BIOL2537 Laboratory in Nutritional Science

Name: Hung Wing Tung

UID: 2009053464

Group: 3

Date: 11/3/2011

Practical 2: Protein Quality and Urea Production in Saliva

Objective: To demonstrate the quality of protein of a test meal by determining the concentration of urea in saliva by spectrometry analysis

Introduction:

After digestion of proteins from diet, amino acids will be released. These amino acids are either used to synthesize tissue protein or converted into nitrogenous breakdown products, mainly urea. The extent of catabolism, and incorporation of dietary amino acids into tissue protein are determined by several factors, such as the individual need for growth and repair, energy and protein adequacy of the diet and the essential amino acid composition of the food protein (protein quality). The protein quality of a food depends on whether the essential amino acid composition matches with the ideal pattern, which can be used with 100% efficiency for tissue protein synthesis.

For any dietary amino acids not used for protein synthesis will be available for urea production theoretically. Therefore, the amount of urea produced after ingestion of a food usually reflects the protein quality of that food. Since urea is a small molecule that readily diffuses through all body fluids, in this practical, the urea concentration in saliva at various time intervals will be used to test the protein quality of 3 different foods (eggs, cheese and peanuts).

Procedures (Materials and Methods): As shown in lab manual.

Results:

Graph (a) The standard curve of urea concentration

Table (a) Diluted, original and change in urea concentration in saliva after consuming cheese at different time intervals

Time intervals (hr)

0

0.5

1.0

1.5

Diluted urea concentration (umol/ml)

0.14

0.13

0.15

0.13

Original urea concentration

(umol/ml)

1.12

1.04

1.20

1.04

Change in urea concentration (umol/ml)

0

-0.08

+0.08

-0.08

Table (b) Class result of total and change in average urea concentration in saliva after consuming 3 test meals at different time intervals

Time intervals (hr)

0

0.5

1.0

1.5

Average urea concentration (umol/ml)

Peanuts

1.52

2.54

2.45

2.92

Cheese

1.68

3.07

2.49

2.85

Egg

1.61

2.11

2.35

2.29

Change in average urea concentration (umol/ml)

Peanuts

0

+1.02

+0.93

+1.40

Cheese

0

+1.39

+0.81

+1.17

Egg

0

+0.50

+0.74

+0.68

Table (c) Change in urea concentration in saliva after consuming 3 different test meals throughout the experimental time

Type of food

Peanuts

Cheese

Egg

Change in urea concentration throughout the experimental time (area under curve) (hrumol/ml)

+1.35

+1.43

+0.81

Discussion:

Change in urea concentration in own saliva sample at different time intervals

Referring to Graph (b), my individual change in urea concentration in saliva did not have significant change after the consumption of cheese. The fasting (0 hr) urea concentration was 1.12 umol/ml. After consuming the cheese, it decreased to 1.04 umol/ml at 0.5 hr, and then increased to 1.20 at 1.0 hour and eventually dropped to 1.04 umol/ml again at 1.5 hour. However, all these changes were too little to reflect the protein quality of the cheese.

Comparison of mychange in urea concentration with class results

My results were deviated from class results that my urea concentration did not have significant change throughout the experimental time, while the class results had a fluctuated but general and significant increase in urea concentration. Besides, my directions of changes were totally opposite to class results. First, the urea concentration in class saliva samples increased after consuming the cheese for 0.5 hr, but my results was a reduction in urea concentration. When the class results showed a significant drop in urea concentration at 1.0 hr, my urea concentration increased a bit at same time interval. At 1.5 hr, when the class urea concentration increased, my urea concentration dropped a little.

The difference between mine and class results can be attributed to personal factors. For instance, although I ate all assigned cheese according to my weight and energy needs, I may have greater protein needs for tissue growth and repair than the others and thus more amino acids from the cheese protein were absorbed and incorporated for the synthesis of tissue protein. Therefore, less urea were produced and diffused to the saliva. Besides, the difference might be due to the presence of drinking water in my mouth cavity during collection of saliva samples, which greatly diluted the urea concentration in my saliva.

Difference in the change of average urea concentrations in different test meals

From the class results, the change in average urea concentrations in saliva samples for all test meals increased. This was because part of the dietary amino acids wasused for urea production, causing more urea diffused to the saliva.

