January's store shelves are groaning with new diet books, all promising to finally reveal the universal formula for weight loss.
That promise, however, is as empty as the calories in a Timbit. It's not going to happen.
"People want to hear that there are 'good' and 'bad' foods, and that you simply need to avoid the bad foods and only eat the good foods and you'll be fine," says Yoni Freedhoff, the founder of Ottawa's Bariatric Medical Clinic and one of Canada's foremost obesity clinicians.
Over the past few decades, we've tried cutting carbs, eliminating fat - or meticulously optimizing the ratio between them; we've eliminated meat and subsisted on liquids; we've even eaten according to our blood type. The result? When Statistics Canada went out and actually measured thousands of Canadians (instead of trusting them to tell the truth about their weight) between 2007 and 2009, they found that 61 per cent of us were overweight.
As we begin a new decade, obesity researchers are turning away from this search for "good" foods - a quest that has led us down a nutritional rabbit hole, in which the rich complexities of the human diet are reduced to didactic edicts that change every few years.
Instead, they're focusing more on the physiology and psychology of why people eat what they do, how societal forces influence their choices, and what they can do to change. The code hasn't been cracked yet, but here's what we do know about weight loss.
For starters, it's clear that losing weight isn't just about eating certain foods. Last fall, Mark Haub, a nutrition professor at Kansas State University, subsisted for 10 weeks on 1,800 calories a day - about 800 fewer than usual - of Twinkies, Doritos, brownies and other convenience-store staples. In the process, he lost 27 pounds and improved health markers such as cholesterol levels.
While Dr. Haub's experience runs counter to the advice in anti-carb polemics like Gary Taubes' new book, Why We Get Fat, his slim result was no surprise to obesity researchers.
"Of course you can lose weight by eating pizza and junk food," says Jean-Philippe Chaput, a scientist in the Healthy Active Living and Obesity Research unit at the Children's Hospital of Eastern Ontario Research Institute in Ottawa. "And you can gain weight by eating apples, if you eat more apples than you eat pizza."
There are many compelling nutritional reasons to choose apples over pizza and Twinkies. But if you're primarily concerned about weight, researchers have found that people can successfully shed pounds on pretty much any diet, as long as it involves a reduction in calories.
A 2005 study published in the Journal of the American Medical Association, for example, followed 160 overweight volunteers assigned to the Atkins, Ornish, Weight Watchers or Zone diet. After a year, the average weight loss was about six pounds, but there was no statistically significant difference among any of the diets.
Instead, the results showed the best predictor of weight loss was how closely the subjects had managed to stick with their prescribed diet. The true mark of a successful diet, after all, isn't losing weight, it's keeping it off - and that's where the Twinkie diet, and any other diet that you're not prepared to maintain for the rest of your life, fails.
The most famous equation in nutrition is "calories in = calories out." But the right-hand side of the equation is more complicated than most people realize.
If you add one 60-calorie cookie to your daily diet, simple math suggests you'll gain weight for the rest of your life at a rate corresponding to about a pound of fat for each 3,500 calories. But, in fact, as you gain weight, your body has to spend more energy maintaining the new tissue, using up some of the excess calories. A paper published last year in JAMA calculated that the weight gain would level off at six pounds after a few years.
Unfortunately for dieters, the same mechanism works in reverse when you cut calories. Your body becomes more efficient as you lose weight, and soon stabilizes at a new level. As soon as you resume your previous intake, your weight shoots back up.
Researchers are trying to identify the subtle but insistent cues that drive how many calories we consume - and their latest results reveal just how complex these factors are.
Last year, Dr. Chaput published an analysis of data from 537 participants in the Quebec Family Study. He found that a collection of unlikely factors - short sleep duration, emotional eating patterns and low dietary calcium intake - predicted the risk of obesity better than the amount of fat in the subjects' diets or how much vigorous exercise they did.
His data confirmed earlier findings about the link between sleep and obesity: A 2008 study found that even a single night of shortened sleep raises levels of ghrelin, the gut hormone that signals when it's time to start eating. Similarly, just two nights of short sleep causes a drop in leptin, the hormone that tells you when you're full.
There are other ways to manipulate your appetite. In a study published last year in the Journal of Clinical Endocrinology & Metabolism, volunteers ate identical amounts of ice cream in either five minutes or 30 minutes. (The latter group was fed in seven portions so the ice cream didn't melt.) The slow eaters reported feeling more full, and showed higher levels of peptide YY and glucagon-like peptide, two hormones that, like ghrelin, signal you to stop eating.
Of course, knowing how fast to eat still doesn't tell you what you should eat. There are some basic ideas that most scientists agree on, such as the fact that protein is more filling than carbs. Dr. Freedhoff suggests taking in at least eight grams of protein with every meal and snack - the equivalent of a cup of milk or about 30 almonds.
Generally, whole, unprocessed foods that keep you feeling full tend to be low on the glycemic index, which means their energy is released into your body over a longer period of time. Mr. Taubes' core idea, that refined carbs cause a damaging spike of glucose that can affect insulin function (and thus fat storage) is backed by quite a bit of science. Several studies have found that people following low-GI diets are indeed more likely to keep weight off a year after starting.
This suggests you should think twice about refined carbohydrates like sugar and white flour, and choose home-cooked over processed meals.
This advice sounds a lot like Michael Pollan's relaxed mantra, "eat food, not too much, mostly plants." Having this kind of flexibility, rather than puritanical rules about eliminating whole classes of food, is crucial to finding a sustainable route to weight loss, Dr. Freedhoff believes, because the bottom line - how many calories you take in - remains non-negotiable.
"The only way a person is going to lose weight and keep it off," he says, "is to like the life they're living while they're actually losing."
Put like that, it sounds like simple common sense. If only it were that simple to pull off.
Workout versus diet
It's generally agreed that if you ignore diet, exercise on its own isn't very effective for losing weight. But there's plenty of evidence that exercise helps keep weight off once you've lost it, according to obesity researcher Jean-Philippe Chaput.
Studies have found that high levels of physical activity lead to "tight coupling" between calories consumed and calories burned. In other words, your hunger mechanism gets more finely tuned.
On the other hand, exercise makes you hungry, which can easily backfire if you fuel a 200-calorie workout with a 250-calorie sports drink. Recent research at the University of Massachusetts-Amherst suggests that very low-intensity activity - think standing desks - can burn a significant number of calories without triggering any rise in appetite hormones.