The biology of obesity: How our ancient brain conspires to make us overeat

By | October 11, 2019

According to a new study, an additional 1.7 million Canadians will be living with obesity by 2023.

Our growing girth is already at historic levels, and we’re among the heaviest countries in the world. Over the past decade, rates of overweight and obesity have increased in Canada, France, Mexico, Switzerland and the U.S., according to a 2017 report by the Organization for Economic Co-operation and Development. More than one in four adults is obese in Canada. Within three years, there will be a total of 8.5 million people in a weight class considered a serious threat to health.

All of this leaves virtually no chance of meeting World Health Organization targets for a zero increase in obesity from 2010 to 2025, a global goal set to address the health impacts of weight gain (such as high blood pressure, type 2 diabetes, heart disease, stroke and certain cancers) as well as the cost to health-care systems, estimated at $ 5 to $ 7 billion in Canada alone.

“The burden surprised me,” says Laura Rosella, co-author of the modelling study and an associate professor in epidemiology at the University of Toronto’s Dalla Lana School of Public Health. “I thought, optimistically, that we have actually been making progress in terms of awareness and efforts to reduce obesity.

“It’s going to get worse before it gets better.”

Why?

It’s not, as comedian Bill Maher recently put it, because people are “eating like a–holes.”

Try to lose weight and the brain fights back, aggressively. Higher levels of the hunger hormone ghrelin are released, sending a single-minded message to the nerves in the hypothalamus: “Get food.” At the same time, the brain blocks satiety, or “I’m full,” signals from the gut and slows down the rate at which calories are burned.

This “famine” effect can last a year or longer as people struggle to keep the lost weight off.

“It is an incredible and efficient response to weight loss,” obesity specialist Dr. David Macklin says with awe.

But Maher’s fat-shaming quip taps into a common misperception: that obesity comes down to some kind of moral failing, a lack of discipline and self-control, and that the solution is as simple as “finding the right diet and working out a ton,” Macklin says.

In fact, it goes much deeper.

“We now have great clarity that obesity is a chronic and complex, progressive, primarily genetically conferred, centred-in-the-brain, environmentally influenced, real medical condition,” sums up Macklin, the medical director of a weight management program for high-risk pregnancies at Toronto’s Mount Sinai Hospital.

The DNA of obesity

The tendency is to blame obesity primarily on poor food choices — sugary drinks, salty, greasy processed foods, staggering portion sizes.

But a growing body of research suggests that the appeal of these foods, as well as the drive to overeat, is rooted in our DNA.

Genome-wide studies have identified hundreds of genes associated with body mass index, waist-to-hip ratios and other traits of obesity, most of them expressed — meaning whether they’re turned on or off — in the brain.

Many of these genes evolved over millions of years to collect and store excess calories as fat whenever food was available, and to keep early humans from starving whenever food was scarce. Except as we’ve shifted from hunter-gatherers to farmers, then farmers to factory workers, food is no longer so scarce.

“In this part of the world, for most people, we don’t have famine anymore, we have only a feast,” says Dr. Sue Pedersen, of the C-ENDO Diabetes and Endocrinology Clinic in Calgary.

Instead of a survival mechanism, gaining excess weight is now a liability. And as scientists are discovering, some of us are more “genetically vulnerable” to packing on the pounds than others, says Macklin.

Part of this is how the brain responds to the hunger hormone ghrelin. In people with a genetic predisposition to obesity, the gut also tends to release fewer quantities of the hormones tied to fullness.

Either way, “If you take people who are the same weight and they have the same metabolic rate and you put everyone on (a) diet, people will lose weight unequally, based on their genetics,” says Macklin.

Furthermore, some people who consume excess calories gain fat. “Other people, their body responds by burning more, by increasing their metabolic rate and taking anything extra and putting it into muscle.”

Even more frustrating for those less prone to burning fat, the further people get from their highest weight, Macklin says, the harder the body fights against losing it.

How strongly we respond to cues in the environment that generate the fundamental drive to eat — the psychological state known as “wanting” — and our ability to control that “wanting,” Macklin says, is heritable as well.

The gut microbiome

The environment inside our digestive tract may also play a critical role in weight gain.

Each of us plays host to trillions of different bacteria, which colonize our intestines immediately after birth and continue to evolve as we age based on what we eat and where we live. These bacteria impact our digestion, the production of certain vitamins and our immune system.

There’s now evidence that people living with obesity have different gut flora than those who are not. According to some scientists, it may be that mircobiota — not just our genes — are reducing the expression of gut satiety hormones.

Although researchers are still exploring exactly how gut bacteria interact with our intestines and the brain, the link appears clear: When mice free of intestinal bugs are fed stool from either obese mice or humans, they put on more weight and body fat than those fed bacteria from the guts of lean mice or humans.

The chemical context

Some antidepressants and newer generation anti-psychotics, drugs Canadians are being prescribed in record numbers, may be behind our rapid and dramatic weight gain too.

Antipsychotics can trigger “hedonic hyperphagia” — eating to excess for pleasure, not hunger. Two years ago, Montreal researchers reported that, after 24 months of treatment, the mean weight of children prescribed antipsychotics for ADHD and other behavioural problems increased by 12.8 kg.

One study published last year in the British Medical Journal found people taking any of the 12 most commonly used antidepressants had an increased risk of weight gain that persisted over at least five years of follow-up. It’s not clear why. Depression, in and of itself, can cause weight gain. And people might eat more as their mood improves. Some believe the drugs may affect metabolism or trigger cravings for carbohydrates. But there are options, Pedersen says. Some anti-depressants are “weight neutral” or even induce weight loss.

The chemicals in our food — particularly artificial sweeteners — may also react with taste receptors or gut bacteria in ways that stimulate more food intake, Pedersen adds.

Recent studies suggest there’s something about the sheer textural and sensory properties of ultra-processed foods that make us eat more of them, and more quickly. (Again, it could be that foods with “hyper-palatable” amounts of sugar, fat and salt are irresistible to the ancient brain.)

Obesity ultimately still comes down to physics, Pedersen says. “If calories in are higher than calories out, weight will go up.” But managing the factors that contribute to that equation is much more complicated than simply sticking to a diet. And a lack of education means that fat-shaming and weight discrimination are as prevalent in medicine as everywhere else.

“Why are obesity rates getting worse?” says Macklin. “When you’re talking about a real disease, and you’re only offering up advice like, ‘Eat less, move more,’ it’s like saying, ‘Listen, I see you have asthma, and it’s severe asthma, but just breathe deeper. Just pull yourself up by your bootstraps and I don’t want to see you back here wheezing.’

“Not only is obesity real, but treatments exist. That should be the messaging to someone with obesity.”

Read the National Post’s ongoing focus on Canada’s obesity epidemic at nationalpost.com/obesity.


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