistance. In addition, our computational analysis was able to set forth hypotheses that led to subsequent biologic validation experiments which provided further insight into the components of the immune system that may contribute to metabolic diseases (i.e., T-cell recruitment).In the end the difference may be due to difference in the pathogenesis and etiology of IGT and IFG just like in humans. Comparing B6 to AKR mice, who are both responsive to diet induced obesity, however the former experiences IFG, while the AKR experience IGT (19), they have this chart which is suprisingly like the one I posted up top:

Why does this difference seem to exist within species? It could be due to variations in your diet, such as nutrients, timing, calories (intake and expenditure) but it could interestingly also be due to genetics (19,20,22).

In a recent paper about free fatty acids (FFA), the author's had this to say (21):The idea that increased adipose tissue mass leads to elevated plasma NEFA concentrations, and these in turn to insulin resistance in insulin target tissues, is appealing and entrenched in the literature. However, many studies over several decades lend credence to a different

picture. As adipose tissue mass expands, NEFA release per kilogram adipose tissue is downregulated, not increased. In many obese individuals, this can lead to normalization of plasma NEFA concentrations. Some elevation of NEFA concentrations is undeniable in certain groups of obese individuals and in type 2 diabetes, and tracer studies tend to show that NEFA delivery to nonfat tissues is increased even if plasma concentrations are not much raised. However, it is clear that insulin resistance, even severe insulin resistance, can exist in obesity without elevation of NEFA concentrations. It is also clear that elevated NEFA concentrations are not necessarily associated with insulin resistance. Two commonly seen examples are women versus men and younger versus older subjects. Women have very significantly raised NEFA concentrations compared with men, yet tend to be more insulin-sensitive and to have better lipid profiles.We believe that more dynamic studies are needed to clarify relationships between obesity and fatty acid kinetics. We suggest that clamp studies may be misleading when applied to NEFA dynamics because in everyday life plasma insulin concentrations are elevated in insulin resistance, and so-called insulin resistance of adipose tissue lipolysis may simply reflect an adaptation to consistently high ambient insulin concentrations. In the absence of a convincing demonstration of excessive NEFA delivery from adipose tissue in the obese state to explain insulin resistance, more emphasis should be put on alternative explanations such as impaired adipose tissue fat storage (12) or dysfunctional regulation of adipokines or adipose-related inflammatory cytokines. We propose that it is time to reevaluate the relationships between adiposity, fatty acids, and insulin resistance.

Indeed inflammation may play a very large role at each of the sites (23). So does FFAs play a role? Maybe but from Frayn's new paper and this one (24) I suspect its role is smaller then previously thought. In (24) the author's concluded that insulin potentiates the response of insulin to glucose regardless of FFA levels in healthy  humans. Indeed as Frayn says (I quoted above) clamp studies are misleading. In the very first study I talked about up top, clamp studies would not be able to discern the dynamic nature of glucose regulation. In chapter 7 of Diabetes Mellitus: A Fundamental and Clinical Text 3rd Edition (25), the potentiation of hyperglycemia to arginine in vitro pancreas cells can be seen:

From a Banting 1990 lecture by Porter this is quoted in the abstract (26):Type II diabetes is characterized by a defect in first-phase or acute glucose-induced insulin secretion and a deficiency in the ability of glucose to potentiate other islet nonglucose beta-cell secretagogues. The resulting hyperglycemia compensates for the defective glucose potentiation and maintains nearly normal basal insulin levels and insulin responses to nonglucose secretagogues but does not correct the defect in first-phase glucose-induced

insulin release. Before the development of fasting hyperglycemia, only first-phase glucose-induced insulin secretion is obviously defective. This is because progressive islet failure is matched by rising glucose levels to maintain basal and second-phase insulin output. The relationship between islet function and fasting plasma glucose is steeply curvilinear, so that there is a 75% loss of beta-cell function by the time the diagnostic level of 140 mg/dl is exceeded. This new steady state is characterized by glucose overproduction and inefficient utilization. Insulin resistance is also present in most patients and contributes to the hyperglycemia by augmenting the glucose levels needed for compensation.Interestingly fatty acids may be released to help the beta-cells (27,28).

