Speaking Science Diet & Nutrition

“You are what you eat” — are you?

Children raised onsorghum have differentgut microbes. Photo: T. Vijaya Kumar  

About 70 years ago when the biologist J.B.S. Haldane was asked which would be God’s favourite life form, he said “beetles,” since there are far more beetles in the world than people. Were he alive today, Haldane would likely change his mind and answer that it would be microbes. Just the number of microbes — bacteria, virus, archae, and other single-cell organisms in human guts (intestine) alone is 100 trillion, tenfold more than the total number of cells in the entire human body. And as a scientist wag remarked, “I was born with my genome, but when I depart, it will be with 100 other genomes.”

Where do we pick up all theses bugs? The baby in the womb is bug-free but exiting through the birth canal, the baby picks up a complex set of microbial populations. Babies born through caesarean section have different microbial composition then those through vaginal delivery. These two are of maternal inheritance, and they largely collect in the intestines of the baby — hence the term gut microbiome.

What do these bugs do? Help, harm or happily coexist — all of these. I remember when our first child Katyayani was born, she fell very sick within a day, dehydrating through loose motions and weakening by hours. The brilliant paediatrician, Dr. Chikarmane, checked the mother Shakti and found that she carried remnants of the pathogen E. histolytica from an earlier infection, which was passed on to Katya. He then cleared Katya of infection using antibiotics, and fed her another set of microbes, Lactobacillus (which help in digesting milk, but which too were lost in the treatment) and restored her health, the ability to digest milk and gain strength. Over time, the newborn acquires microbes in the gut (and elsewhere) through environmental and other sources as well. The composition evolves continuously, but the numbers are maintained. In effect then the human gut is an ecosystem, with microbial genes outnumbering human genes by 100 to 1.

Given this ecosystem, there is a steady give and take between the host and the guests in the stomach. As we eat, so our microbiome and they produce metabolites and waste molecules that affect our metabolism as well. You are not what you eat, you are what your gut bugs eat and generate as products!

Immigrants manipulate the host?

This interaction between the host and the guest goes to such an extent that the guest can “manipulate” the diet of the host. A recent paper with the provocative title: “Is eating behaviour manipulated by the gastrointestinal microbiota? Evolutionary pressure and potential mechanism” has been published in the journal Bio essays (36: DOI 10.1002/ bies. 2014.0071) by a multi-centre group led by Dr Carlo Maley of UCSF, Califonia. They show how our bodies are composed of a diversity of organisms competing for nutritional resources.

Some of them (e.g., prevotella) grow best on carbohydrates and when they do so, they release some chemicals that induce the human host to crave for starchy food. Others such as bacteroidetes grow well with certain fats and in turn induce the host to go for fatty food. How do they do it? The signal molecules they release act not just on the digestive system of the body but also on the pain perception system and the nervous system. They alter the receptors that respond to taste, mood, pain and pleasure. In essence, you are not necessarily the master but are influenced by what your gut microbiome tells you through the signals they release, affecting your behaviour! This guest-influenced host reaction has been suggested to be associated with conditions like obesity, diabetes, allergy (to certain microbial metabolic toxins) and even mood swings and autism.

In an extreme case with rats, studied by House, Vyas and Sapolsky ( PLoS ONE 2011, 6: e23277, free on the web), the researchers found that the microbe Toxoplasma gondii infecting rat guts suppressed the rat’s normal fear of cat smells, thereby making the rat a prey for the cat. The microbes, now housed in the cat’s belly, found it more hospitable! Occasionally we see the reverse, the host reorganizing the composition of the gut micro biome through diet. Japanese people love to eat seaweed, and now one sees specialist microbes that digest seaweeds in Japanese tummies. Likewise, unique microbes that can digest cellulose are found in the guts of African children raised on sorghum in their food.

We thus see how microbes within us tend to dictate terms and affect not only our digestive system and health, but also our moods, feelings and preferences.

“You are what you eat” is a phrase that became a buzz word in a variety of contexts, after Gillian Mckeith ran a BBC TV series with that name, in which she advised people on how to eat appropriate healthy diets. It now appears we need to modify it to “You are what your gut’s bugs tell you to eat.”



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Printable version | Apr 21, 2021 7:03:32 AM | https://www.thehindu.com/sci-tech/health/diet-and-nutrition/you-are-what-you-eat-are-you/article6335336.ece

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