bacteria may hold a key to weight loss -- 3/28/18

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Today's selection -- from This Is Your Brain on Parasites by Kathleen McAuliffe. There are perhaps 10 times more microbes (especially bacteria and viruses) in the human body than human cells. They are collectively known as the "microbiome," and scientists are learning that they play a crucial role in almost all aspects of human functioning. One such area may be weight loss:

"Behold two mice. One is pleasantly plump, the other skin and bones. Yet the thin mouse is by far the bigger eater of the two. It weighs less because, unlike its chubby counterpart, it has no microbes in its gut. Without these helpers to break down its food, most of it passes through its intestine undigested. Though the animal consumes 30 percent more food than the bigger mouse, it has 60 percent less fat.

"Studies of germfree mice leave no doubt that microbes have a big impact on the amount of nutrients that we can derive from food -- an obvious way they control hunger and body weight. But there's more to this story than simply calories in and calories out Intestinal bacteria regulate hormones your own body makes to stoke or suppress your appetite -- for example, ghrelin, the molecule that goads you to get a second serving at the buffet, and leptin, which tells you to push your plate away. It's also suspected that gut bacteria may themselves syn­thesize chemicals that signal brain regions governing satiety. These areas include circuits rich in cannabinoid receptors -- the same neu­ropathways that are involved when a cannabis user gets the munchies.

"Inspired by these insights, many scientists are gambling that our microbiomes may hold the secret to overcoming obesity. At the cen­ter of this flurry of activity is Jeffrey Gordon, a medical researcher at Washington University in St. Louis, who has performed some of the most creative and provocative experiments in the field.

"In 2006, Gordon's team made an important discovery: fat mice had a far larger proportion of one major division of gut bacteria and less of another, while thin animals displayed the reverse profile. Obese and thin humans, to Gordon's fascination, showed the same pattern. Did that mean certain bacteria were making people fat? Or could it be that the excess calories consumed by fat people favored the growth of those strains?

"To untangle cause and effect, Gordon, along with Vanessa K. Rid­aura and others, performed a series of experiments that riveted the scientific community. They launched a search to identify rare sets of twins in which one was overweight and the other skinny (the idea was to minimize the effects of heredity). The researchers then collected bacteria from the subjects' feces and used it to colonize germfree mice that were genetically identical. The animals that got the bacteria from the overweight twins became obese and those that received the bacte­ria from the thin twins remained slender. Next the researchers staged a battle between the two types of microbes by housing both sets of mice in the same cage. Rodents are coprophagous -- a polite way of saying that they eat one another's droppings -- so allowing the two groups to mingle exposed all of them to the fecal bacteria originally collected from both the fat and thin twins.

"The microbial wrestling match culminated in a dramatic upset: The obese rodents lost their excess weight as bacteria from the thin twins muscled out their founding population. The lean rodents stayed lean. The bacteria from the slim twins prevailed in both groups.

"Because all the subjects in this trial were fed standard rodent chow that was low in fat, the researchers wondered what would have hap­pened to the animals if they'd been fed the equivalent of junk food at the time they were exposed to the bacterial mixture. Would the outcome have been the same? The researchers consulted various dietary charts and tables to create food pellets for the rodents that were simi­lar in composition to the sugary, high-fat fare consumed by a large sec­tor of the public in affluent nations. On this diet, obese rodents did not lose weight. Their fattening microbes triumphed over the slimming ones. The lean rodents, however, did not become rotund no matter how much they ate. Their founding population of bacteria protected them from obesity.

"More recently, Gordon's team has discovered that fat mice have im­poverished microbiomes in comparison to those of thin mice, which harbor far more species of gut bacteria. Related research suggests the diverse microbiota of slim mice can extract more calories from the same food, so you'd think these mice would be the ones ballooning in size. But paradoxically, their bacteria break food down into metabo­lites that seem to act like appetite suppressants and energy boosters, so the animal burns off the excess calories and consumes fewer overall.

"These results hold out the fantasy -- and that's all it is for now­ -- that overweight humans could slim down if the right cocktail of bac­teria were somehow introduced into their guts in combination with a brief period of adhering to a low-fat regimen to oust the fattening bugs from their systems. Then, when the colony of the good guys was well established, they'd lose their craving for chocolate mousse cake. Or maybe they could eat three servings of it in a row and still fit into skinny jeans.

"Alas, it probably won't be that simple. In humans, obesity is a com­plex disease, affected not just by obvious players like diet, heredity, and exercise but also by sleep habits, stress, cultural norms, romantic trouble, income, smoking, drinking alcohol, pet ownership, and who knows what other factors. Having said that, gut bacteria could turn out to be tipping the scales very strongly in the wrong direction for some people

author:	Kathleen McAuliffe
title:	This Is Your Brain on Parasites: How Tiny Creatures Manipulate Our Behavior and Shape Society
publisher:	First Mariner Books
date:	Copyright 2016 by Kathleen McAuliffe
pages:	111-114