References

01. J Clin Invest. 1996 May 15; 97(10): 2351–2361.. Effects of a change in the pattern of insulin delivery on carbohydrate tolerance in diabetic and nondiabetic humans in the presence of differing degrees of insulin resistance. A Basu, A Alzaid, S Dinneen, A Caumo, C Cobelli, and R A Rizza02. Diabet Med. 2002 Sep;19(9):708-23.Impaired glucose tolerance and impaired fasting glycaemia: the current status on definition and intervention.Unwin N, Shaw J, Zimmet P, Alberti KG.03. Diabetologia. 2009 Sep;52(9):1714-23. Epub 2009 Jul 10.Pathophysiology and aetiology of impaired fasting glycaemia and impaired glucose tolerance: does it matter for prevention and treatment of type 2 diabetes?Faerch K, Borch-Johnsen K, Holst JJ, Vaag A.04. http://da3.diabetesatlas.org/indexe345.html05. Science. 1980 Feb 22;207(4433):902-4.Sparing of the brain in neonatal undernutrition: amino acid transport and incorporation into brain and muscle.Freedman LS, Samuels S, Fish I, Schwartz SA, Lange B, Katz M, Morgano L.06. Best Pract Res Clin Endocrinol Metab. 2008 Feb;22(1):155-71.Thrifty energy metabolism in catch-up growth trajectories to insulin and leptin resistance.Dulloo AG.07. Diabetes. 1988 Jun;37(6):667-87.Lilly lecture 1987. The triumvirate: beta-cell, muscle, liver. A collusion responsible for NIDDM.DeFronzo RA.08. J Clin Endocrinol Metab. 2011 Aug;96(8):2354-66. Epub 2011 Jun 22.Preservation of β-cell function: the key to diabetes prevention.DeFronzo RA, Abdul-Ghani MA.16. Diabetes. 2010 Nov;59(11):2960-71. Epub 2010 Aug 16.A systems biology approach identifies inflammatory abnormalities between mouse strains prior to development of metabolic disease.Mori MA, Liu M, Bezy O, Almind K, Shapiro H, Kasif S, Kahn CR.17. Diabetologia. 2009 Mar;52(3):514-23. Epub 2009 Jan 14.Failure of dietary quercetin to alter the temporal progression of insulin resistance among tissues of C57BL/6J mice during the development of diet-induced obesity.Stewart LK, Wang Z, Ribnicky D, Soileau JL, Cefalu WT, Gettys TW.18. Diabetes. 2003 Aug;52(8):1958-66.Variation in type 2 diabetes--related traits in mouse strains susceptible to diet-induced obesity.Rossmeisl M, Rim JS, Koza RA, Kozak LP.19. J Gastroenterol Hepatol. 2007 Jun;22 Suppl 1:S11-9.Has natural selection in human populations produced two types of metabolic syndrome (with and without fatty liver)?Caldwell SH, Ikura Y, Iezzoni JC, Liu Z.20. Med Hypotheses. 2010 Mar;74(3):578-89. Epub 2009 Oct 2.Metabolic syndrome: aggression control mechanisms gone out of control.Belsare PV, Watve MG, Ghaskadbi SS, Bhat DS, Yajnik CS, Jog M.21. Diabetes. 2011 Oct;60(10):2441-9.Fatty acids, obesity, and insulin resistance: time for a reevaluation.Karpe F, Dickmann JR, Frayn KN.22. Int J Obes (Lond). 2011 May 3. [Epub ahead of print]Dietary fat intake and polymorphisms at the PPARG locus modulate BMI and type 2 diabetes risk in the D.E.S.I.R. prospective study.Lamri A, Abi Khalil C, Jaziri R, Velho G, Lantieri O, Vol S, Froguel P, Balkau B, Marre M, Fumeron F.23. Am J Physiol Endocrinol Metab. 2011 Jan;300(1):E164-74. Epub 2010 Oct 19.Multi-tissue, selective PPARγ modulation of insulin sensitivity and metabolic pathways in obese rats.Hsiao

