Basic Human NutritionLecture 8

Sports Nutrition

Benefits of physical activity

Assists with weight maintenance Increases lean body mass Improves cardiovascular

performance Improves resistance to disease,

specifically heart disease, diabetes, high blood pressure, stroke and some cancers

Benefits of physical activity continued

Improves mental outlook Improves flexibility and strength

How do you get physically fit?

Four components of fitness: Flexibility Muscle strength Muscle endurance Cardio-respiratory endurance To build fitness, a person must engage

in physical activity. Muscles adapt to activities they are called upon to perform.

Physical Activity Guidelines Frequency of activity – 3 -5 days per

week Intensity of activity: 55 to 90% of

maximum heart rate Duration of activity: 20 to 60 minutes of

continuous activity

Physical Activity Guidelines continued

Mode of activity – any activity that uses large muscle groups

Resistance activity: strength training of moderate intensity at least 2 times per week

Flexibility activity: stretching major muscle groups 2 -3 times per week.

Muscles adapt to activity demands In response to an overload of physical

activity, muscle cells gain strength and size called hypertrophy. If not used, muscles atrophy.

Periodic rest is necessary for muscles to adapt. During rest, muscles build more of the equipment they need to perform the activity.

Benefits of weight training

Weight training builds lean body mass

Develops strength and endurance of muscles and benefits health and overall fitness.

Weight training helps maximize and maintain bone mass

Cardiorespiratory endurance

Increases cardiac output and oxygen delivery

Increases heart strength and stroke volume

Slows resting pulse Increases breathing efficiency Improves circulation Reduces blood pressure

Fuels that support physical activity are: Glucose from carbohydrate Fatty acids from fat Small amounts of amino acids from

protein. The body uses different mixtures of

fuels at different times depending on the intensity and duration of the activity and the body’s own prior training

Body’s use of fuels continued During rest, half of the body’s energy

comes from fatty acids, rest from glucose and a little from amino acids.

During physical activity, muscle glycogen is the major fuel in the first few minutes.

As the activity continues, hormone epinephrine, signals the liver and fat cells to liberate glucose and fatty acids. The muscles pick up the nutrients from the blood.

Glucose use and storage Body’s glycogen stores in the liver

and muscles are limited. Intense activities, like sprinting use

glycogen quickly. Glycogen depletion occurs after about 2 hours of intense activity.

A high-carbohydrate diet enhances an athlete’s endurance by ensuring adequate glycogen stores.

Aerobic use of glycogen During moderate activity, the

lungs and circulatory system can keep up with the muscles’ need for oxygen.

The person breathes easily, heart rate is elevated but at a steady pace.

Activity is aerobic.

Aerobic use of glycogen continued In aerobic activity, muscles extract

their energy from both glucose and fatty acids when both are present together with oxygen.

A little glucose helps metabolize a lot of fat.

Moderate Aerobic activity conserves glycogen stores.

Anaerobic Use of Glucose During intense activity, the heart

and lungs can only provide so much oxygen so fast.

The demand for energy outstrips the oxygen supply

Aerobic metabolism cannot meet the energy needs.

Anaerobic use of glycogen continued

Muscles rely more heavily on glucose which can be partially broken down by anaerobic metabolism.

Muscles draw more heavily on their limited glycogen supply.

Lactic Acid Anaerobic breakdown of glucose

produces lactic acid Lactic acids are fragments of glucose

molecules that accumulate in the tissues and blood.

Nervous system and hormones detect these fragments and speed up heart and lungs to draw in more oxygen to breakdown the fragments.

Activity Duration and Glucose Use

For first 10 minutes – muscles rely on glycogen.

Muscles use up about 1/5 of available glycogen in about 20 minutes of moderate activity.

Body responds by increasing uptake of blood glucose.

Activity duration and glucose use continued In moderate exercise that continues past 20

minutes, the body begins to use fat for fuel, but glucose use still continues.

