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Gut Flora & Obesity

How one may be able to modify one’s own gut flora to facilitate weight loss.

November 12, 2010 |
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Only one in ten of the cells in your body is human. The other 90% are bacteria—we have about a 100 trillion of them on us and in us. The human colon is considered the most biodense ecosystem in the world. We’re just like along for the ride. We exist as one big superorganism, in a mutually beneficial symbiotic relationship—usually.
Collectively, our gut bacteria weigh as much as one of our kidneys and are as metabolically active as our liver. They affect our immune system —we just found out that if you give probiotics to kids, they don’t get sick as much. Our gut bacteria affect our hormonal balance, and they can affect our energy balance, as well. Is obesity linked to our gut flora?
There are some species of bacteria better at extracting calories from our feces than others. Pooped calories end up in the toilet rather than on our hips. But there’s certain obesity-associated bacteria with an increased capacity for energy harvest. Our bodies are trying to get rid of fecal matter, but certain bacteria in our colon can take our waste, break it down further, and release calories that are then absorbed back into our blood stream. So here our body is trying to get rid of it all and the calories are bouncing right back.
Now this was originally all based on mouse studies so we really didn’t know what to think, but finally this year we have some human data and indeed the type of bacteria in our guts is related to body weight and weight gain.
That got some researchers in Austria thinking. Maybe one of the reasons vegetarians are so much slimmer, on average, is because their diets foster more of the lean-type bacteria, rather than the obese-type.
And that’s exactly what they found. They took a bunch of fecal samples from vegetarians, did some DNA fingerprinting, compared it to… omnivore feces and found significantly more of the lean-type bacteria, suggesting a smaller capacity for energy gain from food in vegetarians. How much smaller? Perhaps 2% of daily caloric intake. That doesn’t sound like a lot but it just happens automatically—while we sleep even, and adds up over time. Over a year that may come out to about 5 pounds of fat. And that may not sound like a lot either but that’s exactly how much people tend to put on annually during mid-life years, so that could help explain why as we’ve gone over before, those eating vegetarian don’t seem to get that age-related weight gain that afflicts the rest of the population.

To see any graphs, charts, graphics, images, and quotes to which Dr. Greger may be referring watch the above video. This is just an approximation of the audio contributed by veganmontreal.

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Dr. Michael Greger

Doctor's Note

Please feel free to post any ask-the-doctor type questions here in the comments section and I’d be happy to try to answer them. And check out the other videos on weight loss. Also, there are 1,686 other subjects covered in the rest of my videos--please feel free to explore them as well!

For some context, please check out my associated blog posts: Boosting Gut Flora Without Probiotics and How Probiotics Affect Mental Health

If you haven't yet, you can subscribe to my videos for free by clicking here.

  • Michael Greger M.D.

    Please feel free to post any ask-the-doctor type questions here in the comments section and I’d be happy to try to answer them. And check out the other videos on weight loss. Also, there are 1,449 other subjects covered in the rest of my videos–please feel free to explore them as well!

  • 2RHealth

    For vegans who don’t eat yogurt, what is a good source for probiotics? Probably not Kimchi, as you noted its harmful effects in another video!

    • Michael Greger M.D.

      Commercial yogurt of any kind (soy, rice, cow, or coconut) is an insufficient source of the level of probiotic bacteria found effective in treating diarrheal illnesses, inflammatory bowel disease, and irritable bowel syndrome. A plant-based diet appears to naturally modulate one’s gut flora, but if you need to bring out the big guns for therapeutic usage or to repopulate your gut after a round of antibiotics, allow me to refer you to the advice of one of my medical mentors, Dr. Michael Klaper, who has some great probiotic tips and insight.

      • Art

        Thanks for all of your information. But I was wondering, what about kefir grains? There are milk kefir grains and also water kefir grains used to ferment beverages. And do you know of any reliable comparison of yogurt and kefir, such as how many organisms are found in homemade yogurt and kefir? Thanks again.

  • Carl B. Nelson

    Dr. Greger, many thanks for your fine, concise, science based videos on so many subjects. My wife is battling stage 4 breast cancer. Due to the experience we’ve had I decided to put together a blog ( to share resources I found that have helped my wife to save others from some of the distress we’ve experienced. Many of your videos dealing with cancer are posted on my blog.

    Here’s my question: I’ve probably heard over a hundred times people recommending the fruit and a tea (made from the leaves) of the soursop fruit tree. The common phrase is that it’s “10,000 times more powerful than chemo” (it’s even on YouTube). There’s clearly a bucket of hyperbole in that statement but I’m wondering if the sheer volume of people that place such faith in this magical fruit indicate that there’s at least some truth to the legends. Have you heard of any studies that shed some light of science on this legend? I’d appreciate your help in evaluating if it would be worth including this fruit (and tea from its leaves) in my wifes cancer management treatment plan.

    We are following predominately the integrative treatment system advocated by Dr. Keith Block (author of Life Over Cancer).

  • Nicole Lucas Haimes

    Can you please help guide me for what are the best sources of probiotics? Pills, yoghurt, Kefir? It would be valuable to know this, short of going for a fecal transplant. Thanks!

    • Nicole Lucas Haimes

      P.S. I love your posts, and appreciate your hard work and service. Thank you for all you do!