Study reminds us of the superior performance of low-carb diets over low-fat ones for weight loss

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Where weight loss is concerned, there are many ways to skin a cat. But the two most commonly applied dietary strategies are low-fat and low-carb. This week I came across a report of a study here in which obese women were put on either a ‘low-fat’ or ‘lower-carb’ diet for a period of 12 weeks. Women in this study were supplied with appropriately made-up and calorie-controlled meals.

This is the carbohydrate:fat:protein breakdown of the two prescribed diets:

Low-fat – 60:20:20

Lower-carb – 45:35:20

The report of the study in question does not give us much more detail about these diets, other than the fact that the low-carb diet fatty component came largely from unsaturated fats (such as those found in nuts). We do not know, however, potentially important information such as the quality of carbohydrate in each diet. However, assuming that calorie intake is about 1500 calories per day, and knowing that there are about 4 calories in each gram of carb, we can calculate that the ‘low-carb’ diet offered up about 170 grams of carb.

This is indeed lower-carb than the low-fat diet deployed in this study. However, it’s still actually high in carbs compared to true ‘low-carb’ diets which are often advocated (including by me) for the purposes of weight loss.

Nevertheless, the reported results from this study show that the women eating the lower-carb diet lost an average of 3.4 lbs (1.5 kg) more than the other group (an average of 19.6 lbs v 16.2 lbs). This despite the fact that the women were, supposedly, eating the same number of calories.

I say supposedly because while the women were supplied with their food, there’s no assurance that they ate all of it and/or didn’t eat additional food. However, there is other evidence that suggests a calorie is not a calorie after all. See here for more about this.

The other thing worth mentioning about this study is the fact that it was performed in ‘insulin resistant’ women (as adjudged by fasting insulin levels). Generally, therefore, these women would not do a good job of processing carbohydrate, and would perhaps have most to gain from a lower-carbohydrate diet.

Our ability to learn much from this diet maybe limited, but it at least reminded me of the fat that low-carb diets generally outperform low-fat ones in the weight loss stakes. I reviewed the evidence in the area for my latest book, Waist Disposal. Here’s the relevant passage from the book (referencing has been adjusted for ease):

To date, seven studies have pitted low-carb against low-fat over various lengths of time. The shortest of these lasted three months.6 The average weight loss on the low-carb diet was almost 10 kg. This compared very favourably with the weight loss on the low-fat diet, which averaged just over 4 kg.

The remaining six trials lasted at least six months [1-7]. All of these trials found that after six months, weight loss on the low-carb diet was significantly superior to that on the low-fat diet.

Four of the studies went on for a whole year. Two of these studies did not find a significant difference in weight loss between the two groups at the end of the study. In one study, compliance was monitored, and it turned out that most participants did not cut their carb consumption to the level they were asked to. In another study, there was no checking of compliance at all. In other words, it is just not known whether the study participants restricted carbohydrate to the extent they were instructed to. The two other year-long [4,6] studies did, however, find that the low-carb diet significantly outperformed the low-fat one in terms of weight loss.

To get an idea of the relative effectiveness of low-carb versus low-fat diets, we can tot up the average weight losses with each diet in all the studies, and divide this by the number of studies to get the average weight loss:

• for the low-carb diets, average weight loss was 9 kg

• for the low-fat diets, it was 4.5 kg.

Do you see a relationship here? Yes, that’s right – overall, those on low-carb diets lost precisely twice as much weight as those slowly starving and depriving themselves on low-fat regimes.

In weight and fat loss, there are no panaceas (nothing works for everyone). But the evidence shows that low-carb eating, overall, is a valid and generally effective strategy for weight loss. It should also be borne in mind, I think, that low-carb regimes have been found to lead to more favourable outcomes with regard to, say, the lowering of triglyceride, sugar and insulin levels.


1. Sondike, S B et al., ‘Effects of a low-carbohydrate diet on weight loss and cardiovascular risk factor in overweight adolescents’, J Pediatr 2003; 142(3): 253-58

2. Brehm, B J et al., ‘A randomized trial comparing a very low carbohydrate diet and a calorie-restricted low fat diet on body weight and cardiovascular risk factors in healthy women’, J Clin Endocrinol Metab 2003; 88: 1617-23

3. Foster, G D et al., ‘A randomized trial of a low-carbohydrate diet for obesity’, N Engl J Med 2003; 348: 2082-90

4. Yancy, W S Jr et al., ‘A low carbohydrate, ketogenic diet versus a low-fat diet to treat obesity and hyperlipidemia. A randomized, controlled trial’, Ann Intern Med 2004; 140: 69-77

5. Stern, L et al., ‘The effects of low-carbohydrate versus conventional weight loss diets in severely obese adults: one-year follow-up of a randomized trial’, Ann Intern Med 2004; 140: 778-85

6. Gardner, C D et al., ‘Comparison of the Atkins, Zone, Ornish, and LEARN Diets for Change in Weight and Related Risk Factors Among Overweight Premenopausal Women: The A TO Z Weight Loss Study: A Randomized Trial’, JAMA 2007; 297: 969-77

7. Shai, I et al., ‘Weight Loss with a Low-Carbohydrate, Mediterranean, or Low-Fat Diet’, NEJM 2008; 359: 229-41

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