Gained Weight Overnight? Here Are 11 Likely Explanations

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Losing weight isn’t easy. In most cases, you have to make sacrifices with your diet, and you’re likely consistently pushing your body through rigorous workouts in order to get the scale to trend in the downward direction.

But what happens when you’re doing all the right things with your diet and exercise—or at least you think you are— and you step on the scale only to find you’ve gained weight overnight?

In most cases, overnight weight gain isn’t really indicative of a true increase in fat, but it can be extremely alarming and disheartening to seemingly have gained weight overnight. Plus, it’s quite a common phenomenon because numerous factors can make your weight go up in a day or overnight.

So, if you’re one of the many people asking, “How did I gain 5 pounds in one day?” or “How is it possible I gained weight overnight?” take a deep breath, push your panic aside, and keep reading to learn about the likely explanations for why you suddenly gained weight from one day to the next.

We will cover: 

  • Can You Gain Weight Overnight?
  • Gained Weight Overnight? Here Are 11 Likely Explanations

Let’s get started! 

A person holding their head stressed out because they gained weight overnight.

Can You Gain Weight Overnight?

Before we look into if you can gain weight overnight, let’s look at the basic question: How do you gain weight?

It seems like a simple enough question, but the answer isn’t as simple as you’d expect because the term “weight” is rather nebulous.

When people refer to “weight,” they might either be referring to their total body weight reflected on a scale, or they may be denoting their fat percentage.

For example, imagine that someone steps on a scale and it reads 180 pounds, upon which the person reports that they want to lose weight.

This could easily be achieved by using the bathroom and stepping back onto the scale. Now, the number might read 178 pounds.

Of course, the person has not lost any fat, but the total weight of the body has decreased because urine has been excreted from the body.

The converse is also true and plays a major theme in the reason you may notice you gained weight overnight.

When we talk about gaining or losing weight, total body weight can fluctuate quite dramatically, based largely on your hydration status and point in your digestive cycle.

A person stepping on a scale.

For example, if you have just worked out without hydrating well, your total body water will be decreased due to fluid losses in sweat, so your weight will be lower.

If you just ate a big dinner and have yet to move your bowels for the day, your digestive system will be loaded with food both in the stomach and intestines, significantly increasing the total weight of your body.

However, this partially-digested food is not truly part of your flesh or fat tissue, so while your weight on the scale will go up, you haven’t gained body fat.

When people talk about gaining or losing “weight,” what they are generally trying to refer to (although rather imprecisely) is gaining or losing body fat.

Body fat, called adipose tissue, is a storage form of extra energy in the body. Fat cells store triglycerides, which can then be broken down for energy during rest or exercise when you aren’t readily taking in more food.

One pound of stored body fat contains about 3,500 calories worth of energy.

Therefore, to lose one pound of fat, you have to burn 3,500 calories more than you eat, which is why real fat loss is a drawn-out process. 

It takes time to generate a significant caloric deficit. You cannot simply do one tough workout and sweat out one pound of fat; you are sweating out water weight and burning some number of calories towards losing body fat.

Similarly, to truly gain one pound of fat, you have to consume an excess of 3,500 calories over what you have burned.

A person on a scale, confused by the reading.

For this reason, when you step on the scale bewildered and aghast, asking yourself, “How did I gain 5 pounds in one day?!” rest assured that you did not gain 5 pounds of fat in a day or overnight.

Using bioenergetics, or “calorie math,” because you have to eat an extra 3,500 calories to gain a pound of fat, to gain 5 pounds overnight, you would have to eat 17,500 extra calories in one day over what you burned!

This is physically impossible. 

Even if you are tiny and have a very low basal metabolic rate (BMR), your body will burn at least 1,000 calories or so just to survive.

This means that you would have to eat 18,500 calories to gain 5 pounds in one day.

To put this into context, this would be the equivalent caloric intake of about 185 apples or bananas or 370 Oreo cookies; in short, it’s not going to happen.

Therefore, as we discuss the reasons why the scale reads that you gained weight overnight, the majority of rapid weight gain is water/food-related rather than actual fat tissue you’ve accumulated.

A variety of salty foods such as chips and crackers.