If activity continues, glycogen stores run out.

When glycogen stores are depleted, physical activity can continue for a short time only because the liver makes more glucose from lactic acid and certain amino acids.

Activity Duration and Glucose Use continued With glycogen stores depleted,

exhaustion and hypoglycaemia results and all activity ceases.

To postpone exhaustion, endurance athletes must maintain their glucose concentrations for as long as they can.

Maintaining blood glucose for activity

4 diet strategies:1) Eat a high CHO diet on a daily basis2) Take glucose during activity3) Eat CHO-rich foods after the activity

to boost the storage of glycogen4) CHO loading. Training muscles to

maximize glycogen stores

Carbohydrate loading Is manipulation of activity and CHO

intake to trick the muscles into storing extra glycogen before competitive sports

1 week before competition:first 4 days, athlete trains moderately hard (1-2 hrs/day) and eats a diet moderately high in CHO.

Carbohydrate loading continued

Last 3 days of week, athlete cuts back on activity and eats a very high CHO diet (8 g CHO per kg body weight or 70% of calories)

With CHO loading, athletes can store extra glycogen to fuel activity up to 90 minutes or longer.

Carbohydrate loading continued In hot climate, extra glycogen beneficial as

glycogen releases water when broken down.

Eating a high-CHO meal within 2 hours after physical activity accelerates glycogen storage by 300%.

Timing is important. Eating the high CHO meal after the 2 hours has passed, decreases the rate of synthesis of glycogen by almost half.

Training effect on glycogen

Trained muscles versus untrained muscles: adapt to store more glycogen burn more fat at higher intensities

therefore, require less glucose to perform the same amount of work

Training effect on glycogen continued

A trained person uses less glycogen per minute to support an activity than an untrained person

A trained person can work at higher intensity for longer periods than an untrained person while using the same amount of glycogen

Should athletes eat more fat?

An athlete who eats a fat-rich diet with little CHO will burn more fat during activity, but will sacrifice endurance.

A high-fat diet requires more oxygen to yield energy, therefore more stress is placed on the heart to supply the oxygen to the muscles

Should athletes eat more fat continued…

High-fat diets increase risks of cardiovascular disease

Body fat stores are a more important source of energy for the athlete than fat in food.

Body fat as fuel Fat stores are (theoretically) an

unlimited source of fuel. Early in activity, muscles draw on fat

from 2 sources- fats stored within working muscles and fats from fat deposits under the skin.

Areas that have the most fat to spare donate the greatest amounts of fatty acids to the blood.

Intensity affects fat use Intensity of activity affects

percentage of energy used from fat.

Fat can only be broken down for use aerobically.

When intensity surpasses body’s ability to supply energy aerobically, the body cannot burn more fat. It burns more glucose.

Duration of activity affects fat use… At start of activity, blood

concentration of fatty acids fall Norepinephrine signals fat cells to

break stored triglycerides apart to release fatty acids into the blood.

After 20 minutes, blood concentration of fatty acids rises above normal resting concentration.

Affect of duration of activity on fat use continued

Only during this phase of sustained, moderate activity after the first 20 minutes, do the fat cells shrink and empty out their stored fat.

Training-repeated aerobic activity, stimulates the muscles to develop more fat-burning enzymes.

Fat as energy continued

Trained muscles burn fat more readily.

Heart and lungs become better at supplying oxygen during high intensities and enables muscle to burn more fat.

Protein builds muscle Athletes use protein to build and

maintain muscle and other lean structures.

After physical activity, muscles increase rate of protein synthesis – they build more muscle to perform the activity.

To rebuild itself, muscle must first be broken down then rebuilt.

Protein for muscle building continued… Physical activity with a slight

overload results in larger muscles. Dietary protein supplies the amino

acids to build protein Genetic code in nuclei of muscles

know when protein is needed and which type of protein to support each type of activity.

