1.2. La Coordinación en la política de cooperación para el desarrollo
1.2.1. La Coordinación en la evolución de las Políticas de Cooperación para el Desarrollo
There are eight B vitamins (thiamin, riboflavin, niacin, vitamin B6, folate, biotin, pantothenic acid, vitamin B12). All are involved in the metabolic path-ways for energy metabolism of the macronutrients:
carbohydrate, protein and fat (and alcohol). Their primary role is to act as coenzymes, which are molecules that bind with an enzyme to activate it.
Other micronutrients involved in energy metabolism include choline and the minerals iodine, chromium, manganese and sulphur. Several other minerals and trace elements, including magnesium, iron, zinc and copper, also act as enzyme activators, primarily in glycolytic and oxidative phosphorylation reactions.
Physical activity increases energy expenditure and theoretically increases the requirements and turnover of these micronutrients involved in energy metabo-lism. The B vitamins and zinc assist in the release of energy from carbohydrate, fat and protein. Minerals also act as coenzymes in energy metabolism, for example iodine for regulating metabolic rate in the thyroid gland, chromium for glucose metabolism and iron for cellular energy metabolism. The current data suggest that athletes have only slightly higher requirements for ‘energy’ nutrients than untrained controls, except for iron, levels of which are high in athletes involved in endurance training programmes.
This section considers only those micronutrients that have received the most research attention in relation to energy metabolism in athletes, namely the B vitamins, magnesium, chromium and iron.
B vitamins
Of the nutrients involved in energy metabolism, the B vitamins have received the most attention in athletes, particularly in relation to their primary and interactive role as coenzymes in carbohydrate and amino acid metabolism, the main substrates for providing energy or ATP to the muscle. Table 8.1 indicates other systems where B vitamins and folic acid have a role. For example, vitamin B12, folate and vitamin B6 are closely involved in the synthesis of red blood cells.
Suboptimal intakes and deficiencies of B vitamins are uncommon in athletes and levels are usually no different to those in untrained subjects. B vitamin deficiencies, usually reported in people with malnu-trition, do not usually occur in isolation or in athletes. Subclinical B vitamin deficiencies based on low biochemical indices are more likely to occur in athletes, although there are limited studies available.
Interestingly, marginal thiamin deficiency was reported in cyclists after a simulated trial of the Tour de France. Given the high energy expenditure and high carbohydrate intakes of these athletes, rapid turnover and depletion of thiamin was not
Table 8.2 Minerals involved in body functions that have reported and potential implications for athletic training and performance.
Co-factors for energy metabolism
Nervous and muscle function
Blood health (red cell
function) Immune function
Antioxidant
function Bone health
Fluid and electrolyte balance Macrominerals
Sodium ✓ ✓
Potassium ✓ ✓
Calcium ✓ ✓ ✓
Magnesium ✓ ✓ ✓
Trace minerals
Iron ✓ ✓ ✓ ✓a
Zinc ✓ ✓ ✓a ✓
Copper ✓ ✓a
Chromium ✓
Selenium ✓ ✓a
a Act as coenzymes for endogenous antioxidants.
Adapted with permission from Fogelholm M. Vitamin, mineral and anti-oxidant needs of athletes. In: Clinical Sports Nutrition, 4th edn (Burke L, Deakin V, eds). Sydney: McGraw-Hill, 2010.
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unexpected. Other B vitamins were not significantly depleted. Thiamin has a lower storage pool than other B vitamins, which explains its fast rate of deple-tion. For athletes participating in any long-distance event of high energy intensity, thiamin supplements or perhaps a B multivitamin supplement may be needed, if dietary intake is compromised. Subclinical thiamin deficiency is also associated with increased lactic acid levels during exercise, but marginal defi-ciency, induced by a thiamin-depleted diet, had no measurable effect on working capacity of exercising muscles.
Data on the effects of marginal riboflavin depletion on performance are scarce in athletes. Under situ-ations of depletion or deficiency, urinary excretion of riboflavin decreases which conserves further loss.
Although riboflavin is involved in muscle metabolism and neuromuscular function and is a co-factor in the production of the antioxidant enzyme glutathione peroxidase, no changes to muscle efficiency were reported in athletes undertaking moderate-intensity exercise after 7 weeks of a riboflavin-restricted diet.
Similarly, for athletes involved in wrestling and judo with depleted vitamin B6 status after a weight-cutting period, no effects on performance capacity in terms of anaerobic capacity, speed or strength were reported.
In summary, acute or short-term marginal deficiencies of single B vitamins, identified by blood biochemical measures of their status, have no impact on per-formance measures. However, despite the apparent absence of a performance effect with depleted status of a single B vitamin, aerobic performance capacity may be impaired when there is combined depletion of thiamin, riboflavin and vitamin B6. This highlights the synergistic role of the B vitamins and explains why is difficult to detect any performance effects from single vitamin depletion studies.
Do B vitamin supplements improve performance capacity?
Supplementation with either single B vitamins or multiple B vitamins can elevate biochemical or blood markers and improve a marginal to low micronutrient status, but has no significant effects on other metabolic systems or performance measures in athletes with ade-quate status, with few exceptions. In one study of male athletes, a combination of vitamin B6 supplements together with other B-complex vitamins improved shooting target performance and improved muscle
irritability. These athletes had adequate vitamin B status. However, similar intervention studies in other trained athletes have reported no significant effects of vitamin B6 or B multivitamin supplements on perfor-mance measures compared to controls.
There are only a few well-designed studies pub-lished that have investigated folic acid supplements in relation to sports-related functional capacity in athletes with adequate vitamin folate status. In these studies, folate supplementation slightly increased serum folate levels but did not affect maximal oxygen uptake, anaerobic threshold or other measures of physical performance.
