TIME 4 FAT LOSS SERIES – PART 13 – What is the most effective diet for fat loss?

21 May 2019


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This is the thirteenth in a series of original articles we will be publishing looking at the science behind fat loss. Join our mailing list to be informed when we have added the latest article.

Part 13: What is the most effective diet for fat loss?

Due to the huge variety of diets that now exist, the in-depth discussion of which is beyond the scope of this article, there does not appear to be a simple answer. However, the results of two studies that compared different types of diet should help to make things clearer.


Study 1

The first study by Truby et al, (1) compared the effects of a number of commercial weight loss programmes. These included the Slim-Fast Plan (a meal replacement approach), Weight Watchers Pure Points Programme (an energy controlled diet with weekly group meetings), Dr Atkins’ New Diet Revolution (a low carbohydrate eating plan) and Rosemary Conley’s “Eat Yourself Slim” Diet & Fitness Plan (a low fat diet combined with a weekly group exercise class).  Subjects were allocated to follow one of the four diets or control group for a period of 8 weeks, as this initial phase is the time when people are most likely to adhere to a diet and when the majority of weight changes are demonstrable.



At the end of 8 week trial period, all of the groups had achieved some degree of weight loss with an average of 5.2kg in the Atkins group, 4.7kg in the Weight Watchers group, 3.7kg with Slim Fast, 4.0kg with Rosemary Conley and 0.4kg in controls.

Although all the diet groups had significant weight loss compared to the control group, there was no significant difference in weight loss between the groups.

Remember that energy restriction is essential for fat loss. Bearing this in mind, overall, the Atkins diet gave a 30% reduction in total daily energy intake, Weight Watchers a 38% reduction, and the Slim Fast and Rosemary Conley diet a 37% reduction.

Although the authors concluded that there was no significant difference in weight loss between the groups, the Atkins group did lose most weight in absolute terms. So does this mean that a low carbohydrate, high protein diet is the most effective for fat loss?

It is important to remember that this study, like many others, was conducted over a relatively short period. Would the same results be achieved over a longer period? The authors of study 2 set out to answer that very question.


Study 2

A study by Sacks et al., (2) examined the effects of diets containing different levels of fat, protein and carbohydrate on body weight over a period of two years. 811 overweight adults were randomly assigned to one of four diets:

  • Fat 20% protein 15% carbohydrate (CHO) 65%
  • Fat 20%, protein 25%, and CHO 55%
  • Fat 40%, protein15%, and CHO 45%
  • Fat 40%, protein 25, and CHO 35%.

Therefore, two diets were low-fat and two were high-fat, two were average-protein and two were high-protein. The four diets also allowed for any difference carbohydrate intake may have with levels ranging from 35 to 65% of the total energy content of the diet.

The diets consisted of similar foods and the participants were offered group and individual instructional sessions for 2 years.



After 6 months, participants assigned to each diet had lost an average of 6 kg, which represented 7% of their initial weight.

Most of the weight loss occurred in the first 6 months and after 12 months, all groups, on average, slowly regained body weight.

By 2 years, weight loss remained similar in those who were assigned to a diet with 15% protein and those assigned to a diet with 25% protein (3.0 and 3.6 kg, respectively); in those assigned to a diet with 20% fat and those assigned to a diet with 40% fat (3.3 kg for both groups); and in those assigned to a diet with 65% carbohydrates and those assigned to a diet with 35% carbohydrates (2.9 and 3.4 kg, respectively).

Among the 80% of participants who completed the trial, the average weight loss was 4 kg. At 2 years, 31 to 37% of the participants had lost at least 5% of their initial body weight, 14 to 15% of the participants in each diet group had lost at least 10% of their initial weight, with no significant differences among the diet groups. Change in waist circumference did not differ significantly among the diet groups.

Satiety, hunger, satisfaction with the diet, and attendance at group sessions were also similar for all diets.

Based on these results, the authors concluded that reduced energy diets result in meaningful weight loss regardless of the exact proportion of fat, protein, carbohydrate they emphasise.  This allows diets to be tailored to individuals based on their personal and cultural preferences and may therefore have the best chance for long-term success.



The results of the studies featured in this article suggest that the precise ratio of fat, protein and carbohydrate of a diet seem to have little bearing on the amount of weight lost. Rather, it’s the reduction in energy intake that a diet creates which determines how much fat we can expect to lose. Consequently, if you create a diet based around a selection of healthy foods that you enjoy and that provide the recommended level of energy restriction, then you are liable to lose weight and are more likely to be able to stick it in the long-term than a highly prescriptive and regimented diet. Remember, as we saw earlier in this series, we only need to create a weekly energy deficit of around 3,500 kcal to lose a pound (0.5kg) of fat per week. This can be achieved relatively easily by reducing energy intake by approximately 200 – 300kcal per day, the equivalent of 50g of potato crisps, and expending an additional 300 kcal per day through physical activity, which should not present too great a challenge for most people.

CLICK HERE TO READ: Part 14 – Are there any supplements that can help us to reduce our body fat?