10 Reasons Your Body Weight Increased Overnight

There you are, standing in your kitchen, glass of Pinot in hand, talking to a few of your friends about the latest TMZ blunder. Your husband is in the living room with a few of his friends watching football, eating wings, and spilling beer all over the brand new Polo you bought him for his birthday *sigh*. What a wonderful Sunday night it is. Once the crowd is gone, you eat the last slice of cold pizza (because really, who doesn’t like cold pizza?), put on your pajamas, brush your teeth with your eyes half shut, and crawl into bed. It’s 1am and the alarm is set for 6am. Time to get some shut eye.

The alarm rings LOUD for the third time this morning; oh how handy the snooze button has become. You crawl out of bed, messy hair, morning breath and all, and stumble into the bathroom to get going with your morning routine. First up? Get on the scale! The numbers blink, you take a double take, gasp for air, and step off upset you’ve undone all your hard work for the past 5 weeks. Three pounds in one day?! What the heck!?

Sound familiar? Is this more common in your life than you’d like to admit? Well, that’s okay. Enjoying time, food, and a little gossip with your friends isn’t a crime, just like the weight going up on the scale doesn’t necessarily mean that you’ve gotten any fatter!

Maybe that doesn’t sound familiar to you. You might be the type of person to never slip up on your diet even for social events, holidays, and traveling. But sometimes that number still moves in the wrong direction. Again, this does. not. mean. you’ve gained actual fat.


You heard me! Seeing an increase in weight on the scale doesn’t mean you’ve necessarily gained fat. Weight gain does not equal fat gain. It most certainly could, but in this situation and many others like it, there are more likely reasons why the scale went up that have nothing to do with fat gain.

Let’s assess.

In your half-drunken stooper, laughing with friends until 12:30 am and indulging in an extra slice of pizza, a garlic knot, and a few too many wings, there was not much time between guzzling down some delicious food and falling asleep like a baby does to a bottle of warm milk. Sleep down-regulates metabolism and slows digestion. This makes sense, as there’s not much of a need for energy while you’re passed out dreaming about laying on the beach in the Bermuda. But this also means that food passes through the intestines much slower than when you’re awake and moving around. When it’s time to get up in the morning and weigh yourself on the scale, the large meal you already wish you didn’t have is still in your digestive tract.

Reason # 1: The scale went up compared to the day before because it’s also weighing the giant meal you had before bed!

This very smoothly segues into my next point. The scale in the morning could also be higher than the day before because you haven’t pooped yet. Sorry to get a little crude (but not really). Even if you haven’t had a very large meal before bed, the intestines hold on to quite a few pounds of material (including an entire ecosystem of living bacteria), and if nothing has been excreted yet for the day (or maybe the day before as well…), the scale will read heavier. In particular, foods high in fiber add bulk to the intestines, as well as a good amount of water content in order to keep things moving along.

Reason # 2: If you’re all of a sudden heavier in the morning, it may be because you just need to take a poop!

Side note: at PRS we recommend all our nutrition coaching clients weigh themselves before  their morning poop so as not to produce “poop-scale” anxiety. If you always weigh yourself before you poop, your morning weight *should* be more consistent.

Continuing with the theme of food as the culprit for your sudden weight gain, foods high in carbohydrates are also a common cause for sudden weight gain. I know what you’re thinking, your suspicions of carbohydrates being evil have now been confirmed and you’re doomed to a life of lettuce and cucumbers in order to lose weight. Not so fast my friend!

Reason # 3: The weight gain that occurs after a high carb meal has nothing to do with storing carbs as fat. Instead, when a surplus of carbohydrates are eaten they are stored in the liver and in muscles as glycogen. For every gram of carbohydrate stored, there is also about three grams of water stored as well. This excess water retention is completely normal and a necessary function of cells.

