The Daily Telegraph tells us today “dieting makes people feel depressed because cutting out fatty foods alters their brain”.
The headline is based on Canadian research investigating why and how stopping eating high fat food might lead to cravings for it, and whether this might relate to the effect of a high fat diet on the reward systems in the brain.
They looked at what happens when mice are fed a high fat diet and this is then withdrawn. It found that the mice show increased anxiety-like behaviours and increased motivation to get high-sugar and fat foods. They also show changes in the amounts of certain proteins in the areas of the brain linked with reward sensations. The researchers suggest that these changes may contribute to an “addiction-like” process of repeated relapse to high fat food consumption after a switch to a healthier diet.
Although one of the researchers is quoted in the paper as saying, “the chemicals changed by the diet are associated with depression”, the study did not assess whether the mice showed signs of depression, only anxiety.
This research in mice may give clues as to why sticking to a lower fat diet after a higher fat diet is difficult, but the findings may not be representative of what happens in humans. Much more research is likely to be carried out in understanding the chemical basis for unhealthy food consumption, as being overweight or obese are big health problems in modern society. Hopefully, such research will eventually lead to better ways of supporting people trying to break unhealthy eating habits.
Where did the story come from?
The study was carried out by researchers from the University of Montreal Hospital Research Centre and other research centres in Canada. It was funded by the Natural Sciences and Engineering Research Council of Canada, Canadian Diabetes Association and Canadian Foundation for Innovation.
The study was published in the peer-reviewed scientific journal, Nature.
The Daily Telegraph and Daily Mail both covered this story and suggest that the study explains why dieting makes people feel “depressed”.
This is not what the study looked at, and it did not measure signs of depression in the mice, rather they looked at behaviours that are thought to show anxiety in mice (such as how willing they were to venture into open spaces).
The study certainly doesn’t tell us whether or why dieting might make humans feel “glum” – as the Mail puts it.
What kind of research was this?
This was animal research looking at the emotional, motivational, brain protein changes associated with a change from a high fat to a low fat diet.
The researchers say that eating high fat and high sugar foods stimulates the reward pathways in the brain, and that withdrawal of these foods often results in increased cravings for them, which may explain the failure of many weight loss programmes. Some researchers think that the effects of high-fat food could trigger the same cycle of pleasure and then craving for more pleasure, accompanied with withdrawals symptoms, associated with addiction. However, there is debate about the extent to which this resembles other forms of addiction.
They report that little is known about how these cravings arise, and this is what they were investigating in their study.
Animal research is used when it is not possible to carry out similar studies in humans.
The researchers in this study were looking at changes in protein levels in the brain in response to dietary changes, and it would not have been possible to sample brain tissue from humans for this type of research.
What did the research involve?
The researchers studied 90 adult male mice.They placed half of the mice on a high fat diet, and half on a low fat diet, for six weeks.
The diets contained the same basic ingredients, but the high fat diet included 58% of calories from fat and the low fat diet had only about 11% of calories from fat.
After the six weeks some of the mice were switched to a normal mouse ‘chow’ diet while other mice were kept on their allocated diets.
Both before and after the diet switch the researchers measured the response of the mice to tests assessing their motivation to obtain sugar or high fat food rewards. They also measured the levels of stress in the mice by assessing a stress related chemical in their blood, and levels of anxiety by assessing their behaviour in a maze. They also looked at whether there were changes to proteins involved in nerve cell signalling and learning in areas of the mice’s brains that are associated with reward sensations.
What were the basic results?
After six weeks, the mice on the high fat diet unsurprisingly gained 11.5% more body weight than the mice on the low fat diet.
Mice on the high fat diet showed less motivation to obtain sugar rewards than mice on the low fat diet. They also showed more anxiety-like behaviour, and although they did not show higher levels of the stress-related chemical in their blood in normal situations than mice fed a low fat diet, they did show greater increases in levels of the stress-related chemical in stress-inducing situations, such as being restrained.
When the mice eating a high fat diet were switched to a normal diet they showed increased anxiety and stress levels compared to mice that had been switched from the low fat diet. They also showed increased motivation to get sugar and high fat food rewards. The levels of certain proteins were altered in specific areas of the brains of mice that were fed the high fat diet, and switching them to a low fat diet also led to changes in the levels of these proteins.
Mice that had been on a low fat diet and then switched to a normal diet did not show these changes.
How did the researchers interpret the results?
The researchers concluded that anxiety, reduced response to normally pleasurable experiences, and increased sensitivity to stress, develops in mice exposed to a high fat diet. Removal of the high fat diet increases stress and makes the mice more vulnerable to eating high fat foods.
This interesting research looked at the emotional, motivational and brain chemistry changes that occur on removal of a high fat diet in mice. The findings suggest that switching from a high fat diet to a normal diet increased the mice’s motivation to seek out sugary and fatty foods, and there were also changes in proteins in the mice’s brains.
The researchers also found protein changes in the brain in response to eating a high fat diet, and changes in response to the switch to a normal diet.
The study did not assess how long these changes lasted or whether the levels eventually went back to normal if the mice remained on the lower fat diet.
Despite the suggestions in the papers, the study did not look at whether the mice showed signs of depression after changing their diet.
Ultimately, the research was carried out to try to understand why people who switch from a high fat to a low fat diet may find it difficult to stick with this diet and to find new drugs to target these pathways. For example, there are drugs that can combat, at least to a certain extent, cravings for alcohol and nicotine. So it is possible that a similar drug could be developed to combat cravings for high-fat food. We will have to wait and see if that is the case.
At this stage, the findings of the study are limited to mice, and may not apply to humans. More research looking at the exact roles of the proteins identified as being affected by the high fat diet would be needed to determine how they might relate to the food motivation changes seen. A greater understanding of this may help researchers understand how they could interfere with the process and reduce the effects of removing a high fat diet.
People being overweight or obese is a big health concern, and this type of research helps to explain why it may be difficult to break bad dietary habits. Ideally, future research will help understand how best to support people to do this.