Leslie Beck, a Toronto-based private practice dietitian, is Director of Food and Nutrition at Medcan.
Perhaps you ate more than you intended at your holiday meal – lots of mashed potatoes, stuffing, turkey, gravy, cranberry sauce, pumpkin pie, the works. And this morning, not surprisingly, the bathroom scale registered a few extra pounds from yesterday.
Don't panic. That small weight increase isn't body fat. It's pretty much impossible to gain even one pound of fat from a day of overeating. And no, binging on Thanksgiving dinner didn't slow down your metabolism.
So, that's good news.
The bad news, though, is that if you don't get your diet back on track now, you could easily pack on a few pounds of fat this fall. (Lingering Halloween candy and the November/December string of holiday parties make that a distinct possibility.)
How you gain body fat
It takes days of overeating for accumulated body fat to show up as measurable weight gain.
Once food is digested, its building blocks (such as glucose, amino acids, fatty acids) are absorbed into the bloodstream. Your cells use what they need for fuel and store the rest in fat cells for later use, a process that begins six to eight hours after a meal.
So, yes, consuming more calories than your body needs in one sitting (or over the course of a day) will result in some of them being tucked away as fat. But not enough to move the needle on scale. (Unless you really, really gorge.)
Consider the math. Let's say you typically consume 2,000 calories a day and that your weight is pretty stable. In other words, you're eating enough calories to prevent gaining or losing weight.
Theoretically, one pound of body fat contains roughly 3,500 calories. That means that in order to gain a pound of fat, you'd have to eat 3,500 calories above and beyond what you would normally eat in a day.
To pile on an extra 3,500 calories – and not burn them off – would be a feat. Even a fully loaded turkey dinner with the fixings, pumpkin pie with whipped cream and two glasses of wine clocks in around 2,000 calories. You'd have to eat all of that, plus more, on top of what you usually consume.
Of course, how people's bodies respond to excess calories – and how quickly those calories add extra weight – varies. Scientists are learning that the biology of weight gain is individualized; a person's unique physiological and hormonal responses to food play a role.
Overeating and metabolism
Weight gain after a day of overindulging doesn't occur because you've somehow lowered your metabolism (a concern I sometimes hear). Eating actually causes your metabolism to increase.
Your body burns calories to digest, transport and store the food you eat; this increase in metabolism is called the thermic effect of food. And the bigger the meal, the more calories your body burns.
In general, about 10 per cent of the calories in a mixed meal (one that includes a mix of protein, carbohydrate and fat) are expended to digest and process it. Not enough to put much of a dent in your holiday calorie intake.
Thanksgiving weight-gain culprits
The extra few pounds you're carrying today reflect water weight. When you eat a large quantity of food you also take in extra sodium, which causes your body to retain water.
The carb-loading you did contributed, too. Excess carbohydrates from starchy side dishes and a sugary dessert also caused some water retention.
Some of that weight might also be the meal itself. It takes about 24 hours before your body begins to eliminate undigested food residue (i.e. waste).
So, you've gained a little water weight, not a surplus of fat cells. If you resume your usual healthy diet and drink plenty of water, you'll lose that extra fluid in a couple of days. (It sounds counterintuitive, but drinking water can help flush out excess sodium.)
Heart risks from overeating
Weight gain aside, there are other reasons why you should think twice about eating a supersized meal.
Overeating generates an overload of free radicals, unstable molecules that can harm DNA and other tissues in the body. Research suggests that free radical damage from persistent overeating can promote plaque buildup in artery walls, increasing the risk of heart disease.
Eating a large meal that's high in fat or refined carbohydrates causes a bigger-than-normal rise in blood triglycerides after eating. After-meal (postprandial) blood triglycerides can damage blood vessels and promote hardening and narrowing of arteries.
A large, fatty meal has also been shown to impair the ability of blood vessels dilate properly.
If you have heart disease or you're at risk for it (diabetes, high blood pressure, high cholesterol, obesity, smoking), overindulging isn't a good idea, even occasionally.