A dietitian explains how many calories you need to eat to gain even one pound of fat. 
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We all indulge at times, and this can be part of finding a realistic balance to your healthy eating — whether your occasional indulgence is a juicy burger with fries, homemade chocolate cake, or chips and margaritas. But on holidays, we all tend to eat more than what we typically consume — plus the foods are often rich, indulgent dishes that only get made once a year.

Because of this, I’ve learned to expect an uptick in questions about eating and weight management around this time of year. The questions usually have to do with preventing weight gain or how “bad” or “good” a food is, but the most frequently asked question by far is “How much weight can you gain in a day?”

Most expect me to answer with an amount like one to three pounds, so my response usually surprises people. Overdoing it on a single day usually does not make you gain weight. In fact, it’s really hard to gain a pound of fat in one day from food — even on Thanksgiving! There are two reasons why:

Gaining a pound in a day requires eating way more calories than you think.

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I’ve written before that simply counting calories isn’t an effective weight loss approach, and my stance on that hasn’t changed. Diets of the past were often ineffective because they treated weight loss like a simple mathematical equation: You lose weight if your “calories in” are less than your “calories out” (aka, the calories you burn). While this concept is true, we now know that a lot of other factors (like hormones and genetics) influence this equation.

The basic tenant that a surplus of 3,500 calories equates to one pound of body fat is still true. To gain one pound, you would need to eat approximately 3,500 calories on top of what you already need to eat to meet your body’s needs and fuel activity — and this is more than many realize.

For example, let’s say your body requires 2,400 calories to operate and live an active lifestyle each day. To gain one pound in a day, this means you would have to consume approximately 5,900 calories (2,400 calories to meet daily needs, plus an additional 3,500 calories on top of that).

This is a lot of food — even when you’re talking about Thanksgiving. When I analyzed a large Thanksgiving meal (with ample servings of turkey, stuffing, green bean and sweet potato casseroles, gravy, multiple rolls, a slice of both pumpkin and pecan pie and two glasses of wine), the total was right around 3,100 calories. This total is still way below what equates to a pound of fat if you go back to the 5,900-calorie example above.

Weight gain occurs over a period, not in a single day.

We tend to look at calories in 24-hour increments, which can be helpful by providing a daily framework to eat within each day. However, it also gives the impression that the body stops at 11:59 p.m. and wipes the slate clean for a new day. And this really isn’t how the body — or weight gain — works.

Weight gain occurs from a gradual accumulation of excess calories over time. This period is sometimes just a few days and sometimes it’s over several months.

But most will rarely ever consume enough calories (like the 5,900 calories in the example above) to support a one-pound gain of fat in a single day. And, even if you did, fat mass isn’t added to the body in the next hour or two; it’s a metabolic process just like weight loss.

So why does the scale show you've gained weight the day after Thanksgiving?

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This increase isn’t fat accumulation. Rather, it’s due to temporary water retention and bloating, an effect that’s to be expected anytime you overdo consumption of less healthy foods (especially refined carbohydrates, added sugars, sodium and alcohol).

A better way to think about weight gain (or maybe the likelihood of gaining weight) is to look at it from a week-long perspective. Some days you consume less than what your body needs, and some days you consume more. A lot of time this equals out and weight stays stable. But when you regularly exceed your body’s needs, this excess accumulation will be stored as fat if not used.

This week-long perspective is why I said initially that overdoing it on one day such as Thanksgiving doesn’t make you gain weight. But it’s important to realize that it also explains how it’s possible for calories to accumulate and lead to weight gain if your Thanksgiving eating continues for a few days.

My advice is to avoid the scale until you’ve gotten back on track with heathy eating and hydration for a few days after indulging. Never weigh yourself the day after Thanksgiving.

What does this mean for managing weight over Thanksgiving week?

My best advice is to enjoy the holiday, including the food, but curtail the indulging to one day (or two at the most). Being active in the morning helps me create a mindset that makes it easier to do this, along with staying adequately hydrated.

Most importantly, cut yourself a little slack — even if you feel like you really overdid it. The best thing to do is to just get back on track the next day with your healthy eating habits.