Nutritional supplements: vitamins, minerals and amino acids in type 2 diabetes

Supplement use is widespread and growing. Over 50% of adults in the US population, and more than one-third of those in the UK and Canada, now use nutritional supplements for a variety of health reasons.1-3 Supplement usage by people with diabetes appears to be similar.3, 4 A number of nutritional supplements have been purported to be of specific benefit to people with type 2 diabetes. Many micronutrients influence carbohydrate metabolism and it has been suggested that minerals such as chromium, zinc and magnesium may help to improve glycaemic control, or even assist weight loss. A recent systematic review of the use of such supplements in diabetic management concluded that there is insufficient evidence to draw conclusions about their efficacy; randomised controlled trials of sufficient size and duration are lacking.5 However, these supplements cannot be dismissed as being without effect; many are clearly capable of altering metabolic parameters. The concern is that so little is known of what their effects might be, especially in the long term. Use of chromium is a case in point. Chromium is known to potentiate insulin action, and chromium supplementation has been shown to reduce hyperglycaemia in people with type 2 diabetes.6, 7 However, the amount of chromium needed to do so can be high—up to 1000µg/day—an amount that is many times the US recommended adult intake of 20–35µg/day.8 While there is no evidence that this level of intake is harmful, this is as much due to lack of data as documented lack of effect.8, 9 Concerns about the potential toxicity of chromium picolinate (the most commonly used form of supplemental chromium) which has been associated with renal failure and shown to cause in vitro DNA damage in mammalian cells have yet to be addressed.9 At present, the risk:benefit equation of chromium supplementation is simply not known. The benefits of other mineral and vitamin supplements used by diabetic patients are also questionable. Zinc deficiency can aggravate glucose intolerance but, since excessive zinc intake can also impair insulin secretion, zinc supplements can be counterproductive.10 Vanadium has been advocated as a treatment for diabetes despite the fact that it may not even be an essential nutrient for humans but simply a dietary contaminant.9 Since vanadium is absorbed and can accumulate in body tissues, there is potential for toxicity from supplemental intakes.11 Supplements of B vitamins are commonly recommended for people with diabetic neuropathy but their efficacy has yet to be established and concerns over high intakes of B6 remain.9, 11 Recently, there has been interest in amino acid supplements following studies showing that some amino acids such as leucine and phenylalanine can enhance insulin secretion in type 2 patients.12 However, history advises caution. Use of the amino acid tryptophan in isolated form has caused serious side effects, for reasons that are still not entirely clear.13 Since people with diabetes are at high risk of cardiovascular disease, it might be thought that taking supplements of antioxidants is a sensible measure. However, while numerous studies have shown protective benefits from a high intake of fruit and vegetables, large-scale prospective trials have failed to find any evidence that antioxidant supplementation reduces cardiovascular mortality.14 More worryingly, beta-carotene supplementation is associated with a small but significant increase in all-cause mortality, particularly from lung cancer in smokers.9, 14 Supplements do not appear to be an equivalent alternative to healthy eating.15 National guidelines on the dietary management of diabetes all agree that there is no indication for routine supplementation with any vitamin or mineral.16-18 This does not mean that people with diabetes never require supplements. Severe losses of micronutrients as a result of ketoacidosis or poorly controlled diabetes will need to be replaced. Some diabetic patients will require supplements for non-diabetic reasons such as the need for folate during early pregnancy, or calcium to prevent osteoporosis. Supplements may also be indicated for those who, like the general population, are at risk of micronutrient deficiency as a result of poor appetite, chronic illness or restrictive diets. Unfortunately, those who are most in need of supplements are often least likely to be taking them. Inadequate intakes of vitamins and minerals by institutionalised elderly people with diabetes are often overlooked and uncorrected.19 In contrast, many supplement users are middle-aged, well-educated, health-conscious women who already consume diets of higher than average vitamin and mineral content.20 It is often assumed that, even if unnecessary, supplements will not do any harm. This confidence may be misplaced. Micronutrients do not function in isolation but as part of an intricate network of absorption mechanisms, transport systems and metabolic pathways. High intake of one micronutrient can easily create an imbalance or deficiency in another: zinc can inhibit copper absorption, chromium can impair iron transport, vitamin E can antagonise the effects of other fat soluble vitamins.9 It is very difficult to create such distortions by means of food intake but very easy to do so by use of supplements. Megadose preparations providing many times the recommended daily intake are a particular risk but inadvertent overdosage also often occurs through use of different types of supplements containing the same nutrients (e.g. fish oils and antioxidant preparations may both contain vitamins A and E). It is understandable that people with a chronic disorder such as type 2 diabetes should wish to take all the steps they can to safeguard their health. However, supplement use should be based on clinical need, and underpinned by evidence of efficacy and safety. Searching for a magic bullet—the so-called ‘nutraceutical’ which provides a cocktail of micronutrients designed to improve glucose tolerance, insulin sensitivity and lipid metabolism—is probably the wrong approach. All these objectives, and more, can be achieved just as effectively, and far more safely, by diet and lifestyle change, particularly that which corrects obesity. Some people with diabetes spend considerable sums of money on nutritional supplements.3 For most, spending the money on fruit and vegetables would be a much better health investment.

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