G, Chapman J, Ofrecio JM, Wilkes J, Resnik JL, Thapar D, Subramaniam S, Sears DD.24. J Clin Endocrinol Metab. 2011 Dec;96(12):3811-21. Epub 2011 Sep 28.Exogenous insulin enhances glucose-stimulated insulin response in healthy humans independent of changes in free Fatty acids.Lopez X, Cypess A, Manning R, O'Shea S, Kulkarni RN, Goldfine AB.25. http://www.msdlatinamerica.com/diabetes/sid263710.html26. Diabetes. 1991 Feb;40(2):166-80.Banting lecture 1990. Beta-cells in type II diabetes mellitus.Porte D Jr.27. J Clin Invest. 1998 Jun 1;101(11):2370-6.A fatty acid- dependent step is critically important for both glucose- and non-glucose-stimulated insulin secretion.Dobbins RL, Chester MW, Stevenson BE, Daniels MB, Stein DT, McGarry JD.28. J Biol Chem. 2009 Jun 19;284(25):16848-59. Epub 2009 Apr 22.Adipose triglyceride lipase is implicated in fuel- and non-fuel-stimulated insulin secretion.Peyot ML, Guay C, Latour MG, Lamontagne J, Lussier R, Pineda M, Ruderman NB, Haemmerle G, Zechner R, Joly E, Madiraju SR, Poitout V, Prentki M.29. Diabetes. 2003 Jan;52(1):102-10.Beta-cell deficit and increased beta-cell apoptosis in humans with type 2 diabetes.Butler AE, Janson J, Bonner-Weir S, Ritzel R, Rizza RA, Butler PC.

J Pediatr Endocrinol Metab. 2002 Apr;15 Suppl 1:493-501.Impaired beta-cell and alpha-cell function in African-American children with type 2 diabetes mellitus--"Flatbush diabetes".Banerji MA.

Diabetes II PathogenesisIn 1992 McGarry JD published a paper in Science title, "What if Minkowski had been ageusic? An alternative angle on diabetes" (01). In the paper he tells the story of how glucose came to be center-stage in this whole diabetes thing. Minkowski was the scientist who saw that flies collected around the urine of pancreatomized dogs and then he proceeded to tasting it and found it was sweet. McGarry wonders what would have happened is Minkowski was ageusic, no sense of taste, would he have smelled the acetone instead and maybe research would have focused on the lipid metabolism which also seems to go awry with glucose metabolism. A model of etiopathogenesis is proposed that some unknown factor causes hyperinsulinemia which then leads to insulin resistance which then leads to even more insulin causing a vicious cycle, this model is pictured here:

1. If hyperinsulinemia does indeed precede insulin resistance, what is its cause? 

2. How can  hyperinsulinemia in  the  absence of overt insulin resistance be reconciled with a normal fasting plasma glucose level? Is this because in the basal state most glucose utilization occurs in insulin-insensitive tissues? Might  concomitant hyperamylinemia afford protection from hypoglycemia by promoting Cori cycling of glucose carbon?

3. Does persistent hyperamylinemia in a setting of hyperinsulinemia provide a further stimulus to hepatic VLDL synthesis (by enhancing lactate flux from muscle to liver), thus contributing to the accumulation of  triglycerides in  muscle? 

4. What is the precise defect in muscle glycogen synthesis in individuals with simple insulin resistance? Does it in fact stem from triglyceride (or fatty acyl-CoA) accumulation within the  muscle cell? 

5. How does insulin stimulate glucose uptake into  muscle  and  fat  tissue?

6. What causes  adipocyte  insulin  resistance  in NIDDM?

7. Why do some individuals with  impaired glucose tolerance  never progress to NIDDM? Is this because insulin resistance and the propensity for cell failure are distinct inherited characteristics?

8. What is  the  basis of  cell  failure in NIDDM? 

With regards to issue 1, one of the largest proponents of the idea that insulin resistance precedes hyperinsulinism is DeFronzo RA, who has written many papers with the idea the

"skeletal muscle insulin resistance is the primary defect in type 2 diabetes" (the title of his 2009 paper, (2)).

