Science of Sports Nutrition

Vitamin C

2011-11-30T23:02:00.000-08:00

Vitamin C, also known as ascorbic acid, is a water-soluble vitamin. It is essential, as humans do not have the ability to biosynthesize vitamin C in the body or only in very low, inadequate amounts. Ascorbic acid is reversibly oxidized to dehydroascorbic acid (DHA). Because both forms exhibit anti-scorbutic activity, the term vitamin Crefers to ascorbic acid and DHA.

Vitamin C is a strong water-soluble antioxidant protecting cells and cellular components from free radicals by donating electrons. Vitamin C seems to regenerate other antioxidants such as vitamin E.113 Because of its reducing power, the vitamin functions primarily as a cofactor for reactions requiring a reduced iron or copper metalloenzyme.112 Vitamin C also has a role in the transport of non-heme iron. Vitamin C is necessary for the synthesis of collagen, which is a structural component of cartilage, ligaments, tendons, and other connective tissue. The vitamin is required for the synthesis of neurotransmitters, norepinephrine, and epinephrine and for the synthesis of carnitine, which is essential for the transport of fatty acids into the mitochondria for conversion to energy. Vitamin C functions in the conversion of cholesterol to bile acids and probably in the reduction of folic acid intermediates and the proper metabolism of the stress hormone cortisol.

The vitamin C deficiency disease is scurvy, characterized by symptoms related to connective tissue defects. Symptoms of scurvy include follicular hyperkeratosis, swollen or bleeding gums, perifollicular hemorrhages, joint effusions, arthralgia, fatigue, depression, and impaired wound healing. Scurvy is rare in developed countries, but does appear in children and the elderly on restricted diets and in alcoholics. The RDA for adults is 90 mg/d for men and 75 mg/d for women. For smokers, intakes of 35 mg vitamin C higher than for nonsmokers are recommended daily as smokers generally have lower blood vitamin C concentrations and are under increased oxidative stress from the toxins in cigarette smoke. The vitamin C content of selected fruits and vegetables is found in Table 4.7. Dietary sources of vitamin C include fruits and vegetables. Some foods such as breakfast cereals, sports drinks, and various nutrition bars, are fortified with vitamin C. Vitamin C in supplements is typically freeascorbic acid, calcium ascascorbate, sodium ascorbate, and ascorbyl palmitate.

Adverse effects from vitamin C intakes have been reported primarily with large doses (> 3000 mg/d), which may include diarrhea and other gastrointestinal disturbances. Possible adverse effects include increased oxalate excretion and kidney stone formation, increased uric acid excretion, pro-oxidant effects, systemic conditioning, increased iron absorption leading to iron overload, reduced vitamin B12 and copper status, increase oxygen demand, and erosion of dental enamel; however, the relationship between these effects and excess vitamin C intakes have not been clearly confirmed. Therefore, the UL for vitamin C established by IOM is 2000 mg/d for adults to prevent diarrhea and gastrointestinal disturbances.

Vitamin C has certain biological functions that can influence physical performance. It is needed for synthesis of collagen for ligament and tendon formation, synthesis of carnitine to use fatty acids as an energy source during exercise, synthesis of neurotransmitters for metabolic responses to exercise, and iron metabolism related to anemia and fatigue with consequential decreases in aerobic performance. Thus, adequate status of vitamin C is of importance for athletes. Physically active adults generally have been reported to have adequate amounts of vitamin C intakes and plasma vitamin C concentrations in the range of normal values. However, suboptimal intakes of vitamin C were determined in some athletic groups, such as male wrestlers and female basketball player and gymnasts. Approximately 12% of a group of athletes had lower levels of plasma vitamin C concentrations than the normal range.70 Because physical activity can increase plasma vitamin C concentrations for up to 24 hours, caution should be taken when interpreting lasma ascorbic acid values of athletes.

Niacin

2011-11-22T22:17:00.000-08:00

Niacin is one of the water-soluble B-vitamins and is also known as vitamin B3. The
term niacin is the generic descriptor for nicotinic acid and nicotinamide, which are essential for formation of the coenzymes nicotinamide adenine dinucleotide (NAD)
and nicotinamide adenine dinucleotide phosphate (NADP).