Protein continued Intensity and pattern of muscle

contractions initiate signals for the muscles cells to develop specific proteins E.g., weight lifter – more muscle fibers for

bulk and strength and more enzymes for making and storing glycogen.

A jogger’s cells respond by producing proteins for aerobic oxidation of fat and glucose.

Protein used for fuel

A weight lifter may add between ¼ oz and 1 oz (7 and 28g) of protein each day to muscle mass. This protein comes from dietary protein.

Athletes retain more protein and use a little more as fuel than untrained people.

Protein use for fuel continued The body speeds up its use of

amino acids for energy during physical activity.

Protein contributes about 10% to total energy.

Factors that regulate protein use: Carbohydrate intake Intensity and duration of activity Degree of training

How much protein is needed?

Athletes require only a small amount more than untrained people.

Joint paper by American Dietetic Association and Dietitians of Canada recommends: 1.0 to 1.5g per kg of body weight per

day.

Vitamins and Minerals B vitamins – Thiamine, riboflavin and niacin

are important to energy release from nutrients.

Vitamin B6 and B12 play key roles in the release of energy, liberation of glucose from glycogen and formation of haemoglobin.

No evidence to support increases in any of the above vitamins will enhance performance.

Vitamins and Minerals continued Vitamins C and E- potent

antioxidants. High intensity activity increases

oxygen consumption tenfold which enhances production of damaging free radicals in the body.

Vitamin C protects Vitamin E from oxidation which protects cell membranes against oxidation.

Vitamins C and E continued

Evidence indicates supplements with Vitamins C and E or E alone might benefit performance.

No recommended level as yet, still under investigation.

Minerals continued

Endurance athletes, especially women at high risk for iron deficiency.

Physical activity impairs iron status: Iron excreted in sweat Iron lost through red blood cell

destruction from high impact sports

Physical activity impairs iron status continued

Irion lost in some athletes through small blood losses in digestive tract.

High iron demands of muscles for aerobic metabolism

Habitually low iron intakes, vegetarianism

Sports anaemia Early in training, athletes can develop low

blood haemoglobin called sports anaemia. This is a normal adaptation to physical

activity Aerobic training promotes increases in

fluid in blood which dilutes concentration of red blood cells in a unit of blood.

Sports anaemia goes away by itself even with continued training. True anaemia does not.

Calcium Calcium intakes of females athletes are

often low due to calorie restriction, particularly in dancers and gymnasts where weight is important.

Female athletes at risk for osteoporosis and increased risk for stress fractures.

Though activity generally builds bone mass, extremes of activity may be detrimental to bone health.

Chromium, zinc, copper, magnesium Chromium, zinc and copper have

specific roles in physical activity The excretion of all 3 increases during

physical activity, but not enough is known about effect this has.

Magnesium deficiency affects muscle gains in a given amount of training.

Losses in sweat are the same for trained and untrained individuals.

Sodium and potassium Untrained people lose more

electrolytes: sodium, potassium and chloride than trained people.

With regular physical activity, body adapts and conserves these minerals.

Profuse sweating can deplete body of potassium, but it is easily replaced with a few fruits and vegetables.

Fluids and body temperature Need for water exceeds all other

nutrients. Physical activity accelerates water loss

through sweat and breathing (water exhaled as vapor).

A loss of even 1-2% of body weight can reduce a person’s capacity to do work.

A loss of 7% of body weight can lead to collapse.

Temperature regulation

Heat stroke – dangerous accumulation of body heat with accompanying fluid loss.

Athletes need to drink enough fluid before, during and after an event to prevent dehydration and heat stroke.

Temperature regulation

Hypotehermia –significant loss of body heat. Even in cold weather, sweat losses occur.

Athletes need to drink warm or beverages at room temperature to prevent heat loss and dehydration when engaging in cold weather activities like cross-country skiing.

Fluid needs of athletes

Endurance athlete can lose 2L or more of water in every hour of activity. Digestive system can only absorb about 1L per hour.

