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How Gut Bacteria Affect Weight

With the ever-increasing epidemic of obesity in North America, almost 20 percent of children and more than 70 percent of adults are overweight, which increases risk for heart attacks, diabetes, strokes, arthritis and certain cancers.

A new study of 84 children and teens shows that the heavier ones have more of the types of gut bacteria that convert carbohydrates into fats so readily that they absorb more calories from their food than the thinner children do (Journal of Clinical Endocrinology & Metabolism, Sept. 20, 2016). The participants ranged from ages 7 to 19 and from normal weight to obese, with MRIs used to measure how much fat they had in their bodies. The colons of the heavier children were far more likely than those of the skinny ones to have the eight groups of gut bacteria that have been previously associated with carbohydrate fermentation to short chain fatty acids that markedly increases the number of calories absorbed from food (Nutr Clin Pract, April 2012;27(2):201–214).

Since bacteria in your colon eat the same food that you do, what you eat determines which types of bacteria thrive in your colon. These bacteria are a prime driver of how high your blood sugar rises after meals and how many calories you absorb from the food you eat. Many epidemiological studies show that people who take in lots of sugared drinks and sugar-added foods, foods made with flour (ground whole grains) and other refined carbohydrates are more likely to be overweight and that those who eat lots of fruits, vegetables, nuts and grains that have not been ground into flour tend to be thin.

Gut Bacteria and Carbohydrate Absorption
Carbohydrates are sugars alone, in pairs and in chains of up to millions of sugars bound together. Only single sugars can be absorbed into your bloodstream. You can't even absorb two sugars bound together, so enzymes in your intestines and bacteria in your colon break them apart. For example, milk contains a double sugar called lactose. To split the double sugar into single sugars that can be absorbed, your intestines are supposed to have an enzyme called lactase. However, many people have intestines that do not make lactase, so they cannot break down the lactose into single sugars. Since the lactose cannot be absorbed in the upper intestines, it passes to the colon where bacteria ferment the double sugar, which can cause gas, cramping and diarrhea.

Fiber is the type of carbohydrate formed from long chains of sugars that human intestines cannot split into single sugars. Since fiber cannot be absorbed in the intestines, it passes to the colon. There are two types of fiber: soluble and insoluble fiber. Insoluble fiber cannot be absorbed at all so it passes unchanged from your body, but soluble fiber is readily broken down by certain bacteria in the colon. These bacteria break the soluble fiber into short-chain fatty acids that are easily absorbed through the colon to provide extra calories. The colons of obese people contain a higher concentration of the types of bacteria that break down soluble fiber so that they absorb more calories from the food that they eat. When these bacteria are present, you can gain extra calories not only from the soluble fiber but also from the other sugars and undigested food particles that are bound up with the soluble fiber and that would have passed through undigested.

How Processing Foods Contributes to Obesity
The plants we eat contain lots of fiber, much of which cannot be absorbed and passes from your body contributing no calories whatever. When food manufacturers strip fiber away from plant parts, the resulting foods are more easily broken down in the intestines and are quickly absorbed. For example, whole grains are seeds that have carbohydrates bound in a tight fiber capsule that slows the breakdown of the carbohydrates into absorbable sugars. After you eat cooked but unground whole grains, blood sugar levels barely rise. On the other hand, when you eat foods made from whole grains that have been ground into flour (bread, pasta, many dry cereals and so forth), you have no capsule to keep you from breaking down the carbohydrates and your blood sugar can rise very quickly.

Can Changing Colon Bacteria Help to Control Weight?
A review of several studies showed that eating a diet based on plant foods and restricting refined carbohydrates and most other foods appears to be a safe and effective way of changing colon bacteria to help control weight and high blood sugar (Gut Microbes, Jan-Feb, 2012;3(1):29-34).

Currently, the best way to change your colon bacteria to favor the types that do not increase absorption of calories is to eat a diet that includes lots of foods that are not quickly absorbed. However, many people are unwilling or unable to lose weight or control weight just by restricting refined carbohydrates and eating a diet based on lots of plants that have not been processed.

The Future Potential of Stool Transplants
Two studies are currently being conducted to see if taking pills containing freeze-dried feces from skinny people will cause obese people to have fewer high-calorie-absorbing bacteria in their colons. Dr. Elaine Yu of the Massachusetts General Hospital will test adults and Dr. Nikhil Pay of McMaster Children's Hospital in Ontario, Canada will test children.

Animal studies have shown that germ-free mice fed stool from obese mice gain more weight than those fed bacteria from the guts of lean mice.
* Transferring the feces of skinny mice to fat mice changed their colon bacteria to ones that are less likely to convert soluble fiber to acetate and other short-chain fatty acids, and decreased calorie absorption and lowered blood levels of acetate and insulin (Nature, October 9, 2016; 534 (7606):213-217). Blood levels of acetate are increased by absorbing more calories from food and eating a lot of fat. Insulin levels are increased by high rises in blood sugar and high insulin levels are associated with increased risk for obesity and early diabetes.

* Jeffrey Gordon of Washington University in St. Louis showed that feeding mice a diet high in fat and low in fruits, vegetables, and fiber changed their colon bacteria to a more fattening type and made them fat. Then he showed that placing skinny mice in the same cages as the fat mice fed this terrible diet made them fat, presumably because mice eat each other's stool.

* Giving baby mice antibiotics makes them 15 percent fatter than mice not given antibiotics, presumably from changing their colonic bacteria.

* Rob Knight of the University of Colorado Boulder and Maria Gloria Dominguez-Bello of N.Y.U. have shown that human babies delivered by C-section are fatter and more likely to suffer diabetes than those delivered vaginally, probably because vaginally-delivered babies acquire bacteria from their mother's vaginas, while C-section delivered babies do not have that opportunity.

My Recommendations
North Americans are in the midst of an epidemic of obesity and diabetes that is still increasing. Emerging evidence suggests that obesity and diabetes are driven to a significant degree by the types of bacteria in the colon. To a large extent, a person's diet determines the type of bacteria that live in his or her colon.
* A diet that encourages the growth of bacteria that help to prevent and treat obesity and diabetes has plenty of vegetables, fruits and seeds (beans, nuts and whole grains that have not been ground into flour).
* Foods that encourage growth of the types of colon bacteria associated with obesity and diabetes include sugared drinks, sugar-added foods, foods made from flour (ground whole grains) and all other refined carbohydrates. 

October 16th, 2016
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About the Author: Gabe Mirkin, MD

Sports medicine doctor, fitness guru and long-time radio host Gabe Mirkin, M.D., brings you news and tips for your healthful lifestyle. A practicing physician for more than 50 years and a radio talk show host for 25 years, Dr. Mirkin is a graduate of Harvard University and Baylor University College of Medicine. He is board-certified in four specialties: Sports Medicine, Allergy and Immunology, Pediatrics and Pediatric Immunology. The Dr. Mirkin Show, his call-in show on fitness and health, was syndicated in more than 120 cities. Read More
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