Approximately 200 enzymes, primarily dehydrogenases, are dependent on the
coenzymes NAD and NADP, which both serve as a hydrogen donor or electron acceptor.
53 NAD functions as a coenzyme with enzymes involved in the oxidation of fuel
molecules such as glyceraldehyde 3-phosphate, lactate, alcohol, 3-hydroxybutyrate,
and a-ketoglutarate. NADP functions as a hydrogen donor in reductive biosynthesis
such as in fatty acid and steroid syntheses, and is involved in the pentose phosphate
pathways. In addition, NAD acts as donor of adenosine diphosphate ribose (ADPribose)
for the post-translational modification of proteins and for the formation of
cyclic ADP-ribose. Two enzymes, mono ADP-ribosyl transferase and poly ADPribose
polymerase, function to transfer ADP-ribose from NAD to an acceptor proteins
including histone and histone proteins. These proteins seem to function in DNA
replication and repair and in cell differentiation.

The niacin deficiency disease is known as pellagra, whose symptons include pigmented
rash developed by sunlight exposure, changes in the digestive tract causing
vomiting, constipation or diarrhea, a bright red tongue, and neurological symptoms
including apathy, depression, fatigue, headache, and loss of memory. The symptoms
of pellagra are often referred to as the four D’s: dermatitis, dementia, diarrhea,
and death. Niacin deficiency is rare in industrialized countires except in malnourished
alcoholics and in individuals having disorders of tryptophan pathways.

In that trypophan can be converted to niacin, niacin contents in foods and dietary
recommendations are commonly expressed as mg of niacin equivalents (NE). Sixty
milligrams of dietary tryptophan is considered equivalent to 1 mg of niacin. Thus, 1 mg
NE is equal to 1 mg of niacin or 60 mg of dietary tryptophan. The recommendations for
niacin intake as RDAs for adults are 16 mg NE/d for men and 14 mg NE/d for women.
Niacin is widely distributed in both animal foods and plant-derived foods. The dietary
sources of niacin are yeasts, meats, poultry, fish, nuts, and enriched products such as
cereals and grains. The dietary sources of the amino acid tryptophan include lean meats,
fish, poultry, and nuts. In supplements and in food fortification, niacin is generally found
as nicotinamide, while nicotinic acid is often used as a cholesterol-lowering agent.

Large doses of nicotinic acid are used in the treatment of hypercholesterolemia.
The pharmacological doses have been shown to significantly lower total serum
cholesterol and low-density lipoproteins and increase high-density lipoproteins.
Despite the therapeutic effect of nicotinic acid, side effects are associated with these
large doses. The side effects are flushing of the skin with a concomitant itching and
feeling of heat, liver cell damage (hepatotoxicity) including elevated liver enzymes
and jaundice, and gastrointestinal problems such as heartburn, nausea, and vomiting.
Because flushing is the first observed adverse effects after large doses of nicotinic
acid, as this symptom is observed at lower doses than other adverse effects, the
IOM selected flushing as the most appropriate endpoint on which to base a UL.
Nicotinamide is generally better tolerated than nicotinic acid, and does not generally
cause flushing. However, a UL for niacin is considered protective against potential
adverse effects of nicotinamide. The UL for niacin is 35 mg NE/d for adults, and
is limited to niacin that is obtained from supplements or fortified foods. There is no
evidence of adverse effects from naturally occurring niacin from foods.

Niacin functions in energy metabolism like other B vitamins; therefore, exercise
performance could be affected by niacin status. However, there is no evidence of
inadequate niacin status in athletes or physically active individuals. Large doses of
nicotinic acid have been reported to decrease mobilization of free fatty acids (FFAs)
during exercise, which could affect exercise performance because circulating
FFAs provide energy in the muscle during exercise. A decrease in FFA availability
with large doses of niacin as nicotinic acid does not impact physical performance
unless carbohydrate sources are limited. Although experimental data are lacking,
the IOM has recommended at least 10% increase in the niacin requirement to allow
for increased energy utilization and the physical size of individuals who exercise
vigorously.

Energy Balance

2010-11-09T20:10:00.000-08:00

“Energy balance” is a catchphrase that is commonly used by exercise physiologists, nutritionists, and dieticians. It refers to the number of calories (energy) that we take in and whether it is above or below the amount we need to live. Depending on which side of the scale this falls, it will determine whether we are gaining or losing weight. Energy is consumed in the diet through protein, carbohydrate, and fat intake.

