When you binge on Thanksgiving, what happens inside your body?

Oh, Thanksgiving ... what a unique and strange holiday. A time-honored tradition that recognizes the ability of Americans to eat massive quantities of food. And maybe something about saying thanks too.

But what actually happens inside your body when you overeat common Thanksgiving foods? Let’s take a look at your fat stores and gut microbiome, pitting the facts against what’s spread by your well-meaning relatives.

1) You DON’T gain much fat

Let’s look at a sample Thanksgiving meal: turkey, fixin’s, and a two-pumpkin-pie minimum. We estimate that should bring many people up to around 3,500 calories for the day, which is actually quite a bit lower than the “4,500 calories” number often thrown around by mass media.

One day of overeating isn’t likely to cause much fat gain. A pound of fat contains 3,500 calories of energy, and a typical Thanksgiving overeating day would only exceed caloric needs by about 1,000 calories. Moreover, the media claim that “you need to eat 3,500 fewer calories to lose a pound of fat” isn’t quite factual, and conversely, eating 3,500 calories above baseline won’t necessarily make you gain a pound of fat.

For example, eating a large meal in a short period of time causes a bit more of the calories to be released as heat instead of stored in the body, compared to eating normal-sized meals spread out over time. A famous study overfed 12 sets of twins for three months, with enough calories to cause a 35-pound increase in each twin. One set of twins only gained 10 pounds, others gained more, and only one set gained 30 pounds. None gained 35 pounds. So genetics (and lifestyle) can play a major role in determining how much weight you gain, at least after long-term overfeeding.

Overeating for a day, even by one or two thousand extra calories, won’t cause much fat gain. Not to mention that many overeaters won’t eat as much as usual the next day. Also, the common refrain of “3,500 extra calories leads to a pound of fat gain” is pretty much bunk.

It also matters what exactly you overfeed on. Different people have different amounts of carbohydrate they can store as glycogen, with an average around 300 to 500 grams. If your glycogen stores are low (for example, after a day or two of low-carb eating or a couple long runs), you can eat around a pound of carbohydrate, and almost all of it will be stored as glycogen, rather than as fat. Carbs are usually eaten with fat and protein though, especially on Thanksgiving, so let’s take a look at overfeeding with those macronutrients.

Protein can be very helpful for fat loss, through suppressing appetite and boosting thermogenesis. In fact, a recent study showed that eating four times the recommended intake of protein, over four months, didn’t lead to significant fat gain. Whereas carbs can be stored as glycogen, and protein can suppress appetite and boost heat production, fat does not come with any benefits for fat storage. Which makes sense, because fat is … fat. It’s not cumbersome for the body to store it right up as fat droplets. And the fourth calorie source, alcohol, isn’t going to provide any advantage either. The body will easily burn up alcohol for energy (to avoid alcohol toxicity), dampening the oxidation of fat and other fuel sources. Not only that, but alcohol can increase your appetite in the short term.

Overfeeding on protein (e.g. turkey) will cause less fat storage than overfeeding on alcohol (e.g. wine) or fat (as is plentiful in delicious pumpkin pies, not in the low-fat abominations). If you’re prone to overeating on Thanksgiving, it may be wise to load up on a bunch of turkey first, to help with appetite suppression.

2) You DO gain a lot of water weight

You wake up the day after Thanksgiving, weigh yourself, and glumly say hello to your five new pounds of bodyweight. Don’t beat yourself up, you didn’t gain five pounds of fat (see section one for details). There are a few things that can add up to extra pounds that are not body fat. Namely: pee, poop, water, and sugar stores.

Hopefully you weighed yourself after voiding your bladder, because that pee can easily weigh half a pound. And if you also voided your intestines (i.e. pooped), that can be an additional third of a pound to two-thirds of a pound.

Water and sugar stores are a little less straightforward. Blood volume, and total body water in general, can vary quite a bit due to salt intake, medications, and exercise. Carbs, when stored as the sugar in glycogen, have water molecules attached to them. This makes glycogen quite heavy. When you consider that glycogen stores can vary from up to 500 grams of sugar all the way down to 50–100 grams, making the total weight (including water) vary from several pounds down to a pound or so, you can see how easy it can be to quickly lose or gain a couple pounds.

Add in minor factors like clothing weight (a few tenths of a pound at least) and the lack of accuracy of certain bodyweight scales, and suddenly it’s possible to add 4.7 pounds of not-fat, and 0.3 pounds of fat!

Fat gain typically occurs slowly over time, in weeks or months rather than hours or even days. On the other hand, the weight of body fluids can change rapidly with salt and carbohydrate intake, exercise, and other factors. Eating a typical Thanksgiving meal can easily increase your body fluids, tricking you into thinking you’ve gained lots of fat.

3) You MIGHT adversely affect your gut microbiome

The gut microbiome has been linked to a ton of conditions, ranging from anxiety and depression to obesity. Which makes sense, considering that you are what you eat, and that behind the scenes YOUR GUT BACTERIA ALSO EATS WHAT YOU EAT.

And what you eat sits in your gut, an organ that plays a major role in almost every physiological system, from the nervous system to the immune system. For example, over 90% of your serotonin is in your gut, and gut serotonin plays a role in regulating body weight.

Providing your gut bacteria with too much food might be unhealthy for a variety of reasons. One is that excess energy could promote certain bacteria over others, reducing overall bacterial diversity, which would drive further mechanisms that promote weight gain (like reduced satiety after a meal). A rat study shed further light on the effect of binging, showing that rats who binged for part of the week and ate healthy the rest of the week still ended up with screwed-up microbiomes that were similar to those of rats who ate junk food constantly.

Lastly, eating a lot of flour- and sugar-rich food may be uniquely bad for your microbiome. These dense foods lack cells, unlike less-processed plants and animals, and provide your gut with a huge blast of carbohydrate all at once. This isn’t something human guts are used to, and it could promote SIBO (Small Intestinal Bacterial Overgrowth, a common condition where there’s too much bacteria where it shouldn’t be, too high up in the intestine), among other problems.

Overeating is bad for the microbiome, especially overeating acellular carbohydrates (flour and sugar). Thanksgiving meals can be quite rich in these acellular carbs. However, nobody knows the exact effect of a one-day binge on the microbiome. On the other hand, people likely to uncontrollably binge on Thanksgiving are more likely to binge on other days as well.
Don’t lose hope! Occasional overeating isn’t inherently unhealthy, and Thanksgiving could involve large heaps of turkey and mashed potatoes rather than three slices of pumpkin pie and two of apple pie. Again, load up on the healthy (or healthy-ish) food first, so you don’t go too crazy with the unhealthy stuff.
Some form of intermittent fasting could be a good idea for holiday periods. From fasting for 16 hours and eating during an 8-hour period each day, to the extreme of fasting completely every other day (a.k.a. alternate day fasting), many forms of fasting have shown safety and efficacy for weight loss. Plus you get to eat a really big and satisfying meal to break the fast.
It’s not necessary or healthy to starve yourself after Thanksgiving. But mixing up periods of eating more and eating less could be a good idea if you’re watching your weight, and it could promote the ability to diet flexibly. Recent research has shown that even severe caloric restriction for a day doesn’t mean your appetite hormones will go into overdrive, so don’t equate intermittent fasting with the notorious “yo yo dieting”. Remember that overeating involves both physiological factors and a large psychological component … which you may be able to chip away at with practice.

Go forth and eat deliciously, and perhaps abundantly! But be informed, and always keep your health in mind.

Published By on  - Last Updated on 2017-03-28 14:43:43