Beneficial gut bacteria boost the immune system to prevent obesity in mice, regardless of diet.
The Western lifestyle, with its abundant fast food, is wreaking havoc with our waistlines and sending many of us to early graves. A high fat, high salt, low-cost diet has been fuelling an obesity epidemic in industrialised nations and, increasingly, in developing countries. While the consequences of obesity, such as an increased risk of developing type 2 diabetes, cardiovascular disease, fatty liver, arthritis and cancer are well-known, public health solutions are thin on the ground.
A team of researchers from the Massachusetts Institute of Technology and the Aristotle University of Thessaloniki in Greece now believe that probiotic bacteria – such as you might find in yoghurt – could be the answer.
Previous population studies have clearly shown that fast food contributes to obesity. Yoghurt, on the other hand, can help to prevent the expanding waistline that comes with age. To find out why yoghurt has a slimming effect, the researchers fed mice that were genetically predisposed to obesity a range of diets.
Mice fed a ‘fast food’ diet that was high in fat and sugar and low in fibre and vitamin B and D quickly became obese. In contrast, mice fed three servings per week of commercially available probiotic yoghurt – in addition to as much fast food as they wanted – stayed lean. Even mice on a more balanced control diet were more svelte if they ate their three weekly servings of yoghurt.
How could this be?
The researchers honed in on the probiotic microbes in the yoghurt. To rule out any effect of protein, vitamin D and other compounds that might be present in the yoghurt, the researchers fed mice a single strain of probiotic bacteria, Lactobacillus reuteri, by adding it to their water supply. L. reuteri is naturally found in the intestines of many animals, including some but not all humans. Once again, mice supplemented with the probiotic were protected from developing obesity, even if they were eating a ‘fast food’ diet.
They weren’t simply eating less, either. Mice sipping water laced with lactic acid bacteria were eating the same amount of calories as mice drinking plain water.
The gut is home to a vast community of trillions of microbes that help us to digest food and manufacture vitamins, among other benefits yet to be fully understood. The complex microbial ecosystem in our gut is known to fluctuate with diet in humans, and the researchers found that the same was true in the mice – their gut microbes changed after just a few weeks on the ‘fast food’ diet.
Neither the yoghurt nor the purified L. reuteri were able to re-balance the gut microbiota in mice eating ‘fast food’. There was some other mechanism that gave the probiotics their obesity-inhibiting effects.
The fatty adipose tissue that pads out our figure when we put on weight is far from benign. In obese people, and their chubby mouse counterparts, adipose tissue is in a state of chronic inflammation. This is part of the reason why obesity can cause system-wide problems such as type 2 diabetes and cancer.
Gut microbes are essential for the proper development of the immune system. Mice raised in an environment devoid of bacteria fail to form mature immune lymphoid tissue. Not surprisingly, inflammation and immune system function were altered in mice fed a ‘fast food’ diet. In particular, a sub-population of immune cells in the intestine were pumping out more of the pro-inflammatory chemical interleukin 17 (Il-17), than healthy mice. Supplementing the diet with L. reuteri tempered the Il-17 levels, and also modulated other immune markers that were over-active in the mice eating ‘fast food’ alone.
The immune system connection between poor diet and the protective effects of probiotics was confirmed in mice lacking specific components of their immune system. In these animals the probiotic benefits of L. reuteri were non-existent – they gained weight just as they would have without the probiotic supplements. But when the researchers transplanted these mice with ‘primed’ immune cells from the guts of mice fed a diet of ‘fast food’ with probiotics, the beneficial effects were restored.
The researchers suggest that a vicious cycle is established in obesity, where diet affects the gut microbes, which in turn affects proper functioning of the immune system. This leads to weight gain, which compounds problems by again impacting on the immune system. The same process could also be occurring in people. Targeting the components of the immune system directly to treat obesity is challenging because of the complex feedback mechanisms at play.
Probiotics could be a cost-effective, drug-free solution that breaks this inflammatory–adiposity cycle by nudging the immune system towards a healthier state that can prevent the kilos from piling on.
Poutahidis T, Kleinewietfeld M, Smillie C, Levkovich T, Perrotta A et al (2013). Microbial reprogramming inhibits Western diet-associated obesity PLoS ONE DOI: 10.1371/journal.pone.0068596