Title: Endurance Athletes and Supplementation
Key words: Daily energy requirements, supplements, blood nutrient status, free radicals, chromium, selenium, zinc
Date: July 2000
Category: 12. Sports
Author: Dr M Draper
Endurance Athletes and Supplementation
Introduction The daily energy requirements of elite athletes can be 2-3 times greater than untrained people. Most athletes need 40-60 % of their intake1 from carbohydrate (4.5 - 6.0gms/kg ,with intense training up to 10 gms/kg.) although the Zone diet advocates 40/30/302. Various papers give reviews and practical guidelines for ways to improve nutrition3,4. However, there appears to be concern that certain athletes may take up to 15 different types of supplement (cited in 5) often in quantities well above the recommended daily amounts. The rationale appears to have three types of logic:
(a) to compensate for poor diet or lifestyle
(b) to meet high nutrient demands during heavy exercise or
(c) to produce a direct ( ergogenic ) effect on performance.
Definition of Dietary supplement
A dietary supplement 'is a product which provides specific nutrient(s), usually in a more concentrated form or different composition than is normally found in food'. This does not strictly differentiate between those that are intended to correct perceived or actual subclinical deficiencies from those that are intended as a nutrititional ergonomic aid (eg creatine). I will concentrate on those that are taken to replace deficiencies in order to obtain a physiological response. A review of 51 studies6 showed an overall mean prevalence of athletes' supplement use of 46 % (rates range from 6-100%) so it is a common practice.
There appear to be three fundamental questions:
Is there evidence that supplementation helps athletes who have
(a) a poor diet
(b) a good diet but are susceptible to infections
(c) a good diet and their performance in competitions is improved.
There is minimal evidence for (a) or (b) in the literature and there is more evidence for macronutrient (eg CHO) manipulations7 or combined energy and micronutrient intake8 improving performance than micronutrient (vitamins or minerals) alone unless this was correcting a specific deficiency (eg Iron9, Zinc10 or Chromium11).
An interesting study approached the question in a different way and asked if athletes taking 6-8 months of mineral and vitamin supplementation showed alterations in their blood nutrient status. The conclusion was that there were increases in vitamins but not of minerals12. No well-being or performance parameters were measured. The additional intake of antioxidants for UK athletes has obvious theoretical advantages:
(a) in the UK average selenium intakes in the normal population are low (29- 39 microgms per day 1994 MAFF data)
(b) endurance exercise generates an increase in free radical generation.
However, a 10 week study in Germany on male student volunteers undergoing endurance exercise training failed to show any beneficial effect from organic selenium supplementation on training- induced adaptions13. A criticism of this study would be that organic selenium levels would probably have not reached equilibrium (up to 6 months). The supplementation of Vitamin E has been shown to alter certain indices of lipid peroxidation14 but not improve performance.
Conclusions Assuming an adequate dietary intake of macronutrients and in spite of the lack of unequivocal evidence to support my recommendations, I would advise a 70 kg male UK endurance athlete to take daily supplements of: (1) Chromium 50 - 100 microgms
(2) Selenium 100 microgms
(3) Zinc 5 mg and consider Vitamin A , C and E according to his dietary intake of fresh fruit and vegetables.
For vegetarians and female endurance athletes, I would want to see blood ferritin levels before advising on Iron supplementation.