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

“Thankgiving is a holiday that celebrates the ability of Americans to eat massive quantities of food.”

That may not be the official definition, but come on, that’s true, isn’t it? And most of us are fine with that. Or would be fine with that, if not for the several pounds of fat we put on over just a couple of days. How is that even possible?!

Answer: It isn’t. A two-day binge can have a profound effect on your gut microbiota, as we’ll see, but not so much on your fat stores.

Weight Gain vs. Fat Gain

In one study, college students gained just over a pound over Thanksgiving break (a couple of weeks). Of course, by itself, one pound isn’t much to worry about, but you’ll undoubtedly find someone to point out that if you ate like that all the time, you’d gain more than 28 pounds in a year!

Or would you? A three-month study fed 12 sets of twins enough calories to cause a 35-pound increase in each twin. Within a pair, twins gained similar weight. However, between pairs, variability was high. The pair who gained the least gained 10 pounds; the pair who gained the most gained 30 pounds; no pair gained the predicted 35 pounds. So it appears that genetics (and lifestyle) play a major role in determining how much weight you gain, at least after long-term overfeeding.

Of course, a two-day binge doesn’t qualify as “long-term overfeeding,” and the results of short-term overfeeding are even less predictable. For instance, the aforementioned students who gained just over a pound over Thanksgiving break didn’t have to widen their belt a notch: their waist size decreased by nearly half an inch on average—which is good news for them since waist size is a better predictor of health problems than BMI.

Good news for them, puzzling news for us. After all, there are 9 calories per gram of fat, so if, on a given day, we eat 3,500 calories when we only needed 2,500, the 1,000 extra calories should morph into 111 g (about a quarter of a pound) of fat.

But it doesn’t work that way. There’s a limit to the amount of calories your body can process into fat stores in a given amount of time. If you eat beyond that limit, some of the extra calories will be burned off as heat and some will end in the crapper. Weighting yourself before and after visiting the toilets after a Thanksgiving meal can be an illuminating experience. The average morning pee weighs half a pound. The average poop weighs a third of a pound. After a Thanksgiving meal, you can easily beat those numbers, and you’ll go more frequently too.

And then there’s water. Blood volume, and total body water in general, can vary quite a bit with exercise, medications, salt intake, and carb intake. To store the carbs you ingest, your body must transform them into glycogen, a polysaccharide of glucose, then attach the glycogen molecules to water molecules: 3–4 grams of water per gram of glycogen. Considering that an adult weighting 70 kg can store around 100 g of glycogen in the liver and 400 g in the muscles, it follows that if you eat enough carbs to fully fill your glycogen stores, you’ll be carrying an extra 1.5–2 kg (3.3–4.4 pounds) in water weight. Incidentally, that’s why “detox diets” appear to be so effective: they make you lose a lot of (water) weight in a short amount of time.

Thanksgiving means eating a lot … and gaining a lot of weight; but little of that weight is fat. Most of it is water and soon-to-be-poop. 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.)

Eating a lot of carbs means storing a lot of water. Obviously, what you eat matters. 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 carbs and almost all of it will be stored as glycogen, not as fat. Carbs are usually eaten with fat and protein, though, especially on Thanksgiving, so let’s take a look at what happens when you overeat those macronutrients.

Protein can support fat loss, notably by suppressing appetite and boosting thermogenesis. In fact, a four-month study showed that eating four times the recommended intake of protein didn’t lead to significant fat gain. As for fat, it does't have any benefits with regard to fat storage, but it still beats alcohol with regard to fat gain, not because your body will store alcohol as fat, but because your body readily burns alcohol for energy (to avoid toxicity), thus 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.

Whatever your favorite foods are, however, you won’t gain much fat from overeating for a day. But what does the evidence say about overeating over the longer holiday season? A study in college students found that their body fat percentages jumped by about 1% from before Thanksgiving to after New Year’s Day. This finding is consistent with what we’ve been saying, which is that significant fat gain is something that occurs over weeks to months, not over a day.

Don’t take that to mean that fat gain is inevitable during the holidays, either! Most participants in a six-week study thought they had gained weight over the holidays … yet most hadn’t. So just feeling that you’ve packed on the pounds doesn’t mean you actually have!

Well, at least you can trust your scale. Except, that is, when you can’t. Not only are scales not always accurate, but even when they are, they can lie by telling the truth, since, between the variatiations in water stores we’ve mentioned, the current stage of your digestion, and minor factors such as clothing weight (a few tenths of a pound at least), your scale’s verdict can vary quite a bit even within a day.

Significant body fat is gained in weeks or months, not in hours or even in days. On the other hand, water weight can vary 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. Even over the holiday season, however, not everyone gains fat, and feeling like you’ve gained weight doesn’t always mean you have.

The Gut Factor

The gut 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. Moreover, the gut microbiota has been linked to a ton of conditions, ranging from obesity to anxiety and depression.

What you eat is also what you feed your gut bacteria. If you eat too much, the excess energy could promote certain bacteria over others, reducing overall bacterial diversity, which could drive further mechanisms that promote weight gain (such as a reduction in postprandial satiety).

And of course, here again, your choice of food matters. If you eat a lot of flour- and sugar-rich foods — dense foods that lack cells, unlike less-processed plants and animals — you bomb your gut with a huge blast of carbs. This isn’t something human guts are used to, and it could promote SIBO (Small Intestinal Bacterial Overgrowth, a common condition characterized by your having too much bacteria where it shouldn’t be, too high up in your intestine), among other problems.

Rat experiments show that hypercaloric high-fat diets aren’t great for the gut microbiota, either: in a matter of days, such diets cause changes in the gut microbiota that cause changes in parts of the brain that regulate satiety. The result? Fat gains.

Even worse: rats who eat junk food for part of the week and eat healthy the rest of the week can still end up up with screwed-up microbiotas similar to those of the rats who eat junk food constantly. So while, in itself, a single-day binge isn’t likely to affect your gut microbiota on the long term, a few binges a week could — even if you aren’t a rat.

If you overeat, your gut microbiota is going to change for the worse, especially if you binge on acellular carbohydrates (flour and sugar). Thanksgiving meals can be quite rich in those acellular carbs. Hypercaloric high-fat diets too affect the gut microbiota, causing changes in the brain areas that regulate satiety. Nobody knows the exact effect of a one-day binge, but people who tend to binge uncontrollably on Thanksgiving are more likely to binge on other days as well, and that’s where the real danger lays.

So What Should You Do?

Celebrate Thanksgiving, not with a rich, delicious meal, but with six to twelve glasses of lemonade with maple syrup and cayenne pepper. (No, please, don’t!)

As it stands, 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.

Finally, some form of intermittent fasting could be a good idea during the holiday season. It is neither necessary nor healthy to starve yourself after Thanksgiving, but alternating periods of eating more and periods of eating less could be a good idea if you’re watching your weight, and it could develop in you the ability to diet flexibly.

Recent research has shown that one day isn’t enough for even severe caloric restriction to cause your appetite hormones to go into overdrive, so don’t equate intermittent fasting with the notorious “yo-yo dieting”. Remember that overeating involves not just physiological factors but also a large psychological component — which you may be able to chip away at with practice.

From fasting for 16 hours each day to fasting completely every other day, many forms of intermittent fasting have shown safety and efficacy for weight loss. Plus, you get to eat a really big and satisfying meal to break the fast!

Now go forth and eat deliciously, and perhaps abundantly! But stay informed, and always keep your health in mind.