DeFronzo cites many convincing and supportive studies in regards to this pathogenesis, including the fact that in normoglycemic tolerant (NGT) first degree relatives of type 2 diabetics, in euglycemic clamps most of the decreased glucose uptake is due to a decrease in nonoxidative glucose metabolism (glycogen production). DeFronzo also cites a study he performed before where healthy individuals without any family history of T2DM (CON) was compared to healthy individuals with a family history of T2DM (FH+) and found that at physiological levels of free fatty acids that T2DM patients experience, CON experienced a decrease in glucose clearance, while FH+ experienced no change (3). Then in another study 7d reduction of free fatty acids with the use of acipimox resulted in improvements of glucose homeostasis mostly due to non-oxidative glucose metabolism. The mechanism:Elevated plasma FFA concentrations cause insulin resistance in muscle by substrate competition (increased FFA oxidation restrains glucose oxidation in muscle by altering the redox potential of the cell, i.e. Randle cycle) (40), inhibiting the insulin signal transduction system (41), and impairing glycogen synthesis (9) via direct inhibitory effect of fatty acyltransferase-coenzyme A on glycogen synthase (42, 43).

The original FFA levels of these two studies is this:

Discordant effects of a chronic physiological increase in plasma FFA…. (3)

Sustained reduction in plasma free fatty acid concentration improves insulin action …(4)

Fasting FFA (µmol/l): CON: 580±76 (MA/C:8/2) FH+: 631±77 (MA/C:7/0)

Fasting FFA (µmol/l): Pre-acipimox: 515±64 Post-acipimox: 285±58

From (4) one can see that the decrease in FFA was quite substantial, even to lower levels then healthy controls then (3), thus whether this has anything to do with actual pathogenesis of diabetes I find questionable based on this evidence. Based on this evidence DeFronzo proposes:

The NGT offspring of two type 2 diabetic parents also manifest marked adipocyte resistance to the suppressive effects of insulin on lipolysis. One could argue, therefore, that the adipocyte represents the primary tissue responsible for the insulin resistance. According to this scenario, the elevated plasma FFA levels produce insulin resistance in muscle and liver and impair β-cell function. Adipocytes in the NGT offspring of two type 2 diabetic parents also secrete excessive amounts of inflammatory and insulin resistance producing adipocytokines that could initiate/exacerbate the insulin resistance in skeletal muscle. Whether FFA play a role in pathogenesis has been called into question by Frayn, who by the way did a lot of important work in the FFA field for diabetes (4). The article begins with a short history of FFA and the citation of study (5) which is a prospective study which showed that baseline FFA was not related to diabetes however that diabetes caused increased FFA. Then goes on reviewing evidence from their group and others showing limited relationship between fasting FFA levels and insulin resistance/obesity. In the conclusion they say:In summary, there is mixed evidence for the notion that obesity is associated raised postprandial, diurnal, or nocturnal NEFA concentrations. There is need for more studies in this area, and close attention must be given to the nutritional and methodological elements of such studies.The idea that increased adipose tissue mass leads to elevated plasma NEFA concentrations, and these in turn to insulin resistance in insulin target tissues, is appealing and entrenched in the literature. However, many studies over several decades lend credence to a different picture. As adipose tissue mass expands, NEFA release per kilogram adipose tissue is downregulated, not increased. In many obese individuals, this can lead to normalization of plasma NEFA concentrations....However, it is clear that insulin resistance, even severe insulin resistance, can exist in obesity without elevation of NEFA concentrations. It is also clear that elevated NEFA concentrations are not necessarily associated with insulin resistance. Two commonly seen examples are women versus men and younger versus older subjects. Women have very significantly raised NEFA concentrations compared with men, yet tend to be more insulin-sensitive and to have better lipid profiles.

This idea that FFA release is downregulated as adipose tissue size increase was found among Pima Indians in 1991 (8) [Note:the author's here wondered if this resulted in the obesity]:

What I find odd is that again in Pima Indians even the diabetics (C) experienced decreases in FFA levels as compared to nondiabetics (A and B) (9):

Boden G has been a very large proponent of this whole NEFA thing and in a 2011 review article explains his reasoning (10). In it he states:Nevertheless, to be considered a physiological link between obesity and insulin resistance, an adipose tissue-derived factor should meet the following three criteria:1. The factor should be elevated in the blood of obese people.2. Physiologic elevations of its blood levels should increase insulin resistance.3. Lowering of elevated blood levels should decrease insulin resistance.

