The recent consumer interest in books such as The Zone, Body-for-Life and Dr. Atkins’ Diet Revolution, has prompted confusion among public and professionals alike as to the best way to lose body fat. These diets recommend a reduction in carbohydrate intake and subsequent increase in protein consumption — disputing the typical view suggesting that high carbohydrate diets are the best way to lose fat.

As with anything challenging conventional wisdom, these books are surrounded by a good degree of controversy (which never hurts sales). However, despite the arguments, it appears that a diet containing moderate levels of carbohydrate and protein could be a far more effective way to drop body fat.

Let’s start with the basics.

Firstly, it’s worth remembering that the the main determinant of weight loss is the extent to which caloric intake differs from caloric output. In other words, energy expenditure (e.g. physical activity) must be greater than energy intake (in the form of food). When expenditure matches intake, body weight will remain the same.

This was recently demonstrated by a research team at New Zealand’s Otago University [2]. They persuaded a group of 32 endurance-trained cyclists to alter their diets for 12 weeks. One half of the group consumed a high fat diet (47% of total calories came from fat). The rest consumed a high carbohydrate, low fat diet (69% of total calories came from carbohydrate).

Body fat levels remained unaltered over the three-month period, simply because energy intake was indentical to energy expenditure. That’s the bottom line. If you’re not losing fat, it’s because energy intake matches energy expenditure.

But (and here’s where it gets interesting), a number of recent trials show that the composition of the diet can have a big impact on fat loss. Specifically, diets that provide an identical number of calories but contain different levels of protein, carbohydrate and fat can vary widely in their effect on the rate of fat loss.

Confused? I’ll explain.

The majority of studies have played around with the level of protein and carbohydrate in the diet. While it’s certainly not true that one diet is suitable for everyone, there is a growing body of evidence suggesting that more protein and less carbohydrate will accelerate fat loss — even if the caloric content of both diets is identical.

A recent paper, published in the International Journal of Obesity, compared the effects of two different diets on weight loss [1]. Thirteen subjects consumed either a high (58% of total calories) or low (25% of total calories) carbohydrate diet. Four weeks later, the low carbohydrate group lost over a third more weight than their high carbohydrate counterparts.

But here’s where you have to be careful. You see, when most people talk about weight loss, what they really mean is fat loss. When your carbohydrate intake drops, the body turns to stored carbohydrate (called glycogen) for energy. Because glycogen also stores three times its weight in water, low carbohydrate diets lead to rapid water loss.

The result? It appears as though you’ve lost a lot of weight — in reality, you’re simply losing water. That’s why short-term studies reporting weight loss (rather than fat loss) aren’t much help, so here’s one a little more relevant.

A Danish research team, tracking subjects for six months, report that a reduction in carbohydrate intake leads to greater fat loss [6]. A group of 60 subjects were assigned to a high protein or high carbohydrate diet. The results are shown in the table below.

Dietary analysis revealed that the high protein group had eaten less food, which accounted for the greater fat loss. This was probably due to the fact that protein has a higher satiating (pronounced say-she-ate-ing) effect than carbohydrate. In other words, subjects feel less hungry when consuming a diet high in protein. It also takes far more energy to digest protein — which would have increased energy expenditure in the high protein group.

The last finding is particularly interesting, especially for individuals whose rate of weight loss has slowed. A well-established response to energy restriction is a reduction in metabolic rate, which can drop by up to 20% in response to a restricted calorie diet [7] — making further fat loss very difficult. However, diets high in protein can prevent this metabolic ‘slowdown’.

For example, a research group at Aberdeen’s human nutrition unit gave subjects a restricted calorie diet for seven days [7]. One diet derived 36% of its energy from protein, while the other two contained 15% protein. The drop in 24-hour energy expenditure in the high carbohydrate groups was almost double that seen in subjects consuming the high protein diet.

A research group from the University Hospital Geneva also report that diets containing less carbohydrate promote greater fat loss [3]. The results are shown in the table below.

Although both diets contained an identical number of calories, subjects consuming the low carbohydrate diet lost 11% more fat than those on the high carbohydrate diet. The reduction in waist circumference was also 20% greater in the group receiving the low carbohydrate diet.

Of considerable interest is the reduction in blood insulin levels seen in subjects consuming the low carbohydrate diet, which dropped by almost twice as much as the high carbohydrate group. Insulin is a hormone that blocks fat "burning", via its action on several enzymes that play a role in converting fat into energy.

As such, the lower levels of insulin resulting from the low carbohydrate diet might explain the increase in fat loss.

One major criticism of low carbohydrate diets is the concern that reductions in glycogen storage limit the capacity for regular exercise. Although this may be true for elite athletes performing high volumes of training, it doesn’t necessarily apply to individuals who are exercising and dieting primarily to reduce body fat.

Your body has a remarkable ability to adapt to the fuel source provided to it. Give it an abundance of carbohydrate, and it will simply ‘burn’ more glucose, reducing its reliance on fat. For example, after four weeks on a low (36%) carbohydrate diet, Italian scientists were unable to find any adverse effects on exercise performance [5]. Cyclists switching from a high (51%) to a low (37%) carbohydrate diet did so without suffering a drop in training performance or aerobic capacity [2].

Restricted calorie diets deriving a larger proportion of their energy from protein (30-40% of total calories), and less from carbohydrate (30-40% of total calories) can accelerate fat loss when compared to their high carbohydrate (60% of total calories) counterparts.

Low carbohydrate/high protein diets also promote greater feelings of satiety (in other words, you feel fuller for longer), minimise the drop in metabolic rate and reduce fasting insulin concentrations — leading to greater fat loss. As such, they do offer an alternative course of action for individuals unable to find success with the traditional high carbohydrate approach to weight loss.