The low carbohydrate theme says that cutting down on carbohydrates and therefore reducing blood sugar levels will cause the pancreas to produce less insulin (a hormone related to tissue storage of macronutrients, including sugars, fats and protein). This forces the body to burn fat stores and produces a reduction in body fat level. Whilst this process can occur in the body, it is only likely to provide an advantage for fat loss when taken to the extreme, say less than 100g of carbohydrate per day.(1) Here, the absence of carbohydrate is likely to stimulate an increase in circulating glucagon (a hormone related to greater fat utilisation), but will not stimulate an increase in insulin.

There are also other factors that influence weight loss. If you strictly follow a low carbohydrate, high protein diet you'll lose weight quickly. Most of the initial weight loss is due to a loss of liver glycogen, a store of carbohydrate. Liver glycogen, and then eventually muscle glycogen is used up because low-carb diets don't provide enough glucose to maintain normal blood sugar. Water molecules stored as part of glycogen are also lost when glycogen is converted into glucose.(2) Therefore, most of the initial drop in weight is due to water loss, not body fat reduction. The diet just appears to be successful because the bathroom scales say so.

Sticking to a low carbohydrate diet can become difficult. Low carbohydrate, high protein diets are quite limiting in the selection of foods you can consume. Even though an increased protein intake is likely to act as a strong appetite suppressant(3) , and may increase energy expenditure after a meal via greater post-prandial thermogenesis(4) , a diet without bread, pasta and rice is restrictive and can become boring. When dietary restraint falters and carbohydrates are consumed, glycogen stores are quickly replenished and subsequently weight is regained. If you do manage to stick to the diet for more than a few days, you may well continue losing weight because most low carbohydrate diets are also low in energy, including the well-publicised Zone Diet. Any diet where you consume less than you expend helps weight loss.(5) Unfortunately, credit for success in this situation is often wrongly attributed to the ratio of fat, carbohydrate and protein.

Now to the question of whether a low carbohydrate diet increases sports performance. Recent studies show that reducing carbohydrates may hamper rather than boost performance. It's well established that endurance performance (i.e., over an hour) is compromised when muscle glycogen is depleted.(6) The situation is like starting off on a long drive with a near-empty fuel tank - you won't get far.

Even performance of short-duration, high-intensity exercise, taking only seconds or minutes, can be compromised. Scientists believe the increased acid environment in the muscles created when you eat a low carbohydrate, high protein diet makes it harder for muscles to function. They also think it becomes harder to control the build-up of lactic acid, which also reduces the ability to sustain intense physical effort.(7) Low carbohydrate diets also present problems after exercise. If you limit the amount of carbohydrate consumed after strenuous activity you're likely to compromise glycogen replenishment. Avoiding insulin production by limiting carbohydrates at this time is counterproductive, as adequate glycogen storage could not be achieved without it.

Are low carbohydrate diets healthy? Diets like the Dr Atkins' New Diet Revolution that allows no more than 20g of carbohydrates per day - the amount in one and a half slices of bread - certainly aren't. These types of diets can reduce the variety and amount fibre and antioxidant-rich, plant-based foods consumed (8), which could contribute to an increased the risk of developing various cancers. Supplementation is not the answer, as pills cannot supply all the beneficial chemicals found in plants. A high protein diet may have other health implications and its long-term effects on osteoporosis and kidney function have not been adequately studied.(9)

In summary, some extra protein (i.e. an increase from around 15% to 25% of energy intake) may be useful as an appetite regulator, particularly for dieters who have reduced their intake of protein-rich foods like dairy products, meat and fish.(10) As our task is to help clients achieve permanent healthy eating habits, not short term fat loss a diet that's reduced in fat, has adequate protein and is moderate to high in carbohydrate is the most prudent prescription.

References

1. Cheuvront, S.N. (1999) The Zone Diet and Athletic Performance. Sports Medicine, Vol. 27., No. 4, 213-28.

2. Brosnan, J.T. (1999) Comments on metabolic needs for glucose and the role of gluconeogenesis. European Journal of Clinical Nutrition, Vol. 53, Suppl 1, S107-S111.

3. Rolls, B.J. (1995) Carbohydrates, fats, and satiety. American Journal of Clinical Nutrition, Vol. 61: 960S-967S.

4. Robinson, S. M. etal. (1990) Protein turnover and thermogenesis in response to high-protein and high-carbohydrate feeding in men. American Journal of Clinical Nutrition, Vol. 52, pp72-80.

5. Golay, A. (1996) Weight-loss with low or high carbohydrate diet? International Journal of Obesity, Vol. 20, pp1067-72.

6. Cheuvront, S.N. (1999) The Zone Diet and Athletic Performance. Sports Medicine, Vol. 27., No. 4, 213-28.

7. Maughan, R.J. etal. (1997) Diet composition and the performance of high-intensity exercise. Journal of Sports Sciences, Vol. 15, pp265-75.

8. Lean, M. etal. (1997) Weight loss with high and low carbohydrate 1200 kcal diets in free living women. European Journal of Clinical Nutrition, Vol 51., pp243-8.

9. Skov, A.R. (1999) Randomised trial on protein vs carbohydrate in ad libitum fat reduced diet for the treatment of obesity. International Journal of Obesity, Vol. 23, pp528-36.

10. Skov, A.R. (1999) Randomised trial on protein vs carbohydrate in ad libitum fat reduced diet for the treatment of obesity. International Journal of Obesity, Vol. 23, pp528-36.