To support criterion 1, he cites two studies (11,12), one from 1969 and one from 1963. I have access to (11) and for the obese group they included mostly women and as we now know, women have higher FFA levels that doesn't actually tell us much. In another text from 1968 (13) the author's again made the same mistake by plotting both males and females on the same graph to determine FFA levels relation to % desired weight. I re-plotted the data:

And I find it hard-pressed to draw a relationship within gender groups. What about studies that use intralipid heparin infusions? Again while this does show that FFA seem to play a role, the high levels and increased glycerol levels (that aren't found in controls) do not represent what actually happens, however this does not rule out its role. As Frayn says more research needs to be done to flesh out the real relationship.

The Concept of Insulin Resistance

One of the mental blockades for me has been which comes first, insulin resistance or beta-cell defect, and how does hyperinsulinism come into all of this. A relationship between insulin sensitivity and insulin secretion was found, it turned out to be a hyperbolic function (13). In a group of Pima Indians followed longitudinally nonprogressors stayed on the curve because their insulin secretion increased to overcome the "insulin resistance," however progressors to DM fell down towards the origin, meaning that the insulin secretion wasn't enough to overcome the insulin resistance, thus representing beta-cell dysfunction.

As (14) explains in the introduction, this relationship supposes that for most that maintain normoglycemia as insulin resistance increases, the insulin secretion increases to compensate, however as the beta-cell fails, insulin secretion cannot compensate and hyperglycemia develops. Based on this reasoning the curve for glucose impaired patients would have a lower slope, since for the same insulin sensitivity, insulin secretion is not enough to compensate, thus would be lower as compared to normoglycemic disposition index. However in (14) with a larger population (NGT (n=1,123) or IGR(n=156) (age 44 +/- 8 years, BMI 25+/-4 kg/m2, mean +/- SD)) to study from they found that in IGR patients, it was actually parallel:

Parallel means that hyperglycemia is most likely not due to the failure of insulin secretion to overcome insulin resistance but that the parallel curves indicate an intrinsic beta cell defect, which is the same at all insulin resistance levels, thus insulin resistance may not be a part of the pathogenesis (the latter part is my interpretation). Their conclusion:Although our study clearly demonstrates that tonic (i.e. the fasting secretory tone) and phasic (e.g. glucose sensitivity) properties of beta cell function, as determined by OGTT, are substantially independent of one another and have a different role in glucose intolerance, it cannot clarify the underlying cellular mechanisms. In particular, our data do not address the question of whether the changes observed in IGR participants reflect proportionate differences in beta cell mass or function alone (or a mixture of the two).

In conclusion, we have shown that in non-diabetic individuals insulin resistance is associated with an increase of specific variables of the insulin secretory response to glucose challenges. In particular, the adaptation to insulin resistance involves the fasting secretory tone, but not glucose sensitivity. On the other hand, beta cell glucose sensitivity plays an essential role in determining glucose levels during an OGTT. In states of reduced glucose tolerance, this function is markedly impaired, while there is little evidence of a defective relationship with IS. Therefore, our results suggest that hyperglycaemia is the consequence of intrinsic beta cell function deficiency rather than of a defect in the mechanisms of compensation for insulin resistance. This conclusion is based on cross-sectional data; future prospective analyses of the RISC cohort may further clarify the role of beta cell dysfunction in the development of hyperglycaemia.

Thus if this is true and their data holds, DeFronzo's idea that insulin resistance causes hyperglycemia after beta-cell failure may not be true. Some other pathological mechanism is at play here. Finding out the pathological mechanism is important for two reason: 1) this tells us how to avoid Diabetes and associated illnesses, and 2) directs us towards research to cure diabetes. If insulin resistance doesn't play a large role in the etiopathogenesis of diabetes, then why the billions of dollars spent in this field? If a beta-cell defect is the main cause, then money should be spent on preventing failure, and our dietary choices should be chosen to prevent this failure.