In summary, based on the limited research pub-lished, vitamin B supplements taken as either single vitamin supplements or as a multivitamin B complex are unlikely to significantly affect or improve energy efficiency, oxygen uptake or performance capacity in athletes who are not deficient.
Magnesium and energy metabolism
Magnesium is a major mineral in bone and is involved in protein synthesis, enzyme action, muscle function including oxygen uptake, nerve impulse transmission, electrolyte balance and the immune system. Its role in substrate and energy metabolism in athletes has been the focus of most research in trained subjects. Strenuous exercise initiates a redistribution of magnesium in the body with a corresponding increase in magnesium loss via sweat, faeces and urine. These losses are significantly higher in athletes than untrained controls, which may increase requirements in athletes by up to 10–20%
higher than usual. Magnesium deficiency, and even marginal depletion, can impair oxygen delivery and thus the ability to undertake and complete submaximal exercise, which can reduce endurance performance. Recent evidence suggests that habitual magnesium intakes of below 260 mg/day for male athletes and less than 220 mg/day for female athletes may result in magnesium deficiency. These cut-offs are slightly below the EAR (Estimated Average Requirement) for men of 330 mg/day (19–30 years) and women of the same age of 255 mg/day. The EAR is the ‘amount of a nutrient consumed on an average daily basis that is estimated to meet the requirements of half the healthy individuals in a particular life stage and gender group’. This cut-off is used by health professionals to examine the probability that usual
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intake of a nutrient is adequate (or inadequate) in an individual subject. Clearly, the threshold of ade-quate intake and hence requirement in physically active individuals such as elite athletes is indicative of higher magnesium requirements than population benchmarks.
However, according to biochemical indices and dietary surveys of athletes, magnesium status appears satisfactory and is not compromised by these higher requirements, with some exceptions. Magnesium is a mineral in the chlorophyll molecule, which is used for photosynthesis in plant leaves. As expected, green leafy vegetables are good sources. As a mineral, it is absorbed from the soil and concentrates in other areas of the plant, particularly in the grain (or seed) so legumes, nuts, seeds and foods made from cereal grains (wholegrain) are rich sources. Highly processed and refined cereal grain foods are low in magnesium. Like all covalent minerals, absorption (or bioavailability) is decreased when magnesium-rich foods are consumed together with foods magnesium-rich in phytates and oxalates (see Table 8.5).
Do magnesium supplements improve performance capacity?
Magnesium supplements do not consistently increase serum magnesium levels or improve physical perfor-mance in physically active individuals with low to adequate magnesium status. Magnesium supplements of 250–360 mg/day as magnesium aspartate or up to 500 mg/day as magnesium picolinate or magnesium oxide for 3–4 weeks in athletes who have low but not deficient indices of magnesium status have been shown in some studies to improve muscle function, cardiovascular function and work efficiency. Hence, there may be a beneficial effect for magnesium supplements in athletes during periods of high-intensity training involving glucose as the pre-dominant substrate for metabolism. Nevertheless, other studies fail to show any beneficial or at least measurable enhancing effect on performance using magnesium supplements. The reason for this may be that magnesium fluxes or redistribution of magne-sium associated with exercise is highly variable between individuals and seems to alter with the type of exercise performed (i.e. aerobic vs. anaerobic).
Clearly, magnesium, like the B vitamins, does not work in isolation and exerts an independent effect on metabolic pathways.
In summary, magnesium supplements may be required in athletes at risk of suboptimal intakes or during periods of very high intensity workouts, when requirements are highest. The prevalence of low status is highest in the population in people consum-ing low energy intakes, so athletes involved in weight class sports or those who follow very low energy diets for whatever reason are likely to be at risk. Further research is needed in different groups of athletes undertaking varying levels of exercise intensities to determine the magnesium status and level of magne-sium depletion at which energy systems are compro-mised. Adverse effects of magnesium depletion on immune function and oxidative damage also need further investigation.
Chromium and energy metabolism
Chromium is an essential trace mineral and has many roles in metabolism. In relation to sports per-formance, its role in enhancing the action of insulin, which is required for uptake of glucose and amino acids into the cell, has implications for enhancing glucose oxidation and recovery. Other claims in relation to its action on insulin are increases in muscle mass and strength.
It has been proposed that chromium supple-mentation during exercise, mainly as chro mium picolinate, the most active form, enhances carbo-hydrate metabolism and promotes glycogen re- synthesis, hence speeding up recovery of fuel reserves.
Studies on these effects have not supported this hypothesis. Adding chromium picolinate to a sports drink provided no additional effect on carbo hydrate metabolism above that of the carbohydrate content in the sports drink. Other claims of habitual use of chromium supplements to increase muscle mass and strength and reduce body fat have not been substantiated in well-designed studies using a control group (see Chapter 9). Athletes with restricted energy intakes are most at risk of low chromium intakes.
Food sources of chromium include wholegrain cereals, eggs and poultry.
Iron and energy metabolism
Iron has many functions but is best known for its role in blood health (see following section). Iron is also a co-factor in several biochemical reactions involved in oxidative energy production, which occurs within
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the mitochondria, where it is a component of oxidative enzymes and respiratory chain proteins.
Clearly, iron has a strong functional role in maintain-ing the energy release from macronutrient substrates needed to support aerobic and endurance capacity.
When iron stores are exhausted, the functional iron compartment in the cells then becomes affected and the oxidative capacity of the muscle is compromised.
Recent evidence suggests that even marginal iron deficiency can reduce maximum oxygen uptake and aerobic efficiency in the muscle cells (the functional site) and decrease endurance capacity (see section Does low iron status (iron depletion) affect perfor-mance and other health outcomes?, p. 74).