Title: Endurance Athletes and Diet
Key words: dietary strategies, supplementation, eating disorders, post-event recovery, electrolyte losses, isotonic sports drinks, glutamine, amino acids, overtraining syndrome, high protein diets, iron, calcium, amenorrhoea, hypo-oestrogenism, GI symptoms, free radicals, mucosa, hypoxia
Date: Sept 2000
Category: 12. Sports
Author: Dr van Rhijn
Endurance Athletes and Diet
The nutritional intake of elite endurance athletes is a critical determinant of their athletic performance1, making proper education, understanding and guidance regarding their dietary requirements essential. They have high nutritional demands, requiring careful, individually tailored dietary strategies to cater for specific physiological needs, a world apart from just ‘eating enough foods’. The need for dietary supplementation definitely arises in this gruelling sport, which will be further discussed.
Endurance athletes have an increased demand for carbohydrates, yet there is a tendency to manipulate their energy intakes (eating disorders common2) to achieve minimum fat levels3. An intake goal in the order of 7-10g/kg/d (60% energy) requires a deviation from the typical western diet. Low bulk, concentrated CHO intakes are needed before competition, during4 and in post event recovery phases, though the type of CHO, size and frequency of the meals (grazing) and awareness of micro- and macro-nutrient content need consideration. Potential fluid and electrolyte losses5,6 are enormous, requiring careful, individual replacement strategies before, during and after an event to avoid gastrointestinal upsets7. Isotonic sport drinks8,9 prevent dehydration and enhance endurance10 and performance11.
Endurance athletes induce tremendous stress on their bodies and the characteristic depletion of skeletal muscle glutamine pools12,13 impairs immune function14,15 for 6-9 hours after severe exercise. They are more vulnerable to infection16,17,18 during this recovery period as damaged muscles are unable to respond to the increased demand. Glutamine can be considered as an “essential” amino acid in athletes who may be suffering from “overtraining syndrome”19, requiring supplementation to replenish the deficit and its immuno-enhancing effect has been confirmed by numerous studies20. Increased metabolic protein demand during exercise21 for growth, muscle repair, amino acid oxidation and nitrogen losses do not equate to an improved performance by high-protein diets22,23. Immune response can be jeopardized not only by deficiencies but also by excessive intake of certain nutrients24.
Long distance runners are prone to iron deficiency 25 (various mechanisms) which can have a detrimental affect on performance26. Calcium levels are often sub-optimal, risking development of the ‘Female Athlete Triad Syndrome’27, the interrelation of eating disorders (weight loss), amenorrhoea (hypo-oestrogenism) and osteoporosis, increasing vulnerability to osteoporosis and stress fractures28. Many athletes run the risk of sub-optimal intakes29 of Fe, Ca, Zn, Mg, Cu and the B-group vitamins. Athletes require regular monitoring, dietary advice regarding Ca and Fe rich foods30,31 (not easily achieved in practice) and may require supplementation32,33 to compensate for increased demands34.
The Role of the Gut
Up to 30-50% of exhausting endurance event participants suffer gastrointestinal (GI) symptoms35, such as reflux, stomachache, vomiting, acid reflux, bloating, bleeding and diarrhoea. Inability to adapt to physiological stress imposed during endurance exercise37 can result in breached integrity of the GI mucosal block38, delayed small intestinal transit, reduced absorption39 and an increased colonic transit. Blood flow is diverted away from the gut40,41 (enhanced by dehydration) in favour of an increase to muscle42. This may adversely affect absorption, increase secretion (Zn) and makes the mucosa very vulnerable to energy deficits, hypoxia, free radical damage43 and pathogenic toxins. Small feedings during exercise may prevent GI blood flow from falling to critical levels. The biochemical adaptation to exercise44 may not be sufficient to compensate for deficiencies, despite an adequate diet. Ingestion before exercise of large meals of foods rich in dietary fibre45, fat, protein, caffeine, salt, hypertonic drinks and dehydration, hypoglycaemia and low CHO loading46 can aggravate the symptoms47.
Endurance runners, with low energy and nutrient intakes below RDA levels are at risk of deficiencies. Professional nutritional education and guidance on appropriate dietary regimes to supply their higher demands for nutrients should suffice. However, some athletes will require supplementation of CHO, electrolytes and certain key nutrients.
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