In the presence of excess calories, the body will subsequently convert and store them as triglycerides in adipose tissue. Over time, if this process continues, obesity results. Obesity as defined by some as a body mass index (BMI) greater than 30. However, because BMI is not an accurate reflection of adipose mass,1 it may not be the best objective measure of obesity. Obesity results as a consequence of increasing both the size and number of adipocytes. As stated, the overconsumption of macronutrients contributes to obesity.

However, not all macronutrients contribute to obesity in an equal manner. For instance, a highfat meal that results in a positive energy balance will stimulate fat storage without a subsequent match in fat oxidation. Furthermore, the deposition of excess dietary triglycerides into adipose tissue is associated with a very low metabolic cost (0–2%). In contrast, Flatt et al. reported that the conversion of carbohydrate into fat is an energy-requiring process in which 25% of the energy content is lost as heat. Of the three macronutrients, protein requires the most metabolic cost to be converted to and subsequently stored as fat. In studies involving twins, energy intake and obesity has been explored. Authors studying both monozygotic and dizygotic twins reported that there may be macronutrient-specific familial influences, but not conclusively. Also, greater-than-average caloric intake was associated with increased levels of body fat, despite likely genetic influences on both phenotypes. However, the authors concluded by stating that more research is needed to determine the genetic influence on energy intake. Finally, Tholin et al. studied genetic and environmental influences on eating behavior in a large cohort of monozygotic and dizygotic twins. The study concluded that genetic factors are of great importance of in the eating behavior in young adult male twins.

In conclusion, it has been reported that the increase in the prevalence of obesity has concomitantly occurred with an increase in portion sizes of foods that we consume. Furthermore, it has been shown that eating low-energy-dense foods such as fruits and vegetables helps sustain satiety while concurrently reducing energy intake, which appears to be a more effective weight loss strategy than fat reduction and decreased portion sizes. Energy expenditure, also known as physical activity, is one of the two components of the energy balance equation. Both energy intake and energy expenditure are profoundly important when discussing the etiology of obesity. Energy expenditure is composed of three categories: basal metabolic rate, thermic effect of food, and physical activity. Physical activity can also be broken down into two distinct sub-classes. The first is activity thermogenesis, which is volitional exercise. The second category is coined non-activity exercise thermogenesis, which is all of the activity that one performs that is not related to “sporting-like” exercise. Examples of this include fidgeting, housework, etc. It should be noted that non-activity exercise thermogenesis is often difficult to quantify. Activity thermogenesis accounts for approximately 15–50% of total daily expenditure in the sedentary to very active population respectively.

It further has been estimated that spontaneous minor activity performed during the day can account for 20% of the differences of energy expenditure in a 24-hour time frame. Equally important, Castaneda et al. reports that minimal amounts of spontaneous physical activity are a major predictor of accumulating fat mass during overfeeding in humans. An inverse association between physical activity and weight as a result of epidemiological studies has also been shown. Similarly, Meredith et al. reported a negative association between aerobic exercise at 65–80% maximal oxygen (VO2max) uptake and body composition. Additionally, a meta-analysis demonstrated that weight training as well as aerobic exercise is effective in facilitating weight loss, but also can increase or maintain lean mass. Thus, adding physical activity to promote weight loss encourages favorable changes in body composition. Sedentary lifestyle is commonly mentioned as a significant cause of the mounting prevalence of obesity.
In a study by Slentz et al. researchers reaffirmed what many have already suspected. If individuals partake in a modest exercise program similar to those suggested by the Centers for Disease Control and the American College of Sports Medicine, significant increases in visceral fat can be avoided and exercise that is a modest increase beyond their recommendations can facilitate significant decreases in visceral, subcutaneous, and total abdominal fat without changes in daily dietary intake. In a study published in 2005, researchers looked at the comparison of monozygotic twins in relation to lipoprotein and weight variation between vigorously active and sedentary siblings. The authors reported vigorous exercise could possibly reduce genetic influences on body mass index. Therefore, it is of critical importance to investigate energy expenditure in relation to obesity, especially when it has been reported that approximately 22–29% of adults report that they engage in no leisure-time physical activity.