Study shows cutting carbs does not enhance fat burning

There is a movement towards low-carb hi-fat (LCHF) diets, for weight loss and diabetes reversal. This is the same as the Banting diet, Atkins diet, South Beach diet and numerous variations. These diets are quite attractive because they are relatively simple – no carbs, eat high fat meats and dairy products. But do these diets really help you burn fat faster? They are often favoured by meat lovers, but hard to do for vegetarians, especially those limiting or avoiding dairy products.

There is little doubt that you can lose weight on these diets. But three important questions remain unresolved. These are:

  • Do you lose weight and stabilise blood sugar control just because you are eating less calories?
  • Is an LCHF diet any more effective than a low GL (glycemic load) diet, as I have advocated, where you reduce carbs but really emphasise choosing low GL carbs, plus other ways to lower the GL such as combining protein with carbs and increasing soluble fibres?
  • If you switch the body’s metabolism to run on ketones, the energy source derived from fat, do you burn more fat, lose more weight, in other words ‘expend more calories’.

There are four reasons why I don’t buy into LCHF diets

1. Firstly, because numerous studies have shown (a) that the weight loss is attributed to less calories – not that theres’ anything wrong with that. It may well be that eating in this way reduces appetite. It certainly reduced sugar cravings and many get fat due to sugar addiction. However, low GL diets do the same thing equally well.

2. Secondly because every clinical trial in which one group is put on a low GL diet and the other an Atkin’s style diet, have shown equal weight loss, although one study showed slightly more fat loss on the low GL diet, and better cardiovascular risk reduction, compared to a high protein diet.

Harvard’s Professor of Nutrition, David Ludwig, one of the world’s leading expert on carbohydrates, agrees. “Replacing added sugars and refined starchy foods with unprocessed carbohydrate, healthful fats and proteins may provide many of the benefits of a very low carbohydrate diet, without having to eliminate an entire class of nutritious (and delicious) foods.”

3. The third reason is that I have yet to see any evidence that switching the body’s metabolism over into ketosis, that is burning ketones for energy instead of glucose, does anything magical. Atkins claimed that, somehow, the body would lose more weight/fat by expending more calories (more out than in) but studies didn’t confirm this. However, this is not an easy thing to study. You’d have to have two groups on exactly the same ‘calories in’ from diet and exactly the same ‘calories out’ from exercise in a ‘metabolic chamber’ which measures the calories/energy you burn as heat loss, as well as measuring weight and fat loss.

This week the results of such an experiment were released at the International Congress on Obesity in Vancouver.

Dr Kevin Hall, an expert in metabolism, set out to test the idea that it is carbs that makes us fat and switching carbs for fat would have a positive result. He took seventeen obese volunteers who were willing to spend a considerable amount of time, two days a week, in a metabolic chamber during this two month study. In the first month their diet was adjusted to give them exactly the amount of calories they burnt, so no weight loss. (Actually, they were taking in slightly less than they needed, and losing a small amount of fat, equivalent to 300kcals energy loss.) In the second four weeks the calorie intake, and their activity (calorie expenditure) and their protein intake was kept exactly the same. All that was changed was cutting carbs to 5% of calories and replacing it with fat, equalling 80% of calories. The theory that removing almost all carbs would drive the body to break down fat and you’d lose more fat, more weight and increase ‘calories out’. This is an expensive and thorough study, funded by the Nutrition Science Institute and the US National Institutes of Health (NIH).

When the volunteers were switched onto the LCHF diet they lost, in the first week, about 1.5kg (3lbs). However, the rate of fat loss was negligible. It actually slowed down, compared to the previous weeks. That means that the initial weight loss was not fat loss.


The body stores glucose as glycogen. We have, on average, 0.5kg of glycogen stored in the body. However, every gram of glycogen carries with it 3 grams of water, meaning that if you depleted all your stores of glycogen, you can lose up to 2kg. This is what happened, as expected, in the first the first week when volunteers were starved of glucose in favour of fat.

But, after that, this study found no evidence that the LCHF diet enhanced fat loss. Watch this interview with the chief researcher who explains the study and its results.

So, metabolically speaking, there is no advantage in cutting all carbs and replacing with fat.

4. The fourth reason I’m sticking with my low GL approach, which also emphasises increasing micronutrients – vitamins, minerals, essential fats and other key diet components that help the body’s metabolism operate at optimum – is that, based on my experience with thousands of people, this approach works very well and is much easier to sustain, with a wider variety of delicious foods, and hence more pleasurable.


Learn more

Our 100% Health Club members can read more about this in last month’s newsletter article ‘Confused about carbs?’ and find out the results of our 8 week low GL diet ‘trial’ in this month’s 100% Health newsletter. Find out more about the low GL diet here.