Food and diet

'It's not what you eat, it's when you eat' claim

The Daily Telegraph tells us “why dieting is all in the timing”. Sticking to strict mealtimes is apparently the key, rather than snacking on healthy food.

Sadly for those of us who enjoy a daily dose of high-fat fast food, the true story behind the headlines is a little more complicated.

The research is actually in mice and researchers were trying to see if the body clock could have an impact on metabolism, which, in turn, could affect factors such as body fat.

One group of mice was provided with a high-fat diet, which they only had access to at set times during each day over the course of 18 weeks.

The mice were then compared to three different control groups – unlimited access to a high-fat diet; time-restricted access to a low-fat diet; and unlimited access to a low-fat diet

The surprising result was that mice in the time-restricted high-fat group put on less weight than mice in the unrestricted low-fat group – even though the researchers calculated that both groups of mice ate the same amount of calories.

A possible interpretation of the results is that it may be possible to ‘train’ your metabolism. If you eat your three meals at the same time every day, your metabolism ‘knows’ to work harder to burn off the fat (though this is, of course, pure speculation).

It should be pointed out that the mice in the restricted high-fat diet still put on weight, just not as much weight as expected. So if you are trying to shift the pounds, then a strict timetable won’t help you if you do eat high-fat food. And you will still be prone to the health risks, such as heart disease, that are associated with a high-fat diet.

Where did the story come from?

The study was carried out by researchers from The Hebrew University of Jerusalem and was published in the peer-reviewed Journal of the Federation of American Societies for Experimental Biology (FASEB). No sources of funding are reported.

The Telegraph is representative of this research, but fails to mention until several paragraphs in that the research was actually conducted in mice. The introductory paragraphs suggest that the same time-restricted high-fat diet can be adopted in humans, which should not be assumed.

Even if you choose to make that assumption, the reporting of the study also does not make clear that a time-restricted high-fat diet will still cause you to gain weight, just at a slightly slower rate.

The paper also doesn’t mention that the research was actually published several months ago.

What kind of research was this?

This was animal research designed to investigate the theory that disrupting the body’s biological clock – which regulates hormones and other body systems (circadian rhythm) – results in obesity and metabolic disorders such as diabetes. To investigate this, the researchers aimed to look at the effect of restricted feeding (RF) times and unrestricted feeding (i.e. open access to food), in mice. The intention was that RF would limit the time and duration of food availability, but would not restrict the available calories compared to those with free access to food (the Latin for free access is ad libitum so researchers used the acronym ‘AL’ as a convenient coding device).

Animal studies such as this can be a useful indication of how biological processes may work in humans, but humans and mice are not the same. No implications for human health should be taken directly from this animal study.   

What did the research involve?

The researchers took young male mice and fed them freely for two weeks. They were divided into four groups for a further 18-week period:

  • high fat-restricted feeding (HF-RF)
  • high fat-unrestricted feeding (HF-AL)
  • low fat-restricted feeding (LF-RF)
  • low fat-unrestricted feeding (LF-AL)

The HF diet was said to be based on soybean oil and palm stearin and 42% of the calories came from fat. The LF diet contained soybean oil and 16% of the calories came from fat. The restricted feeding time was between four and eight hours after the lights were turned on each day. Daily food intake and bodyweight were monitored once weekly throughout the experiment, and samples from the blood and liver were taken at regular times during the day from anaesthetised mice. The activity of mice in the cage was also monitored. 

What were the basic results?

The researchers found that compared to the mice given open access to the high fat diet (HF-AL), those with restricted access to fatty food (HF-RF) had 18% lower bodyweight and 30% lower cholesterol.

The HF-RF mice also had improved sensitivity to insulin, suggesting they could better regulate their blood sugar. They were also observed to have altered the expression of certain genes involved in regulating the body clock.

Those with restricted access to fatty food (HF-RF) also surprisingly had 12% lower bodyweight and 21% lower cholesterol compared with mice given open access to low-fat food (LF-AL). They also had improved sensitivity to insulin, albeit not as great a difference as compared to the HF-AL mice. Both HF-RF and LF-AL mice were actually observed to consume the same amount of calories.

When looking at hormone levels it was found that the HF-RF group also demonstrated reduced levels of stress hormones compared with mice given a low-fat diet at restricted times (LF-RF).

How did the researchers interpret the results?

The researchers say that their findings ‘suggest that timing can prevent obesity and rectify the harmful effects of a high-fat diet’.


This research has caught the attention of the media, but the findings do not mean that people should feel free to eat a diet high in saturated fats even if they are doing so at restricted times of the day. Animal research such as this can be a useful indication of how biological processes may work in humans, but humans and mice are not the same. Looking at a direct comparison, the restricted feeding time related to mice having oily meals available between four and eight hours after the lights were turned on each day. This can hardly be seen to directly reflect a human eating three high-fat meals a day and not snacking between meals, as the media seems to have interpreted. The pattern of change to hormones and other biological markers that were seen in the mouse body may also not be the same as what would happen in humans. We can also gain no indication of what would happen to the longer-term health and disease outlook of a human following such a restricted time high-fat dietary pattern.

Before overweight and obese people think that it is now OK to put chips, curries and burgers into every meal plan, it must be remembered that these foods tend to be high in saturated fats. While sticking to mealtimes and not snacking between meals may be a good idea, it is preferable if meals contain a high proportion of fruit and vegetables and balanced amounts of carbohydrate and proteins that are low in sugar and saturated fats. High-fat diets and obesity are clearly linked to cardiovascular disease and many other chronic diseases and cannot be recommended.

In conclusion, eating at regular times may mitigate some of the health risks associated with a high-fat diet, but only to a very limited extent. Sadly for fans of high-fat fast food it is very much not a case of 'It’s not what you eat, it’s when you eat'.

NHS Attribution