Should we consider the microbiome to be another organ?

In this feature, we hear from two United Kingdom-based researchers whose work focuses on obesity and metabolism: Dr. Petra Hanson and Dr. Thomas M. Barber. They discuss the microbiome, the marketing hype behind probiotics, and the future direction of research.

Dr. Hanson and Dr. Barber work at Warwick Medical School and the University Hospitals Coventry & Warwickshire National Health Service (NHS) Trust. They are both members of the Warwick Obesity Network.

Browsing the shelves of many shops these days reveals a growing consumer infatuation with probiotics that has moved well beyond yogurt. Probiotics are in scores of supplements. They infuse shampoo, toothpaste, skin care products, and snacks for both humans and pets. They are even a feature of some anti-allergy mattresses.

The probiotic buzz stems from the growing scientific attention to and recognition of the importance of our gut microbiome — the collection of bacteria that lives in our large intestine.

Researchers are examining the gut microbiome for its potential to benefit countless aspects of both physical and mental health. This potential generates excitement about the prospect of improving our health — and, possibly, stemming the obesity epidemic — by improving our gut bacteria.

Such are the possibilities that researchers at the University of Warwick have concluded that the medical community should consider the gut microbiome almost like an organ in its own right.

As with other organs, the gut microbiome has the capacity to make us unwell if we do not properly nurture it. Conversely, it holds the power to promote health and well-being if we take care of it correctly.

“We know that the human microbiome is crucial in healthy physiological processes. Our research shows that it plays many and varied roles — for example, in the normal development of the immune system, in the mediation of inflammatory pathways and metabolic processes, and in the regulation of appetite.”

– Dr. Hanson

A recent high profile and high quality study in Nature Medicine, for example, documents new, significant connections between health and gut biomes, linking certain microbes to healthy and unhealthy outcomes.

Some bacterial species appear to be linked to a lower appetite, lower body weight, and reduced overall inflammatory status. Recent research from Warwick Medical School has shown that other bacterial species are associated with an unfavorable metabolic status. Moreover, scientists have recently linked a certain microbiome pattern to more healthy aging.

To date, we have identified only about 1,000 of what we believe are likely to be millions of microorganisms in the human body.

By the age of 3 years, the gut microbiota is established, but we know that various factors can change its diversity and development. These factors include host genetics, diet, age, mode of birth, and antibiotics, as well as probiotics, fecal microbiota transplants, and prebiotics.

“So far, data from human-based studies are mainly observational in nature. We still lack enough evidence to say that healthier, more diverse microbiomes cause greater metabolic health; we can only say that these microbiomes are associated with better outcomes. This is a very different standard.”

– Dr. Barber

Amid greater scientific inquiry and growing public interest, marketers sell a lot of products on the back of unproven promises. To cut through the clutter of advertising claims and analyze the benefits of the array of items on the shelves, we need to distinguish between fads and facts. Here, we give our verdict on what the latest science tells us.

Probiotics

Scientists believe that probiotics act by preserving the balance of the normal intestinal microbiota and improving the immune system. Recent research suggests that certain types of probiotics may benefit specific disease states.

Despite the popularity of probiotics in recent years, there is insufficient evidence about the benefits of probiotics and their safety, including the possible side effects.

Fecal transplants

Recent work has examined the benefits of transplanting fecal bacteria from healthy donors into people with intestinal diseases in an attempt to restore the function of healthy gut microbiota.

Such transplants may treat various diseases, including inflammatory bowel disease, irritable bowel syndrome, constipation, metabolic diseases, autoimmune diseases, allergic disorders, and chronic fatigue syndrome.

These procedures are far more effective than probiotics — with the effects of transplants on microbiota lasting for some 24 weeks, compared with 14 days for probiotics.

Prebiotics

Plant-based foods are a source of prebiotics, which stimulate the growth of gut bacteria. Research has shown that prebiotics have three features:

• They resist absorption in the digestive tract.
• The microbiome can ferment them.
• They can have a positive health effect through either direct or indirect action of the microbiome.

Dietary fiber, predominantly from plant-based foods, is the main source of prebiotics. People can classify dietary fiber as one of two types: soluble, which helps lower cholesterol and glucose levels, or insoluble, which promotes the movement of material through the digestive system.

The primary sources of soluble fiber are fruits and vegetables. Although cereals and whole grain products provide insoluble fiber, most high fiber foods contain both kinds of fiber.

The current recommendations for dietary fiber intake for adults in most European countries and the United States are 30–35 grams per day (g/day) for males and 25–32 g/day for females, but most people’s diets fall short of this. Recent research from the Warwick scientists highlights the extent of the existing gap.

These findings suggest that most of us should increase our dietary fiber intake by about 50%.

As the team’s recent research has underscored, the health benefits of dietary fiber are widely recognized. They reduce innumerable health issues, including excess body weight, chronic inflammation, depression, and the risk of cardiovascular disease and colorectal cancer. Importantly, this work also highlights the beneficial effect of dietary fiber on the human microbiome.

We are still learning exactly how this all takes place. However, we know that dietary fiber enhances the production of key microbial molecules that set off a reaction that promotes overall health and well-being.

While the interaction between the gut microbiota and the brain is still unclear, it is likely to be complex and multidirectional. We are yet to discover the precise mechanisms.

Insights about the wider role of the microbiome in overall human physical and mental health are on the horizon. In the meantime, the take-home message for the general public remains simple:

Eat plenty of fiber. Eat diverse, unprocessed foods. Eat fruits and vegetables. You will have better metabolic health, as well as a more diverse gut microbiome.

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