Thus if insulin resistance does not need to be present for hyperglycemia to be present and beta-cell dysfunction is required, then etiologically you could have insulin sensitive T2DM, and then beta-cell dysfunction is the primary defect in the pathogenesis to diabetes. Ultimately even in the definition of Metabolic Syndrome has been questioned because a pathophysiological basis (e.g. insulin resistance) cannot account for the "syndrome" and

thus may provide no use for patients or clinicians (15). In a great review paper (16) the author's review evidence that, as McGarry hypothesized, hyperinsulinism is enough to create both more hyperinsulinism and insulin resistance. So from this paper we have insulin resistance due to hyperinsulinism, but again as McGarry asks what was the initial cause of the hyperinsulinism. However hyperinsulinism can be looked at from two perspectives. From the persepctive of insulin resistance, it could be due to increased secretion to overcome it, however from a viewpoint of insulin secretion, there could be a relative deficiency when normalized for glucose levels, I suspect this is the case since insulin secretion is most likely the initiating factor but I don't have the data to back it up.

Insulin resistance is most likely just a cover for obesity, however most research has focused on IR, however all these billions of dollars while improving insulin resistance doesn't actually cure or stop progression. To truly stop progression, the beta-cell dysfunction has to be addressed directly. Even with healthy folks variations in measurement of IR can differ up to 600% (17). The article claims that 50% can be attributed to obesity, while the other half is attributed to genetics and exercise, which I agree with. What I find most convincing of all despite, the variation in IR, the problems with measurment of IR, is that for years the genetic origins of IR has been tracked down to pathways that can be explained by overnutrition, however genetic defects for beta-cell dysfunction (19) and obesities interaction with the environment is much stronger (18).

In terms of insulin secretion an odd result I came across from McGarry's papers was that when animals were fasted and FFA acid levels were lowered, insulin secretion is impaired to both nonglucose and glucose stimulation (20). When the pancreas were isolated from the animal the addition of palmitate returned insulin secretion to normal, however in fed reduction of FFA levels did not alter insulin secretion.

Thus there may have been some evolutionary relationship between FFA levels and insulin secretion, and what we see in diabetics may be a left over mechanism (gone awry) in which increased FFA delivery should increase insulin secretion, however because of the beta-cell defect the negative feedback doesn't exist.

Based on the fact that the lack of a possible lipid moeity inside the beta cell (supplied by free fatty acids) affected glucose, leucine, arginine, and glibenclamide and proposed this:The current data indicate that the site of fatty acid interactionis late in the secretory cascade and likely occurs at the level of Ca2+ entry into the b-cell or at a point distal to this step (perhaps at the stage of fusion of the insulin granule with the plasma membrane or exocytosis)Again in rats a more recent study also traces beta-cell dysfunction back to the "ATP/Ca(2+) and lipid signaling, as well as free cholesterol deposition." And again the importance of lipids were shown by the same author's on different mice and cells (22). In human autopsy studies up to a 40% decrease in beta-cell mass can be found (23) and the cause of the apoptosis is most likely protein misfolding due to amylin (24). Cerasi E has done a ton of research in this field of beta-cell dysfunction and in a recent article titled "What ails the beta-cell?" says this:In addition to cell-autonomous (intrinsic) defects that can be demonstrated long before disease onset [3], there are cell-nonautonomous contributors to β-cell dysfunction: first and foremost hyperglycemia, also lipids, other hormones, autonomic nerves and the ever elusive cytokine set, not to mention changes in endothelial function and vascular flow, that are unlikely to relieve the β-cell's predicament [4]. Nor are changes limited to the insulin secretory aspects of β-cell function. Over time, the number of β-cells undergoes changes too, although their extent is unlikely to be large, and their modality is somewhat shrouded in

