London - Are you really in charge of your appetite and weight? It might seem a strange question, but new research suggests that we may not choose our own diets. Instead, the billions of bacteria in our guts drive us to consume the nutrients that they need to survive.
We need gut bacteria to help digest our food, keep our organs healthy and our immune systems strong.
Now it seems these bacteria also drive our appetite, cravings and moods to get us to eat what they want, consuming the nutrients they prefer, rather than simply living off what we might otherwise choose to eat.
This is the conclusion of a broad review of scientific studies, including laboratory tests on human cells and gut bacteria, as well as observations of human eating and behaviour. The authors, of the University of California, San Francisco (UCSF), concluded different species of bacteria need different nutrients. For example, some bacteria prefer fats, others want sugars.
“Bacteria in the gut are manipulative,” says Dr Carlo Maley, the study’s leader and director of the UCSF Center for Evolution and Cancer. “There is a diversity of interests represented in the microbiome (the community of trillions of “friendly” bacteria in our bodies). Some are aligned with our own dietary goals, others not.”
So, while we may want to shed kilos by cutting down on, say, fats, our gut bacteria may decide otherwise and cause us irresistible cravings for fatty foods.
The authors believe the bacteria may do this by releasing chemicals into our gut that influence our appetites. Those signals could influence our bodies and behaviour, as the gut is linked by powerful pathways with our brains and hormonal systems, in particular the vagus nerve, which connects 100 million nerve cells in the digestive tract to the base of the brain.
In this way, they may alter our taste receptors, producing toxins to make us feel bad when we eat foods they can’t use, and releasing chemical rewards when we eat their favoured nutrients, says the study. These may not only affect our tastes but also our morale, it adds.
Gut bacteria may play a part in deciding our risk of obesity. In another experiment with mice, Jeffrey Gordon, a professor of biology at Washington University, St Louis, raised genetically identical baby rodents in a germ-free environment, so their bodies were bacteria-free. Then, he populated their guts with intestinal bacteria collected either from obese women, or their slim, non-identical twin sisters.
The mice were fed identical diets, yet the animals that received bacteria from an obese twin developed more body fat, Professor Gordon reported in journal Science in 2013. He believes the obese mice lacked bacteria that helped their bodies to stay lean, perhaps by influencing how quickly they burned calories. When the team then gave the overweight pups bacteria from the thin twin group, they returned to a healthy weight.
Last month, a research team at King’s College London revealed the make-up of our stomach bacteria is as individual as our genes - a discovery that may ultimately help those who can’t seem to lose weight, by prescribing individualised bacteria-changing therapies.
The team examined the genes of more than 400 UK twins, as well as their stomach bacteria. They found that identical twins (who share identical genes) have far more similar bacterial colonies in their guts than non-identical pairs.
This points to a genetic link, says Tim Spector, professor of genetic epidemiology at King’s College London, one of the researchers. “There is a match between genes and the bugs that grow in our guts,” he says. “It is like having gardens with different types of soils. Different plants grow in each, as the bacteria and the gut environment are most suited to each other.”
The differences may have profound effects on our risk of becoming overweight or developing serious illness. This is because gut bacteria create chemicals vital to healthy functioning and, while some stomach bugs will happily grow in anyone, others will only thrive when genetically compatible with their host.
It appears these picky bacteria have the strongest influence on our wellbeing. “One class of gut bacteria - the firmicutes - are the most genetically-influenced,” says Professor Spector. “They tend to have more functions important to human bodies.
“These firmicutes create one-third of the metabolites in our bodies - chemicals that drive our cells with sugars, fatty acids, proteins and vitamins they generate from the food we eat.
“These send messages around the body and are very important to us.” Moreover, he adds, they are vital to health: “They produce more short-chain fatty acids, chemicals such as butyrate, which act on our immune systems and brains. They are important in determining risks of arthritis and heart attacks.”
Such bacteria seem to be a double-edged sword. “They can both protect against these diseases and increase your risk of developing them. How they do all these things is yet to be fully determined.”
Unravelling such mysteries could herald new generations of disease-fighting therapies. We already know our lives would be virtually impossible without our gut microbes, says Professor Spector. “Experiments show mice cannot survive without them. They have no immune system and have to eat 30 percent more calories just to stay alive.”
Professor Spector describes the idea of bacteria driving individual appetites and cravings as “an interesting hypothesis”. He adds: “The question is, how much does it happen and how relevant is it?” It has been shown that microbes can significantly change appetites and behaviours in other species.
“We know microbes can zombify insects to change their behaviour entirely to suit their needs,” says Professor Spector.
“Experiments with mice show you can increase their anxiety and depression levels by altering their gut microbes. It is hard to do similar studies on people, so the experimental evidence in humans is weak, but it is theoretically possible.”
Already, however, scientists are developing yoghurts loaded with bacteria believed to benefit our immune systems. In future, such foods may also boost levels of gut bacteria that could help protect our hearts and other organs, and overcome cravings induced by the bugs already in our stomachs.
“Unlike a genetic finding which you can’t really change, you can alter your microbes,” says Professor Spector. “Someone could develop a probiotic, stick this in yoghurt and bingo.” This hope remains some way off - not least because there are still many question marks over whether the bacteria in such products actually reach the gut intact and in useful quantities.
In the meantime we can all make crucial changes to improve our health-enhancing, weight-improving balances of stomach bacteria, says Professor Spector, whose book on this subject - The Diet Myth: The Real Truth About Food - will be published next year.
The first change is to avoid taking broad-spectrum antibiotics unnecessarily. “We take far too many, most commonly for colds, where they have no effect, as colds are caused by viruses, not bacteria. This is arguably related to the obesity epidemic. Mice and rats put on weight if you give them antibiotics. In the US, farmers give these drugs to healthy cattle to fatten them.”
It’s thought antibiotics have this effect as altering the microbe population in our guts may have a knock-on effect on metabolism - studies have shown mice treated with antibiotics extracted more calories from food than mice not given the antibiotics.
Other research has found children given antibiotics during the first six months of life go on to be fatter than those who were not.
Professor Spector says we can also improve our gut bacteria levels by giving them better “fertiliser”, such as dietary fibre from fresh vegetables and wholefoods, such as wholemeal bread and brown rice. This can help all classes of bacteria to thrive.
“The more diverse the bacteria, the more chemicals and metabolites they produce - and the more beneficial they may be for us,” he says. “Fibre benefits us as it feeds these bacteria, whereas processed food gets absorbed by the digestive tract before it gets down to where the microbes live.”
Professor Spector is focusing his research on building a massive database about the bacteria found in individual Britons’ stomachs. He has set up an experiment open to the public: the British Gut Project.
“We are at the tip of the iceberg,” says Professor Spector. “Gut bacteria could explain a lot of things that have confused us about diet, weight and behaviour for years.” - Daily Mail