Niacin

Niacin is involved in energy production from carbohydrate, protein, and fat; glycogen synthesis; and normal cellular metabolism through its active coenzymes. These enzymes, nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP), are essential for normal muscle function. Niacin deficiency is well documented in human populations suffering from famine or monotonous intakes of unenriched grain products, but there is no evidence of niacin deficiency in athletes.

Niacin is found in meat, whole or enriched grains, seeds, nuts, and legumes. Body cells have the capacity to synthesize niacin from tryptophan (60 milligrams of tryptophan yields 1 milligram of niacin), an amino acid found in all high-quality protein foods (e.g., meat, fish, poultry). Given the broad spectrum of foods that contain niacin, it is relatively easy for people to consume the DRI of 12 to 14 milligrams per day, or 6.6 niacin equivalents (NEs) per 1,000 calories. NEs are equal to 1 milligram of niacin or 60 milligrams of dietary tryptophan; you can obtain niacin directly from food or indirectly by consuming the amino acid tryptophan. The NE unit of measure takes both sources into account.

A deficiency of niacin results in muscular weakness, loss of appetite, indigestion, and skin rash. It is possible to produce toxicity symptoms from excess niacin intake; symptoms include gastrointestinal distress and feeling hot (becoming red faced or flushed). It may also result in a tingling feeling around the neck, face, and fingers. These symptoms are commonly reported in people taking large doses of niacin to lower blood lipids.

In studies evaluating the performance effects of niacin supplementation, endurance was reduced because the excess niacin caused a reduction in fat metabolism.^7-9 This resulted in a greater reliance on carbohydrate fuels (glucose and glycogen) to support physical activity. Because the storage of carbohydrate is limited, athletes taking niacin supplements had lower endurance. To date, there is no evidence that the requirement for niacin is increased with physical activity.

Vitamin B₆ refers to several compounds (pyridoxine, pyridoxal, pyridoxamine, pyridoxine 5-phosphate, pyridoxal 5-phosphate, and pyridoxamine 5-phosphatepyridoxine) that all display the same metabolic activity. Vitamin B₆ is most concentrated in meats (especially liver) and is found also in wheat germ, fish, poultry, legumes, bananas, brown rice, whole grain cereals, and vegetables. Because the function of this vitamin is closely linked to protein and amino acid metabolism, the requirement is linked to protein intake (the higher the protein intake, the higher the vitamin B₆ requirement).

The adult requirement is based on .016 milligrams of B₆ per gram of protein consumed each day1 and is satisfied in those consuming typical protein intakes. When you consider that high-protein foods are also typically high in vitamin B₆, those consuming protein from food (regardless of the amount) are most likely to have adequate B₆ levels as well. However, many athletes consume additional protein in purified supplemental forms (protein powders, amino acid powders), making it conceivable that athletes with high supplemental protein intakes may have an inadequate B₆ intake.

Vitamin B₆ functions in reactions related to protein synthesis by aiding in the creation of amino acids and proteins (transamination reactions) and is also involved in protein catabolism through involvement in reactions that break down amino acids and proteins (deamination reactions). It is involved, therefore, in manufacturing muscle, hemoglobin, and other proteins critical for athletic performance. The major enzyme of vitamin B₆, pyridoxal phosphate (PP), is also involved in the breakdown of muscle glycogen for energy through the enzyme glycogen phosphorylase.

A chronic deficiency of vitamin B₆ leads to symptoms of peripheral neuritis (loss of nerve function in the hands, feet, arms, and legs), ataxia (loss of balance), irritability, depression, and convulsions. An excess intake of vitamin B₆, primarily from consumption of supplements, does lead to toxicity symptoms that have been documented in humans. These symptoms are similar to those seen in B₆ deficiency and include ataxia and severe sensory neuropathy (loss of sensation in the fingers). Toxicity symptoms have been documented in women taking supplements that, on average, equal 119 milligrams per day to treat premenstrual syndrome and several types of mental disorders.^10,11

There is a theoretical basis for investigating vitamin B₆ and athletic performance. B₆ is involved in the breakdown of amino acids in muscle as a means of obtaining needed energy and in converting lactic acid to glucose in the liver.¹² Vitamin B₆ is also involved in the breakdown of muscle glycogen to derive energy. Other functions of vitamin B₆ that may be related to athletic performance include the formation of serotonin and the synthesis of carnitine from lysine. There is evidence that some athletes may be at risk of inadequate vitamin B₆ status.^13-15 Poor B₆ status reduces athletic performance.¹⁶

Because many athletes are always looking for that extra edge, there is an understandable attractiveness to natural substances that are legal. Vitamin B₆ is sometimes marketed as one of those natural and legal substances; besides its importance in energy metabolism, it is linked with the production of growth hormone, which can help increase muscle mass.¹⁷ It appears as if the combined effect of exercise and vitamin B₆ on growth hormone production is greater than either of these factors individually.^{18, 19}

Before athletes take vitamin B₆ supplements, the following factors should be considered:²⁰

• Most athletes have adequate vitamin B₆ intakes and a healthy vitamin B₆ status.
• Those athletes with poor vitamin B₆ status are generally those with inadequate energy intakes.
• A greater proportion of female athletes and athletes participating in sports that emphasize low body weight (gymnastics, wrestling, figure skating) are likely to have inadequate energy and protein intakes and, therefore, inadequate vitamin B₆ intakes.
• High doses of vitamin B₆ have been shown to have toxic effects.
• While poor B₆ status is associated with reduced athletic performance, there is no good evidence that consuming more than the recommended intake has a beneficial effect on athletic performance.²¹
• Vitamin B₆ supplementation does not appear necessary to enhance athletic performance if a balanced diet with adequate energy is consumed.²²

All things considered, these factors should encourage athletes to consume an adequate intake of energy substrates rather than resort to supplements of vitamin B₆.