mystery. The relationship between β-cell mass and type 2 diabetes is also clouded by the intrinsic difficulty of assessing endocrine islet mass, by the large individual variation even among normal subjects, by the fact that human studies are by virtue of necessity limited to dead people, and by the uncertainties of quantifying immunohistochemical determinations [5]. To complicate matters, clinical outcome data show that ‘therapeutic’ interventions may affect β-cell function while doing little to reverse this progressive deterioration, beyond providing stopgap reliefIn another of his articles (27) he has this very insightful paragraph:Four decades after our initial thoughts about T2DM development [61], we would like to present a modified view on how we see prediabetes. A ‘prediabetic’β-cell is a perfectly normal β-cell, but its capacity to adapt to increased demand of insulin production and/or mass expansion is in the lower-end of the normal biological variability. Thus, the ‘prediabeticity’ of the β-cell is entirely context-dependent: until a given degree of nutritional overload and/or insulin resistance is reached, this β-cell functions normally and maintains normal glucose homeostasis. The ‘given degree of load’ may be a function of the genetic background of the β-cell. To take an example, the T variant of TCF7L2 gives a 40% increased risk of developing T2DM [59]. However, this is true for the world of today, with its excessive nutrition and physical inactivity; we postulate that had the genome-wide association studies been performed in the immediate post-World War 2 Europe, TCF7L2 would not have emerged as a T2DM risk factor, because the T-homozygous β-cell would have been perfectly capable of handling the limited nutritional load of the epoch. These considerations lead us also to propose an alternative to the ‘thrifty gene hypothesis' [62], which sees an evolutionary advantage to obesity/insulin resistance: the ‘lazy gene hypothesis'–gene variants that limit the functional capabilities of the β-cell had no evolutionary disadvantage as these β-cells, over the whole period of evolution, were never subjected for extended periods to the excessive demands of a nutritional overload, except for the past half-century.[For more reading about mechanism of beta-cell failure read (28)]

Since a large amount of focus for diabetes has been placed on glucose, its other metabolic functions has largely been ignored, especially with regards to protein kinetics, which the majority of studies being done by Marliss EB (29-31). Whether this part of the metabolism has an important role in pathogenesis I do not know yet, but it seems to be a compensatory mechanism for the reduced uptake of glucose, and may also indeed be one of the reasons why hyperglycemia may be present in the first place as suggested by Porter (previous post here). [It may also be a way to signal bad genes by reducing skeletal muscle expansion (32)]

In a great paper (33) the author's found that even in the normoglycemic range a loss of beta-cell dysfunction occurs in adults. In a commentary (34) a researcher says results like this calls for moving IGT and IFG into diabetes, and normoglycemic dysfunction as pre-diabetes, thus treating the disease even earlier and hopefully stopping disease progression (as most likely at the IGT and IFG states it is already too late).

From study (22) based on their study (with FFA and various stimulants of insulin) the author's conclude that the defect most likely occurs at either the Ca2+/ATP or distal to it, while this may be the case I suspect it is something independent but related. That is the synthesis of insulin and its related factor amylin (picture source (36)):

Proinsulin is made first then cleaved later. The presence of a increase in proinsulin/insulin ratio is highly predictive of diabetes progression as shown in the Mexico Diabetes Study (35). Progressing from non-diabetic, to IGT, to NIDDM, proinsulin levels increase respectively signalling beta-cell dysfunction. Interestingly I think this result may account for the lack of relationship between insulin resistance and metabolic syndrome. It could be that the relationship to metabolic syndrome could indeed be instead proinsulin. Indeed in 1988 when Lilly wanted to test proinsulin as a slow release form, its admininistration may have resulted in increased cardiovascular events (38). This relationship with cardiovascular risk and proinsulin is even present in nondiabetic healthy young individuals (39). Thus this whole chase of the concept between insulin resistance causing hyperinsulinemia could in fact be easily explained by the fact that proinsulin has detrimental metabolic effects. So with proinsulin and insulin we have two measures, the fasting measure of both, and the ratio PI/I. An increase in the former means just more insulin, however an increase in the latter is the result of dysfunctional processing of insulin (due to beta-cell defects). If insulin resistance played a role in all of this one would suspect that increased insulin resistance would result in increased PI/I, however when comparing insulin resistance to PI/I, no association could be found (40).

