Is supplementation the right strategy? Or do athletes need to be make preventive changes to their diets?
Exercise stresses many of the metabolic pathways in which micronutrients are required, and training may result in bio-chemical adaptations in the muscles that increase the need for some micronutrients.
Athletes who frequently restrict energy intake, rely on extreme weight-loss practices, eliminate one or more food groups from their diet, or consume poorly chosen diets, may consume sub-optimal amounts of micronutrients and would, therefore, require and benefit from micronutrient supplementation.
According to a study published in March 2016 in the journal, Medicine & Science in Sports and Exercise, the most frequently noticed deficiencies noted are calcium, vitamin D, iron, and some anti-oxidants. Single-micronutrient supplements are generally only appropriate for correction of a clinically defined medical reason like iron supplements for iron deficiency anaemia (IDA).
Iron deficiency, with or without anemia, can impair muscle function and limit work capacity leading to compromised training adaptation and athletic performance. Sub-optimal iron status often results from limited iron intake from sources such as meat, sea food and poultry, and inadequate energy intake.
Periods of rapid growth, training at high altitudes, menstrual blood loss, foot-strike haemolysis, blood donation, or injury can negatively impact iron status. Some athletes in intensive training may also have increased iron losses in sweat, urine, faeces and from intra-vascular haemolysis.
A compromised iron status can negatively impact health, physical and mental performance, and warrants prompt medical intervention and monitoring. Iron requirements for all female athletes may be increased by up to 70% of the estimated average requirement.
Athletes who are at greatest risk – such as distance runners, vegetarian athletes, or regular blood donors – should be screened regularly and aim for an iron intake greater than their recommended daily allowance of 18 mg for women and 8 mg for men.
Athletes with IDA should seek clinical follow-up, with therapies including oral iron supplementation, improvements in diet and a possible reduction in activities that impact iron loss, such as blood donation, and a reduction in weight-bearing training to lessen erythrocyte haemolysis. The intake of iron supplements in the period immediately after strenuous exercise is contra-indicated as there is the potential for elevated hepcidin levels to interfere with iron absorption.
Reversing IDA can require up to six months and, therefore, it is advantageous to begin nutrition intervention before IDA develops. Athletes who are concerned about iron status, or have iron deficiency without anaemia, should adopt eating strategies that promote an increased intake of food sources of well-absorbed iron as the first line of defence.
Although there is some evidence that iron supplements can achieve performance improvements in athletes with iron depletion that are not anaemic, unmonitored supplementation is not recommended. Such supplementation is not considered ergogenic without clinical evidence of iron depletion, and may cause unwanted gastro-intestinal distress.
Some athletes may experience a transient decrease in haemoglobin at the initiation of training due to haemo-dilution, which is also known as “sports anaemia”, and may not respond to nutrition intervention. These changes appear to be a beneficial adaptation to aerobic training and do not negatively impact performance.
There is no agreement on the serum ferritin level that corresponds to a problematic level of iron depletion/deficiency, with various suggestions ranging from 10 to 35 ng/ml. A thorough clinical evaluation in such cases is warranted as ferritin is an acute-phase protein that increases with inflammation.
Vitamin D regulates calcium and phosphorus absorption and metabolism, and plays a key role in maintaining bone health. There is also emerging scientific interest in the bio-molecular role of vitamin D in skeletal muscle, where its role in mediating muscle metabolic function may have implications for supporting athletic performance.
A growing number of studies have documented the relationship between vitamin D status and injury prevention, rehabilitation, improved neuro-muscular function, increased Type-II muscle fibre size, reduced inflammation, decreased risk of stress fracture and acute respiratory illness.
Athletes who live at latitudes above the 35th parallel, or who primarily train and compete indoors, are likely at higher risk for vitamin D insufficiency and deficiency. Other factors and lifestyle habits such as dark complexion, high body fat content, undertaking of training in the early morning and evening when UVB levels are low, and aggressive blocking of UVB exposure (clothing, equipment, and screening/blocking lotions) increase the risk for insufficiency and deficiency.
As athletes tend to consume little vitamin D from food, dietary interventions alone have not been shown to be a reliable means to resolve insufficient status. Supplementation above the current RDA and/or responsible UVB exposure may be required to maintain sufficient vitamin D status.
A recent study of the US-based National Collegiate Athletic Association swimmers and divers reported that athletes who started at 130 nmol/L and received daily doses of 4,000 IU of vitamin D were able to maintain sufficient status over six months, while athletes receiving placebo experienced a mean loss of 50 nmol/L.
Determining vitamin D requirements for optimal health and performance is a complex process. Empirical data is still needed to elucidate the direct role of vitamin D in musculo-skeletal health and function to help refine recommendations for athletes.
Until then, athletes with a history of stress fracture, bone or joint injury, signs of over-training, muscle pain or weakness, and a lifestyle involving low exposure to UVB may require clinical assessment to determine if an individualised vitamin D supplementation protocol is required.
Calcium is especially important for growth, maintenance, and repair of bone tissue, regulation of muscle contraction, nerve conduction, and normal blood clotting. The risk of low bone-mineral density and stress fractures is increased by low energy availability, and in the case of female athletes, menstrual dysfunction, with low dietary calcium intake contributing further to the risk.
Low calcium intakes are associated with restricted energy intake, disordered eating and/or the specific avoidance of dairy products or other calcium-rich foods. Calcium supplementation should be determined after a thorough assessment of usual dietary intake.
Calcium intakes of 1,500 mg per day and 1,500–2,000 IU per day of vitamin D are needed to optimize bone health in athletes with low energy availability or menstrual dysfunctions.
Anti-oxidant nutrients play important roles in protecting cell membranes from oxidative damage. Because exercise can increase oxygen consumption exponentially, it has been hypothesised that chronic training contributes to a constant “oxidative stress” on cells.
Acute exercise is known to increase levels of lipid peroxide by-products, but also results in a net increase in native anti-oxidant system functions and reduced lipid peroxidation.
Thus, a well-trained athlete may have a more developed endogenous anti-oxidant system than a less active individual and may not benefit from anti-oxidant supplementation, especially if consuming a diet high in anti-oxidant rich foods.
The safest and most effective strategy regarding micronutrient anti-oxidants is to consume a well-chosen diet containing anti-oxidant-rich foods. The importance of reactive oxygen species in stimulating optimal adaptation to training merits further investigation, but current literature does not support anti-oxidant supplementation as a means to prevent exercise induced oxidative stress.
Athletes at greatest risk for poor anti-oxidant intakes are those who restrict energy intake, follow a chronic low-fat diet, or limit dietary intake of fruits, vegetables and whole grains.
In summary, athletes should be educated that the intake of vitamin and mineral supplements does not improve performance unless reversing a pre-existing deficiency and the literature to support micronutrient supplementation is often marred with equivocal findings and weak evidence.
Despite this, many athletes unnecessarily consume micronutrient supplements, even when dietary intake meets their needs. Rather than self-diagnosing the need for micronutrient supplementation, when relevant, athletes should seek clinical assessment of their micronutrient status within a larger assessment of their overall dietary practices.
Sports dieticians can offer several strategies for assessing micronutrient status based on collection of a nutrient intake history along with observing signs and symptoms associated with their deficiency. This is particularly important for iron, vitamin D, calcium, and anti-oxidants.
By encouraging athletes to consume a well-chosen diet focused on food variety, sports dieticians can help athletes avoid micronutrient deficiencies and gain the benefits of many other performance-promoting eating strategies.
– Courtesy: The American College of Sports Medicine, the Academy of Nutrition and Dietetics, and the Dieticians of Canada.