Athletes need to drink more in preparation for competition. Extra water is not retained in the body, but ensures maximum hydration at time of event.

Water versus Sports Drinks Plain cool water is sufficient for most

athletes as water: 1) rapidly leaves the digestive tract and

enters tissues and 2) water cools the body from the inside

out. Endurance athletes are exception. Need

water and carbohydrate to replenish glucose stores.

Sports drinks - benefits Taste good , so people drink more Provide psychological benefits and

some essential nutrients for endurance athletes.

Provide fluid to prevent dehydration Provide glucose in right proportion –

about 7%. CHO concentration greater than 10% delays gastric emptying and delivery of water to tissues.

Sports drinks continued

Contain sodium and other electrolytes to help replace those lost in sweat. Sodium helps to accelerate rate of fluid absorption from the digestive tract.

Athletes can replace lost electrolytes from activity with a meal following the activity.

Sports drinks continued

In strenuous activity in hot, humid conditions, heavy sweating coupled with plain water can dangerously dilute sodium in the blood.

In this instance, electrolytes need to be replaced during the activity—sports drinks can do this.

Other beverages- Caffeine

Caffeine in beverages in moderate amounts (2 cups) can enhance athletic performance.

Caffeine may stimulate release of fatty acids early into blood thereby conserving glycogen.

Caffeine

Also a diuretic, can cause headaches, stomach upset, diarrhea, constricts blood vessels and raises blood pressure all of which negatively impacts performance.

Caffeine in national/international sports restricted to <800 mg (5-6cups strong coffee) 2 hours before an event.

Beverages continued Carbonated beverages – not a good idea.

Carbonation bubbles take up room in the stomach, so drink less.

Alcohol – dehydrates and promotes excretion of vitamins such as thiamin, riboflavin, folate, calcium, magnesium and potassium.

Alcohol alters perception, reduces strength, slows reaction and impairs judgement.

Choosing a performance diet

Key concepts: Nutrient density – foods with

maximum nutrients, vitamins, minerals for the energy that they provide. E.g. meat sandwich versus bowl of salad.

Choosing a performance diet

Balance – Athletes need to eat for energy as energy needs are immense- 3000 to 5000 calories per day. But, must still chose foods from all 4 food groups to prevent disease.

Diet should be based on Canada’s Food Guide To Healthy Eating

Pre-Game meal Food should be CHO-rich and the meal

light (300 – 500 calories) Meal should be easy to digest and

should contain fluids. E.g., bread, pasta, potatoes and fruit juice.

Avoid high protein, high fat, high fibre foods before a game –take too long to digest and may cause stomach upset during game.

Pre-Game meal continued

Meal should be completed 3-4 hours before competition to allow stomach to empty.

Special food bars do not provide all nutrients, offer no advantage over food and are expensive.

Male athletes and weight Some sports such as wrestling and

horse jockeys need to “make weight”. This can lead to unhealthy practices of fasting, dehydration through sweat in steam rooms and diuretics to shed water before an event.

These practices are dangerous and diminish performance during and for days after an event.

Female athlete triad

Disordered eating: coaches and athletes have unrealistic expectations for weight for performance.

Coaches may not understand that an athlete’s body is denser: more muscle and bone mass than non athlete, therefore cannot use general population standards for weight

Female athlete triad continued

Amenorrhea: loss of menstrual cycles. In normal population, may occur in about 2% of pre-menopausal women, but in female athletes, prevalence as high as 66%.

Amenorrhea is not a normal adaptation to physical activity – symptom of something wrong.

Female athlete triad continued

Amenorrhea characterized by low blood estrogen levels, infertility and bone mineral losses.

May be due to very low body fat. May be due to vigorous training

combined with low food energy which can result in low estrogen.

Female athlete triad continued Osteoporosis - Low estrogen

contributes to stress fractures and osteoporosis in later life.

Women with anorexia nervosa particularly at risk.

Women with bulimia rarely cease menstruating so may be spared bone loss.