Positive feedback loops

[andrewducker]

...there is a difference in the bacterial taxonomic composition between obese and non-obese humans. They have shown that obese mice and people harbour in their guts a dominant population from the bacterial division Firmicutes. At the same time, lean people (or even those on a weight-loss diet) and lean mice, have less bacteria from the Firmicutes division and more from the Bacteroidetes division.

They found that in obese mice the gut bacterial population contained more enzymes that broke up complex carbohydrates, like starch. Other experiments showed that indeed, the population of bacteria in obese mice break up complex sugars more efficiently; that is, the bacterial populations of obese mice provide their hosts with smaller sugar molecules that are readily absorbed through the gut, creating a vicious feed-forward cycle: if you are a fat mouse, you will get more calories from the same piece of chow than if you are a lean mouse.

Their conclusion was that the human gut bacterial population is intimately connected with what we eat. High poly-carbohydrate foods eventually enrich their consumers’ guts with carbohydrate loving bacteria; and those, in turn, “reward” their hosts with the back-handed compliment of making more simple and easily absorbable carbohydrates available to them, making them fatter.

Consider a slice of whole wheat bread, about 100 calories.* This means that the actual caloric intake from a slice of bread will differ between individuals. Unfortunately, it is the fatter person who will, quite probably, receive more calories from eating the same slice of bread, because his gut bacteria will deliver more available calories to him.

from

Also

Dhurandhar collected blood samples from 52 overweight patients. Ten of them, nearly 20 percent, showed antibody evidence of prior exposure to the SMAM-1 virus, which was a chicken virus not previously thought to have infected humans. Moreover, the once-infected patients weighed an average of 33 pounds more than those who were never infected and, most surprisingly, had lower cholesterol and triglyceride levels — the same paradoxical finding as in the chickens.

With Ad-36, Dhurandhar and Atkinson began by squirting the virus up the nostrils of a series of lab animals — chickens, rats, marmosets — and in every species the infected animals got fat.

“The marmosets were most dramatic,” Atkinson recalled. By seven months after infection, he said, 100 percent of them became obese.

Comments

[autopope.livejournal.com]

Also note: fat is culturally associated with gluttony which is naively associated, in western religious terms, with sin (over-consumption).

I suspect the prevailing assumption that fat people are fat because they're sinful gluttons -- rather than because they're infected with an adenovirus or their gut bacteria have gone apeshit -- may be responsible for a shameful lack of medical research into what could well be a real epidemic, and a medically treatable one at that.

Hypothesis: the worldwide epidemic of obesity really kicked off in the 1970s and 1980s. Has anyone tried to correlate this with increasing access to mass air travel? All those disaster novels about killer plagues spreading by air may have been spot on the mark -- except for the nature of the plague ...

[onceupon.livejournal.com]

Just out of curiousity - what are you basing your dates on? Because no, actually. In the 80s the BMI for obese was moved down, making millions of people fat overnight even though they didn't gain a pound.

The air travel thing is interesting, if you believe that fat can be spread by a virus. But, having read the study.... The subject pool was 502 people. That's, I did the math last night, 1.something*10^-4% of the population which doesn't really give me a lot of confidence in the results at all. Plus, the subjects were drawn from a pool of people visiting a weight loss clinic so they're already self-selected for certain characteristics.

[ciphergoth.livejournal.com]

What does the proportion of the population that they represent have to do with anything? How they are selected, as you say, can be a cause for concern.

[onceupon.livejournal.com]

Because humans aren't a homogenous group - especially Americans (the Melting Pot concept worked in some ways) - and 500 people out of 304 million isn't going to provide the kind of data you need to extrapolate assumptions about an entire population.

[meaningrequired.livejournal.com]

I've seen it in some older family relations, which is "if we have the money, then why are we not able to serve large portions of food". Its as though wealth and food have become analogous... which is in conflict with the sinful gluttons. Maybe there just aint enough religion!

[ciphergoth.livejournal.com]

Interesting. If thin people are getting less out of their food, where is the rest of the energy going?

[andrewducker]

Down the toilet.

[ciphergoth.livejournal.com]

Gosh. Many years ago our biology teacher told us that the calorific value of human shit was very low, lower than most other animals; it's interesting to know that it's higher for some people. Measuring for a correlation between that and BMI seems like a relatively straightforward experiment to do, I wonder if it's been done?

[drdoug.livejournal.com]

Who wants to do shit research? It'd be a crappy job.

(sorry)

[channelpenguin.livejournal.com]

Heat.

Some people give off more heat in response to overfeeding. (I am one). Think I read that adaptation was more common in people with cold-climat ancestry (funnily enough).

[ciphergoth.livejournal.com]

I'm not sure I quite see the mechanism by which the wasted energy goes from food to heat without passing via useful work, could you clarify? Thanks!

[drdoug.livejournal.com]

Oh, no theoretocal problem there. For one mechanism, look up 'futile cycles' in a metabolism text - cells constantly interconvert certain molecules, burning ATP and realeasing the energy as heat each time round the cycle. Ramp up the cycling and your metabolic rate goes up.

[ciphergoth.livejournal.com]

Not having such a text, I have resorted to Wikipedia. Thanks!

[lebeautemps.livejournal.com]

So if its a question of enzymes, can't we just block either the action of or production of the enzyme, and flush people out to let them start over?
I am intrigued.

[onceupon.livejournal.com]

Killing off your gut floral makes you very, very ill. They are quite necessary. You can encourage certain kinds (which is why you see all those Activia yogurt commercials) but I don't think the scorched earth policy is particularly practical. Like, I'd rather be fat (which I am *grin*) than deal with that sort of intestinal disruption.

[lebeautemps.livejournal.com]

I didn't mean scorched earth per se - more of a targetted flush. I know all about gut respect, believe me, but I would rather not be fighting a losing battle with calorie control if there was something that could help.

[soon_lee]

Doesn't that sort of thing (killing of gut flora) happen when one goes on a course of antibiotics?

[robhu.livejournal.com]

This is interesting research. I'd like to see the research repeated independently by others.

I worry though that people will jump on it and argue that the reason that they're overweight is because of something like this. While there are a variety of factors that relate to the weight people do (issues similar to this, we all have different basal metabolic rates), the primary cause seems to be to do with the amount of calories we eat.

[andrewducker]

They could argue that it's _a_ reason, but it's clearly not an entire explanation.

[robhu.livejournal.com]

Yeah, that makes sense.