Have you been dreaming about that second helping of turkey, dressing, mashed potatoes and gravy? Are you planning to eat chocolate now and diet later? You might want to think about this first.
The overwhelming majority of dieters fail. Depending on which study you look at, between 80 and 95 per cent of people who lose weight put it back on within two years.
The reason: most people's bodies fight weight loss — fiercely. And if you manage to lose weight, it fights — fiercely — to get you to put those pounds back on. To boot, it does this in ways you have little or no control over.
One of the most powerful mechanisms your body uses is your resting metabolic rate.
That's the number of calories you burn at rest — to keep your heart beating, lungs breathing and eyelids blinking, etc. And depending on how much exercise you do, it can be between half and three-quarters of the total number of calories you burn in a day.
Biggest Loser effect
But when you diet, your body becomes more fuel efficient, burning fewer calories to accomplish those same tasks. It goes from being a Hummer to being a Honda Civic, in terms of fuel consumption. And that fuel efficiency can be permanent. That's what scientists found in the now well-known Biggest Loser study.
Six years after the show ended, the contestants in Season 8 found that their metabolism was still down an average of 499 calories a day. One man — Dan Cahill — saw his drop by 800 calories a day, even after he'd regained 100 pounds.
Jennifer Kuk studies this phenomenon at the School of Kinesiology and Health Science at Toronto's York University. She says everyone's resting energy expenditure drops while they're dieting. But for about 50 per cent of people, the drop becomes permanent.
The average is about a 200-calorie-a day drop. In extreme cases, it can be four times that much. If you're in the unlucky half of the population, keeping lost weight off is much harder.
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"You have to make sure that you do that much more physical activity, that you have be that much more careful when you do consume your food because you have a smaller window in terms of how many calories you can eat before you potentially store."
The science of hunger
And metabolism isn't the only trick your body uses to prevent weight loss. There are dozens of hormones and peptides that affect what and how much you eat. Scientists are just beginning to unravel the complicated ways they interact. But the effects of two are pretty well understood.
Dr. Arya Sharma is an obesity specialist, a researcher at the University of Alberta and the science director for the Canadian Obesity Network.
Sharma says one of the first things your body does when you go on a diet is raise the level of a hormone called ghrelin. Its job is to make you hungry.
"Ghrelin always peaks just before a meal. It actually induces your eating behaviour. And there's research showing that if you're not eating [dieting], ghrelin levels go up."
The other hormone — leptin — does the other job. It's created by fat cells and tells your body to stop eating because you've had enough. If you go on a diet, you shrink some of those fat cells and produce less leptin. The "stop eating" message gets weaker.
And your brain joins in on the fight, too.
Test of willpower
Stephan Guyenet, a Seattle-based neuroscientist and author, says it is possible to white-knuckle your way past the pastry tray and not eat something — but the strategy doesn't work in the long term.
"The problem with willpower is that it's a limited resource. It's effortful," he says.
"If you have to do it on a constant daily basis, which is what you have to do if you've lost weight and your hunger circuits and your food-seeking circuits are activated, you're going to have to exert that willpower on a continual basis to restrain yourself from eating food."
That's difficult for most people to do, Guyenet says, pointing out you're often using your willpower for other things, like going to work, running errands and looking after your family.
"Those are things that deplete your willpower reserves, such that, at the end of the day, you might not have what it takes to fight those impulses."
The consistently dismal outcomes from diets have a growing number of researchers, like Sharma, coming out strongly against fat-shaming.
They're actively trying to debunk the commonly held idea that people with excess weight are undisciplined or lazy. In fact, science shows they're trying to accomplish something their own bodies don't want them to do.
You can find out more on obesity and weight loss on a CBC Radio documentary called Through Thick and Thin. It airs on CBC Radio One on Dec. 26 at 12 p.m. (12:30 in Newfoundland) and repeats on Jan. 2 at 6 a.m. (6:30 in Newfoundland.) You can also find the program on our website.
Contact us at firstname.lastname@example.org or me @cbcDanielleK.