Gained Weight Overnight? Here Are 11 Likely Explanations

Here are some reasons your weight might jump up overnight:

#1: You Ate a Late Dinner

If you ate your last meal later in the day than you’re used to, or the schedule of your bowels is a little bit off your usual routine, there’s a good chance that your dinner is still kicking around partially digested in your intestines. 

This food mass will certainly reflect a higher weight on the scale, but it is not indicative that you’ve actually gained body fat.

#2: You Are Constipated

A bowel movement can be upwards of 3 pounds, depending on your diet and body size, so if you are constipated or have not defecated in the morning prior to stepping on the scale, it may look like you’ve gained weight overnight. 

Again, this is not true body fat and should be remedied after you use the toilet.

A large sandwich.

#3: You Had a Salty Meal

Sodium causes the body to retain water, so if you eat a salty meal or snack, you can be puffier and heavier overnight.

Even foods that don’t seem particularly salty, such as canned tomato sauce, salsa, soup, and condiments, can be extremely high in sodium, as can any sort of restaurant-prepared food, frozen entrée, processed meat, pizza, fast food, or Americanized ethnic food like Chinese or Indian foods.

#4: You Were Heavy-Handed With Carbs

Another contributing factor to weight gain overnight can be added water weight if you ate a lot of carbohydrates. 

Your body stores about four grams of water for every gram of glycogen you store.

Therefore, if you enjoyed carbohydrates—even healthy carbs like whole grains, legumes, sweet potatoes, and fruits—you might see an uptick in your weight.

A variety of drinks.

#5: You Enjoyed a Few Drinks

Alcohol is technically dehydrating, but the body can overcompensate by holding onto water as if it’s a limited resource.

Additionally, alcohol is metabolized before carbohydrates, fat, and protein, so it can affect your digestion and metabolism.

#6: You Ate More Fiber

Dietary fiber is excellent for your digestion, but it can cause temporary weight gain in terms of the number on the scale.

Fiber bulks up the stool, making it more massive and waterier, so if you haven’t relieved yourself yet, it will seem like you gained weight overnight.

#7: You Were Dehydrated

If you were dehydrated when you previously weighed yourself, such as after a workout or a day of insufficient fluid intake, you can certainly gain weight rapidly once you properly rehydrate your body.

Water is heavy but does not contribute to fat gain as it contains zero calories.

A person doing a Russian twist.

#8: You Did a Hard Strength Training Workout

If you did a hard strength training workout, you might be dealing with some inflammation and water weight that occurs as part of the reparative process for damaged muscle fibers.

#9: You’re Getting Your Period

The hormonal shifts that occur during a woman’s menstrual cycle can affect fluid balance and water retention. 

Right before you get your period, it’s common to hold onto extra water, which can masquerade as rapid weight gain.

#10: You’re Pregnant

Rapid or unexpected weight gain can be a sign of pregnancy.

The weight gain during the early stages of pregnancy has less to do with the actual mass or weight of the new fetus and more to do with hormonal shifts that cause fluid retention. 

If you are displaying other signs of potential pregnancy, consider taking a pregnancy test.

A variety of pills.

#11: You Started a New Medication

Certain medications, such as some antidepressants, antihistamines, and insulin, can cause weight gain, again largely due to changes in fluid balance. 

However, some medications can also affect your appetite and metabolism, so speak with your doctor if you think your medication is causing weight gain.

As can be seen, there are many reasons why the scale might creep up (or even jump up) from one day to the next.

Stick with your diet and stay positive. 

If you are looking to get on track with a healthy diet, we have some great choices for you. Check out our guide to three of the best healthy diets for runners.

A variety of healthy food including fruits veggies, eggs, and grains.
Photo of author
Amber Sayer is a Fitness, Nutrition, and Wellness Writer and Editor, as well as a NASM-Certified Nutrition Coach and UESCA-certified running, endurance nutrition, and triathlon coach. She holds two Masters Degrees—one in Exercise Science and one in Prosthetics and Orthotics. As a Certified Personal Trainer and running coach for 12 years, Amber enjoys staying active and helping others do so as well. In her free time, she likes running, cycling, cooking, and tackling any type of puzzle.

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