Fitness professionals should not recommend low-carbohydrate, high-protein diets due to lack of evidence related to weight management and associated health concerns.

Point 1 - Yes, eating little carbohydrate may result in greater 'fat burning'

The low carbohydrate theme says that cutting down on consumption of carbohydrates reduces blood sugar levels, subsequently resulting in a lower level of insulin production by the pancreas (insulin is a hormone related to tissue storage of macronutrients, including sugars, fats and protein). This in turn forces the body to burn fat stores, resulting in reduction of body fat.

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 With such a diet, the absence of carbohydrate is likely to stimulate an increase in circulating glucagon (a hormone related to greater fat utilisation), without an increase in insulin secretion.

Most scientists would agree that this type of food pattern would create a biochemical environment favourable for fat loss. But they would not recommend the diet for health reasons.

Point 2 - Yes, more protein may increase energy expenditure

The energy cost of digestion and absorption of protein may be greater than that of carbohydrate. Energy expenditure after a meal (post-prandial thermogenesis) has been measured to be greater after a high-protein meal.

Point 3 - Yes, more protein may suppress appetite

In feeding studies, protein generally scores higher on scales of satiety. Of the macronutrients, protein is most likely to suppress appetite. The satiety response to dietary protein may also be inversely related to habitual protein intake. This may explain why some people report less hunger than others when they adopt a higher protein diet.

Point 4 - But, most of the initial weight loss is water

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. 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 indicate a weight reduction.

Point 5 - But, the diet is often low in energy

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. Unfortunately, credit for success in this situation is often wrongly attributed to the ratio of fat, carbohydrate and protein.

Point 6 - But, adherence is likely to be poor

Low carbohydrate, high protein diets are quite limiting in the selection of foods you can consume. 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.

Point 7 - But, the diet could have a negative effect on physical 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. 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.

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.

Point 8 - But, low-carb diets are deficient in important nutrients

Diets like the Dr Atkins' New Diet Revolution that allows no more than 20g of carbohydrates pei day - the amount in one and a half slices of bread - will reduce the variety and amount fibre and antioxidant-rich, plant-based foods consumed, which could contribute to an increased the risk of developing various cancers. Supplementation is not the answer, as pills cannot supply al the beneficial chemicals found in plants.

Point 9 - But, we know little about the long-term physical risks

An increase in protein consumption from animal sources such as red meat and full-fat diary products will result in an increased intake of saturated fat, which may elevate blood cholesterol and contribute to the development of cardiovascular disease. A high protein diet may have othe health implications and its long-term effects on osteoporosis and kidney function have not been adequately studied.

Conclusion

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 low-fat, protein-rich foods like low-fat dairy products, lean meat and fish.

As the role of fitness professionals is to help clients achieve permanent healthy eating habits, not short-term weight loss, a diet that is reduced in fat, has adequate protein and is moderate to high in carbohydrate is the most prudent prescription.

Fitness professionals should not recommend low-carbohydrate, high-protein diets due to lack of evidence related to weight management and associated health concerns.

This article is a summary of Matt O'Neill's 'The Greate Protein Debate' presentation at FILEX, 2001.

References:

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

Long, S. J. etal (2000) Effect of habitual dietary-protein intake on appetite and satiety. Appetite, Vol. 35 : 79-88.

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

Golay, A. (1996) Weight-loss with low or high carbohydrate diet? International Journal of Obesity, Vol. 20, pp 1067-72.

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

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.

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.