LONDON — There is no convincing proof that high protein, low carbohydrate diets bring about long-term weight loss, and indeed, they may even be harmful to health, warns the American Heart Association (AHA) in advisory notice in October 9
Over half the population of the United States is either overweight or clinically obese, and the United Kingdom appears to be facing a similar problem. Since excess body weight is linked to a wide range of health problems, including heart disease, diabetes, hypertension, and some forms of cancer, there's an urgent need to find effective ways of weight reduction.
While pharmaceutical approaches, such as orlistat, may turn out to be helpful, many of those concerned about weight turn to dietary manipulation to address the problem. It is widely agreed that creating an energy deficit — by restricting calorie intake through diet and expending calories through increased physical activity — does promote weight loss. Where there's controversy is over the specific effects on body weight and metabolism of varying the macronutrient composition of the diet.
Fat and carbohydrate are the main macronutrient contributors of energy to the diet and in the average American diet 12–16% of the calories are derived from protein, 34% from fat and 49% from carbohydrate. A restriction in calorie intake from one food type is generally compensated for by an increase in another — thus those on a low-fat diet may not lose weight if they then consume more sugars. The AHA has long espoused the low fat, high carbohydrate approach. But there has also been increasing public interest in the diets advocated by Robert Atkins and others, which promote high protein intake, coupled with limitation — or even elimination — of carbohydrates.
The AHA is now warning against such diets, in which protein typically makes up to 20 to 30% or more of total energy intake, at the expense of carbohydrate intake. Consequently, a high protein diet may therefore achieve weight loss simply through an energy deficit occurring as a result of the type and amounts of food prescribed. A similar argument could be advanced for the apparent success of the 'food combining' approach.
The Atkins Diet centres on a high intake of animal rather than plant proteins, because these contain minimal carbohydrate. But this leads to a corresponding increase in fat intake (the only commonly occurring protein foods containing neither fat nor carbohydrate are egg white and powdered milk). Excess protein is not used efficiently by the body, and can promote an increased metabolic burden on the kidneys and liver. A high protein-high fat diet, deficient in carbohydrate, causes a metabolic shift in favour of ketosis, which has an anorectic effect. Put simply metabolic ketosis happens when cells are forced to burn fat for fuel in place of carbohydrate.
Susan Jebb of the MRC's Human Nutrition Research Unit is in broad agreement with the AHA's Advisory. "People do lose weight on a high protein diet," she says. "But that is only because calories are restricted, and protein is satiating. Balancing calorie intake with calorie expenditure is the only proven way of long-term weight loss. To attribute weight loss to metabolic change is a distortion of scientific fact. If you consume carbohydrate, insulin is secreted but if this is balanced by calorie expenditure, it does not create a problem."
Jebb is less concerned about the potential harm to the kidneys from excess protein intake as by the limitation on carbohydrate in the high protein diet. "This flies in the face of scientific evidence showing that carbohydrate — particularly complex carbohydrates — are good for you. People should consider adopting a long term eating plan that is consistent with both weight control and health. Going on a high protein diet for a few weeks to lose some weight is no good — for on-off dieting is also bad for your health."
The reduction in carbohydrate intake on the high protein diet causes a loss of fluid and therefore rapid initial weight loss — which may be highly motivating. The problem, says the AHA, is that there are no credible long-term studies showing the safety and effectiveness of the high protein approach. And as metabolic experts — and anyone who's ever been on a diet — know, losing weight is easy, but keeping it off is the hard part, so these are the studies that would count.
In addition a diet high in animal protein can also lead to an unfavourable blood lipid profile because of its inevitably high fat content. An increase in low-density lipoprotein (LDL) cholesterol, for instance, which is promoted by a high fat diet and is a cardiac risk factor, is compounded by the low carbohydrate component of the plan, for plant foods high in carbohydrate and fibre can lower LDL. Atkins argues that his plan does not raise cholesterol, so long as excess carbohydrates are not consumed.
Fruits, vegetables and whole grains — not allowed or restricted in the high protein approach — also lower the blood pressure and are implicated in protecting cells from the effects of carcinogens. High protein foods such as meat, poultry and seafood, are also high in purines, which are broken down into uric acid, suggesting that these diets could cause gout in certain individuals. It is also conceivable that high protein intake may impose considerable strain on the kidneys and so speed the progression of renal problems in those with diabetes. A further important consideration with this regime is that the restrictions on fruit and certain vegetables (because of their carbohydrate content) may also lead to deficiencies in vitamins and minerals.
Those who promote the high protein approach often argue that high carbohydrate intake results in elevated insulin levels and storage of body fat. Conversely the AHA advisory suggests that a high protein intake may also stimulate insulin secretion. The problem of insulin resistance — marked by excess insulin secretion — is complex and not well understood (save that it has an adverse impact on health). Calorie restriction, weight loss and exercise can help reduce insulin resistance.
In the wake of the high protein diet controversy the AHA reiterates that a 500 calorie deficit a day will help people lose weight and should be undertaken in the context of increased physical activity to maintain a negative energy balance. Total protein intake should not be excessive — 50 to 100g a day — and should make up no more than about 15% of calories, with fat comprising about 30% and carbohydrate about 55%. A minimum of 100g of carbohydrate, from a range of foods, is recommended. Such a diet should be feasible in the long-term, providing an adequate balance of nutrients to assure future health.
But it seems unlikely that Atkins will let the argument rest here.