The consumption of egg caused the lowest increase in average urea concentration, followed by peanuts, and lastly cheese. This reflects that a higher portion of amino acids from egg protein are incorporated for tissue protein synthesis than cheese and peanuts. Thus,egg has the highest protein quality among the 3 test meals. In fact, egg is regarded as the food having excellent protein quality (a complete protein) since it is rich in leucine, lysine, glutamate, and all other essential amino acids. These essential amino acids cannot be synthesized in the body and are required to taken in from diet. Therefore, when food containing these essential amino acids is consumed, the essential amino acids will be readily absorbed by the body as building blocks of many cellular compounds but not converted to nitrogenous compounds for urea production. This is the reason why less urea was present in the saliva after taking in egg than taking in other 2 test meals.

Although the experimental results showed that consumption of cheese caused a higher increase in average urea concentration than peanuts, theoretically, the urea production owing to intake of cheese should be less than that of peanuts. This is because cheese is a dairy product, which contains allessential amino acids derived from milk protein (also a complete protein), while peanuts only contain legume protein with low concentration of lysine and methionine. More urea should be produced from the non-essential amino acids present in peanuts. The inconsistency between the experimental results and theory might be attributed to some experimental errors, or the fact that the cheese might have been undergone some food processing procedures, and some of these procedureshave caused great milk protein loss.

Possible experimental errors which affect the overall class results

There were several errors that might affect the class results. First, some subjects might have eaten meals within the 3 hours before the consumption of test meals. This caused the measurement of urea concentration in the saliva less reliable since the urea produced might be derived from the food other than the test meals. Second, if the subjects did not consume the test meals according to their weight and energy needs, for example, eating less than the amount assigned, more food protein from test meals might be oxidized for ATP synthesis. This might overestimate the protein quality of the test meals. Third, for subjects who had difficulties in producing saliva, they might rely on drinking lots of water to assist the saliva production. This might lead to the presence of water in the mouth cavity which diluted the urea concentration of saliva, causing overestimation of protein quality of the test meals. Fourth, since the subjects were all unique individuals, they might have different body metabolism and hence different protein utilization. This caused the results of some subjects much deviated from the average. Fifth, the standard curves derived by different groups might be different, so the urea concentrations in saliva determined by the standard curve for one group might not fit with that determined by another standard curve. Sixth, the poor experimental skills of experimenters in preparing the sample solutions might also affect the results.

5 common foods in Hong Kong that are considered as High Quality Protein diet

The recommended dietary intake of all essential amino acids is illustrated in the following table:

Amino acids

Recommended dietary allowances for adults (mg/kg/day)

Histidine

14

Isoleucine

19

Leucine

42

Lysine

38

Methionine + cysteine

19

Phenylalanine + tyrosine

33

Threonine

20

Tryptophan

5

Valine

24

A High Quality Protein must be a complete protein which contains all these essential amino acids in the approximate amounts needed by humans.Most of the high quality proteins come from an animal origin. In Hong Kong, the 5 common foods which belong to High Quality Protein diet are namely egg, milk, cheese, chicken and beef.

Conclusion:

The protein quality of egg was the highest, followed by peanutsand then cheese.

Reference:

S. Gropper, J. Smith, J. Groff. 2009. Advanced Nutrition And Human Metabolism (5th Edition).

Graph (d) Change in urea concentration in saliva after consuming 3 different test meals throughout the experimental time

PeanutsCheeseEgg1.351.430.81

Test meals

Change in urea concentration over time

(hr umol/ml)

Graph (e) Comparison of the my change in urea concenration in saliva aftering consuming cheese with the class results

My results00.511.50-8.0000000000000016E-28.0000000000000016E-2-8.0000000000000016E-2Class results00.511.501.39000000000000010.811.1700000000000002

Time (hr)

Change in urea concentration (umol/ml)

Graph (b) My change in urea concentration against time

Change in urea concentration (umol/ml)00.511.50-8.0000000000000016E-28.0000000000000016E-2-8.0000000000000016E-2

Time (hr)

Change in urea concentration (umol/ml)

Graph (c) The class result of change in average urea concentration in saliva after consuming 3 test meals at different time interval

Peanuts00.511.501.020.931.4Cheese00.511.501.39000000000000010.811.1700000000000002Egg00.511.500.50.74000000000000010.68

Time (hr)

Change in urea concentration (umol/ml)

1