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A weight loss class pictured here.

Eat less, move more. That's been the prevailing weight-loss strategy for years now, but even obesity experts acknowledge that few people who lose weight manage to keep it off.

Meanwhile, in research labs, frogs, mice and zebra fish exposed to minuscule amounts of estrogen replacement drugs, dioxins, bisphenol A and other chemicals are getting fat – very fat.

The phenomenon is so striking that some scientists believe that common chemicals, dubbed obesogens, are messing with our hormonal systems and the natural balance of "calories in, calories out."

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The obesogen theory goes prime time Thursday on the CBC Television series The Nature of Things. In a phone interview, science journalist and filmmaker Bruce Mohun explains the research behind his new documentary, Programmed to be Fat?

We've been bombarded with theories about the obesity epidemic, including the idea that antibiotics have disrupted our gut bacteria and made us fat. Why should we pay attention to this one?

This is the one that makes the most sense to the kind of people who study obesity – endocrinologists and toxicologists and obesity experts. They can repeatedly make mice fat by giving them tiny doses of these chemicals.

How widely accepted is the theory that everyday chemicals are making us fat?

Alison Holloway [endocrinologist at McMaster University]put it nicely. She says, 'Is it plausible that these chemicals are causing obesity? Absolutely. Has it been shown unequivocally? No.' And that's why [funding agencies]are starting to dump more and more money into research in this area because they believe that it's likely that this could be contributing to the obesity epidemic. But they don't know for sure or how much.

The chemical industry says it has been unable to reproduce the link between obesity and low doses of chemicals in animal studies conducted in response to the independent research. Why?

Well, Bruce Blumberg [biologist at the University of California, Irvine]said to me, 'Follow the money.' We did ask industry people to be interviewed – they all refused. If they don't want to talk about this and they're [saying]that they're not getting the same results that Blumberg [and other academic scientists]are getting, well … we decided to go with the people who are unbiased and look at their research.

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How might chemicals amplify the effects of bad habits?

Overeating and lack of exercise are a little more obvious – you know, 'calories in, calories out.' But how those calories are adjusted and what the body does with those calories, that's what the endocrine [hormonal]system does. And when you change the endocrine system very, very slightly, it does it a little better – it bumps up our set point.

What about the 2004 documentary Super Size Me? The filmmaker gained 25 pounds in 30 days simply by eating more junk food. Doesn't this suggest that the Western diet, not chemicals, is the main culprit?

There's no doubt that if you eat enough, you can make yourself fatter. [Scientists]are not questioning that. They're saying, Why are babies under six months of age – newborn babies – fatter than normal over a 20- or 30-year period? Why are animals that live in proximity to people – feral animals, farm animals, lab animals – all about 7 or 8 per cent heavier than they were 50 years ago?

Researchers are measuring chemicals in the bodily fluids of volunteers worldwide to see if chemical exposure affects weight. What do you think they'll find?

A few results have shown a link between higher levels of environmental chemicals in the body and greater propensity to put on weight. [But]nobody is really trumpeting them. [Researchers]feel that they need more people and more studies to really be able to say anything definitively.

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But your documentary makes the link sound conclusive.

When we looked at the interviews I did over a year, [the scientists]are fairly strong in saying, We think we really have a problem here … and we've been working on this for almost 10 years now.

Aren't you concerned the film will encourage people to use chemicals as an excuse to avoid dealing with their weight?

Yes. I'm going to be getting letters from obesity researchers around the world who don't know too much about this theory and have been trying for many, many years to get people to live better lifestyles. They'll say, 'Damn it, now you've screwed everything up.'

How can consumers avoid the 20 or more chemicals, including bisphenol A (BPA) and phthalates, that are identified as possible obesogens?

It's a tricky thing to avoid. Yes, we can avoid BPA to some extent by avoiding handling store receipts. Certainly, pregnant mothers and children [should]eat whole foods and not processed foods, use fewer cans and avoid plastic water bottles. But these chemicals are in all sorts of things – in motorcycle helmets, in car parts. They're ubiquitous.

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This interview has been condensed and edited.

Fat facts

While controversies over the cause of the obesity epidemic rage on, there's no denying the hefty facts. Published data indicate:

-Obesity in Canada has doubled in less than 30 years: 1 in every 4 Canadians is obese.

-In the Western world, every second adult is overweight.

-In the United States, the world's fattest country, 14 per cent of cancers in men and 20 per cent in women are estimated to be caused by excess weight.

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Obesity rates have exploded so quickly that scientists now debate whether sedentary lifestyles and bad eating habits are solely to blame. Here are some theories:

-Antibiotics have disrupted gut bacteria, causing an increase in hunger and greater extraction of calories from the same food, according to research from the University of Colorado, Boulder.

-Certain personality types – including those with a tendency toward negativity, impulsivity and poor self-discipline – predispose people to obesity, found a study from the U.S. National Institute on Aging.

-Consuming high-fructose syrup, which accounts for up to 40 per cent of caloric sweeteners used in the United States, leads to weight gain, particularly in the abdominal area, a team from Princeton University concluded.

-Obesity is a genetic problem: British researchers found that people who carry a variant known as FTO – far more common in Europeans and Africans than in Asians – face a much higher risk for obesity.

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