Diets alone 'will not shift weight'

“Dieting without exercise 'will NOT help you lose weight',” reported the Daily Mail. It said scientists believe that this may be because of “a natural compensatory mechanism that reduces a person's physical activity in response to a reduction in calories”.

This research was carried out in 18 female monkeys fed a high-fat diet for several years. They were then switched to a low-fat diet, reducing calorie intake by about 30% in the first month and 60% in the second. The monkeys did not lose a significant amount of weight in the first month, apparently because they reduced their activity levels. Contrary to the newspaper report, they did lose weight in the second month, even though they did even less physical activity. Another three female monkeys on a normal diet, but exercised on a treadmill, lost a similar proportion of their body weight to the monkeys on the calorie-restricted diet.

This study has several limitations, which increase the likelihood that differences between the groups other than diet and exercise influenced the results. Simply put, weight loss occurs when a person burns more calories than they consume. This can be achieved by either reducing calorie intake, increased physical activity or a combination of the two.

Where did the story come from?

This research was carried out by Dr Elinor L Sullivan and Dr Judy L Cameron from Oregon Health and Science University. The work was funded by the US National Institutes of Health. The study was published in the peer-reviewed American Journal of Physiology, Integrative and Comparative Physiology.

The Daily Mail and Express covered this research. Though both newspapers reported that the study was in monkeys, this was only mentioned partway through the article, and both articles were illustrated with pictures of young women. This could give the misleading impression that the study was in humans. Also, both newspapers suggest that it is not possible to lose weight by dieting alone. This conclusion was not supported by the study, which found that dieting monkeys lost weight in the second month of their diet.

What kind of research was this?

This research investigated the effects of dieting on weight in rhesus macaque monkeys. Though monkeys have many similarities to humans, there are also differences. In particular, when it comes to diet and lifestyle, simulating human behaviour in animals is difficult. Studies such as this may help us understand human biology and behaviour, but the findings may not be fully representative of what would be seen in humans.

What did the research involve?

The researchers used 21 adult female rhesus monkeys aged 9 to 13 years old in their study.

The first experiment involved 18 female monkeys that had had their ovaries removed and had been fed a high-fat diet (35% of calories from fat) for two-and-a-half years. This was meant to simulate the diet of many postmenopausal women in the Western world. These monkeys lived in individual cages during the study. During the first month of the study, the monkeys’ diet was changed to normal monkey food (5% fat), the aim being to reduce the monkeys’ calorie intake by 30% compared to their previous diet. In the second month, the aim was to reduce the calorie intake by a further 30% (that is, a 60% reduction in calories compared to their original high-fat diet). During the study, the monkeys’ physical activity, metabolic rate and weight were measured.

In the second experiment, three adult female monkeys were fed normal monkey food, supplemented with fresh fruit, vegetables and seeds. They lived in social groups in cages that had perches at various heights and had toys available. The monkeys were trained to exercise on a treadmill and did this for one hour a day, five days a week for 12 weeks, at 80% of their maximum capacity. This exercise programme was meant to simulate the recommended levels of activity by the American College of Sports Medicine to prevent weight gain and promote weight loss. The monkeys’ physical activity off the treadmill, metabolic rate and weight were also measured.

The authors analysed the first and second experiments separately, and looked at whether the exercise programme could theoretically compensate for any changes in energy expenditure in the first experiment.

What were the basic results?

In the first experiment, the researchers found that the monkeys ate 44% fewer calories in the first month than in their previous diet, and 68% fewer in the second month. After the first month, there was no significant weight loss, but there was a significant reduction in body weight in the seventh and eighth weeks of dieting. There was an average reduction in weight of 6.4% over the two months, and an average fat mass reduction of 212 grams. During the dieting period, daily activity reduced. This reduction was significant by the fourth week of the diet. Over the two months, physical activity reduced by 26% and metabolic rate also reduced, the equivalent to saving about 68 kilocalories of energy expenditure daily.

In the second experiment, the monkeys trained to use the treadmill lost about 6.1% of their body weight over the three-month exercise programme. Their physical activity off the treadmill did not change. Overall, the exercise programme was estimated to increase energy expenditure by about 70 kilocalories a day. Therefore, the researchers considered that this exercise programme could counteract the reduction in physical activity seen in the diet group.

How did the researchers interpret the results?
The researchers conclude that when the number of calories consumed is reduced by dieting, the body compensates by reducing physical activity, thus resisting weight loss. They say that  and an exercise program of five hours of running per week is sufficient to counteract the diet-induced decrease in activity.


This study suggests that in female “postmenopausal” monkeys, a reduction in calorie intake results in a compensatory reduction in physical activity. This is unsurprising as the body is designed to try and maintain balance, and this mechanism would maintain a balance of calorie intake and expenditure. The second part of the study suggested that an exercise programme could increase energy expenditure enough to counteract this effect. It would have been useful if the researchers had tested their hypothesis by assessing weight loss in monkeys who were dieting but also taking part in the exercise programme, but this was not done. In addition, differences between species mean that these results may not apply directly to humans, particularly due to the complexity of human lifestyles.

There are a number of points to note about this study:

Monkeys in the first experiment lost weight in the second month of their diet, and overall lost a similar percentage of their body weight (6.4%) to the monkeys that exercised for three months (6.1%).
Differences in physical activity and other outcomes between the monkeys in the two experiments may not be entirely due to their diet and exercise. Firstly, the monkeys did not appear to have been randomly assigned into groups. Therefore, there could have been differences between them, other than their diet and exercise, that lead to the differences in weight loss. In addition, monkeys being fed the lower-calorie diet lived in smaller individual cages while dieting, while the monkeys who were exercising lived in social groups in larger cages with toys and perches on the walls. The exercising moneys also had their diets supplemented with fruit, vegetables and seeds, while the monkeys on lower-calorie diets did not. The exercising monkeys exercised for about three months, whereas the monkeys on lower-calorie diets were kept on these diets for two months. It is also unclear whether the monkeys in the second experiment had had their ovaries removed and previously been fed the same high-fat diet as the monkeys in the first experiment. These many differences could have contributed to the observations seen.

The research observed only a small number of female monkeys. This may not be representative of what would be seen if a greater number of animals of both sexes were studied.

This study does not tell us anything particularly surprising. Losing weight requires a person to burn more calories than they consume. Different people may wish to approach weight loss with different combinations of reduced calorie intake and increased physical activity to suit their needs, ability and lifestyle.

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