The explanation for the increase in PI/I ratio has been mathematically modeled (41):Our computer simulations suggest that overloading the endoplasmic reticulum initiates downstream molecular crowding effects that affect protein translational mechanisms, including proinsulin misfolding, delayed packing of proinsulin in secretory vesicles, and low kinetic coefficient of proinsulin to insulin conversion.Basically the overcrowding due to increased need of insulin leads to "adaptations" that result in protein misfolding due to the crowding. The reason for these alterations is most likely due to volume exclusion and steric hindrance (42). In (42) they state:Our results8–10 suggest that an increase of the quantity of misfolded proinsulin to about 50% coincides with a sharp decrease of the conversion of the remaining, native proinsulin to insulin (<30% of synthesized proinsulin). It is known that 50% denatured proteins in a local

environment leads to the onset of massive protein aggregation,42,43 which triggers cell apoptosis.39,40Interestingly in non-human primates at 50% beta cell mass, an inflection point in glucose levels was reached! (47)

In one of Despa's single author papers, this graph is shown:

Figure 1 Pictorial representation of molecular crowding effects on the protein biosynthesis. Under normal physiological conditions (upper), proteins inserted in the ER are folded and transited to the GA. Misfolded species are degraded by the quality control mechanism in the ER. Molecular overcrowding in the ER can increase protein misfolding, overwhelming the quality control mechanism in the ER (lower). Small-volume misfolded protein species (Vm < Vf) are favored by crowding conditions and they can transit much easier to the GA. Accumulation of misfolded species (VM) in the ER leads to a significant dilation of the ER.

What it shows is that as crowding increases the ability to degrade misfolded proteins is decreased, and also the increase in small volume misfolded proteins allow greater transmission to the golgi and out of the cell. In the picture the overcrowded ER is dilated which is in line with observations in real life. Again in their model it shows that as the crowding increases, not only does pro-insulin misfolding increase, but the transmission of amylin is increased, when normally it is <1%. In transgenic mice expressing amylin, when fed a high fat diet their beta-cells did not compensate as did the beta-cells of their wild type brothers (44). Amylin is cosecreted with insulin and if you know anything about Alzheimer's you probably have heard of amyloid. Amylin is special  because unlike insulin it is not globular but for fibrous in shape, thus more likely to form plaques (however other animals don't have the proper sequence to form amyloid (45), thus this finally helps explain to me the large difference in phenotypes).

A recent treatment for NIDDM has been islet cell transplantation, however they always fail quite quickly. This is most likely due to the fact that they are functioning near 100% to maintain glucose levels, thus the overcrowding, ER stress, and amylin/pro-insulin misfolding leads to apoptosis and dysfunction (25,46).

Now back to my favorite study (14):The simultaneous assessment of AIR, OGTT-derived variables and IS provides insight into the possible reasons underlying the dependence of AIR on IS and its impairment in glucose-intolerant states. In fact, our results suggest that AIR is a hybrid variable, related to fasting insulin secretion and fasting glucose. Its relation to IS is lost when these variables are accounted for statistically (Table 4). These associations suggest that upregulation of AIR with insulin resistance could reflect a primary dependence of the fasting secretory tone on IS. The conceptual model could be as follows. First, the rapid discharge of insulin that follows a pulse increase in glycaemia is at least in part dependent on the pre-existing magnitude of the immediately releasable pool. Second, the size of this pool depends on the balance between exocytosis and refilling from precursor pools. Third, an increased fasting secretion translates into an increased size of the pool (by refilling), while an increase in ambient glucose levels tends to decrease the pool by stimulation of exocytosis. And fourth, AIR is also dependent on factors other than the pool size, possibly related to glucose sensing (e.g. calcium signalling).So trying to tie everything together. Due to increased caloric intake and decreased physical activity which leads to increased cholesterol (48), increased oxidative stress, decreased adaptation, this culminates in ER stress, and the constant intake of large amounts of carbohydrates causes hyperglycemia which empties the pool of insulin which then creates more proinsulin and amylin which then leads to overcrowding, and the constant attacks by these various processes lead eventually to apoptosis and dysfunction.

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