Yoghurt story 'hard to swallow'

A headline in today’s Daily Mail stated: “Yoghurt drinks could beat bugs that pile the weight on.” It said scientists have shown that “bugs that live in our stomachs could be causing us to get fat.” The newspaper said the research could lead to probiotic yoghurts that can combat weight gain.

The newspaper’s claim about probiotic yoghurts is misleading. In fact, the study examined the effect of a change in diet on gut flora (microorganisms found in the gut) and weight in mice. The research was well conducted and should help advance investigations into gut flora. However, probiotic drinks such as Yakult, which was mentioned in the Daily Mail , did not feature in the study. While it is easy to see how the newspaper reached its interpretation, it is too great a leap, and the study's relevance to human diet needs more investigation.

While Yakult has had some more publicity because of this study, the research had nothing to do with probiotic yoghurts.

Where did the story come from?

The research was carried out by Dr Peter Turnbaugh and colleagues from Washington University School of Medicine and the University of Colorado. It was funded by the National Institutes of Health and the Crohn’s and Colitis Foundation of America. The authors declare that they have no competing financial interests. The study was published in the peer-reviewed medical journal Science Translational Medicine .

The Daily Mail and The Times reported this research, and both papers made confident statements that the findings show that diets high in fat and sugar are causing obesity. Both papers highlight that this study was in mice. The Daily Mail ’s headline about probiotic yoghurts, such as Yakult, could be misleading as this research did not test probiotics (yoghurts or otherwise) and the researchers do not mention them.

What kind of research was this?

This study had several aspects, one of which was to examine the effect of modifying diet on gut flora and weight in mice. The researchers say that it is difficult to determine the relationships between diet, the behaviour of gut microbes and energy from food because of the effects of genes and environmental factors. By developing an ‘animal model’ of the complex ecosystem in the human gut, the researchers have provided a way forward for further studies.

What did the research involve?

This research involved several different experiments, all of which used ‘gnotobiotic mice’. These are animals which are raised in germ-free environments and deliberately colonised with specific microbes at particular times in their life. Mice such as these are useful in experiments about digestion as they can show how particular microbes and previously germ-free environments affect each other.

In the initial experiments, the researchers attempted to establish a mouse model of the human gut so that they could investigate the effect that diet had on it.

To make the mouse model, the researchers put microbes from human faecal matter into the guts of gnotobiotic mice to establish a human-like gut colony. The mice were fed a standard low-fat diet that was rich in plant matter. Researchers collected faeces from the mice one day, one week and one month after the mice were colonised with the human microbes. After a month, half of the mice were switched to a high-fat, high-sugar Western diet. The two groups remained on these diets for an extra two months, with weekly faecal sampling. The mice were then killed and the components of their guts compared.

In other experiments, the researchers assessed whether the human-like gut flora could be transferred between animals and whether this could be done using frozen faecal samples. In the experiment that the media has picked up on, the researchers examined whether diet-induced obesity could be caused in healthy mice by transplanting into them the gut flora from mice that were fed a Western diet.

What were the basic results?

The researchers developed a mouse model of the human gut using human faecal matter. These microbial colonies could be transplanted into other mice even through frozen faecal samples. This establishes this mouse model as useful for further research.

A Western diet caused mice to gain weight and changed the microbes in their gut. Transplanting the gut flora from these mice to healthy mice led to significant weight gain compared with mice that received gut flora from mice on a non-Western diet, even though there was no increase in food consumption.

The researchers identified the particular bacteria that were predominant in diet-related obesity, having found a bloom in bacteria called Erysipelotrichi and Firmicutes along the length of the gut.

How did the researchers interpret the results?

The researchers say that while human gut communities have been transplanted into germ-free animals many times in previous experiments, they used these methods to show that the human gut colony can be transferred to mice even if the starting material is frozen faeces.

They have also shown that this can be passed between mice and that the complement of microbes changes quickly and dramatically when the diet is switched from a low-fat diet to a high-fat, high-sugar Western diet. This diet-altered gut flora can be transplanted into other animals. Overall, the findings show that this is a potentially useful animal model for further study of the environment of the gut.

Conclusion

This animal study was well conducted. The purpose of the experiment, which is well described, was to establish an animal model for further studies of human diet and of the complex ecosystems that exist in the digestive system.

The study did not examine the effects of yoghurt or other probiotics on weight, as some news reports imply. Nor does it suggest that a probiotic yoghurt will soon be available that can aid weight loss. This extrapolation of the results is likely to have been based on the finding that a Western diet alters the microbial constituents of the gut. Researchers found that when the mice’s diet was changed from a low-fat diet to a Western-style high-fat, high-sugar diet, the component bacteria in their gut changed quickly and considerably. Stating that a probiotic yoghurt could “combat weight gain”, and including a picture of a normal yoghurt drink next to the article, could mislead people.

The results from this study will undoubtedly inform further research and the study has developed an important animal model for this type of research. Probiotics should not be seen as a ‘treatment’ for a poor diet, and eating a healthy, balanced diet and doing regular exercise remain the best ways to prevent overweight and obesity.