What this means for you is an increase in weight on the scale that may be unexpected. Most often this occurs after a very large meal like a plate of pasta with meatballs, or in this instance, pizza, garlic knots and wings. Another scenario where this is likely to occur is when you switch from a lower to higher carbohydrate diet. For example: if on average you consume 150 grams of carbs but then you’re at a halloween party and happen to eat 3 handfuls of candy corn (holler to the small population of people who lurrvvvvvv candy corn!) then you’ve just taken in about 150 MORE grams of carbs than usual. This doesn’t all turn into fat overnight but it DOES hold on to extra water.

There is one more component to food that may be the cause of your rapid, unexpected weight gain over the course of a day or two; Sodium. Anyone who has had a large serving of General Tso's Chicken and a side of fried rice, or a medium movie theater popcorn, has experienced this water retaining effect, typically seen as swelling in your hands and feet or bloating.

Reason # 4:
Sodium is one of the main electrolytes in the human body; a water soluble mineral that helps to maintain a proper electrochemical gradient and normal fluid balance. If there is too much salt in relation to water or vice versa the body must do something to restore equilibrium in your body. When too much salt is eaten, the body increases its water content by holding onto, or “retaining,” more water from food and drinks than usual. The effect is a body that is heavier, sometimes substantially heavier, than usual.

There are so many reasons why you may be a few pounds heavier some mornings than others that are not at all related to food that don’t mean you’ve gained fat.

We all know exercise is good for us. Actually, it may be one of the best things for us. And despite the fact that exercise is most commonly used as a means of losing weight, it can actually cause a transient increase in weight in the acute period post-exercise, especially after resistance training.

I know what you’re thinking: exercise is now the devil and if you wanna lose weight you should just sit around and eat lettuce all day. Not really though! It is completely normal for you to weigh more after a hard bout of resistance training, like a barbell training session or an F45 class, and it comes down to three things.

Reason # 5: Exercise increases blood flow to the muscles being worked. Resistance training in particular is responsible for an effect typically labeled “the pump.” This pump is a natural response to training and is actually beneficial for many reasons (most of which are outside the scope of this article). The cause of the pump is a rapid rushing of blood to the working muscles in an effort to deliver important nutrients as well shuttle out unwanted, and potentially harmful, waste products. The swelling that occurs may also be a mechanism that drives muscle growth. What is important for you to know is that “the pump” can and will last for hours and sometimes days, to some degree, after the session is over, and the increase in fluid volume in muscles will cause you to be heavier when stepping on the scale.

Reason # 6: Exercise increases the production and release of cortisol, the body’s main stress hormone. This is also a natural response to training and is a necessary part of the stress-recovery-adaptation cycle. Cortisol has a profound effect on the body, interacting with almost every type of tissue in some way or another. One of its major effects is water retention. As we’ve learned previously, and a common theme throughout this article, is that this water retaining effect will increase scale weight.

Reason # 7: Exercise induces an inflammatory response as part of the repair process. Microtrauma caused by resistance training, as well as ballistic exercises like running or plyometrics, are necessary to repair in order to recover. Again, with inflammation comes, you guessed it, water retention. Go figure!

By this time you have probably figured out that many of the reasons why scale weight fluctuates day-to-day is due to a fluctuation in body water content. You’ve probably also realized that there are many different reasons why body water content can fluctuate, and in fact there are still more to talk about!

Going back to cortisol, there are two other things that cause elevations in this hormone that will also have a fairly large impact on water fluctuation:

  1. Lack of sleep (Reason # 8)

  2. A high amount of general life stress (Reason # 9)

Both of these factors are fairly prevalent in most people’s lives, so remember that the next time the scale is something you’d rather throw out of your third story window instead of look at.

There is one more key factor that will affect day-to-day weight fluctuations, but it is only a factor for women: Menses.

Reason # 10: The premenstrual water retention that occurs before the menstrual phase of your cycle can cause quite a significant amount of water weight gain, amounting as high as a few pounds in some women. This water is then lost during menstruation. So, if you notice that you weigh the heaviest around the same time each month, this may very well be why!

In our example from the beginning of this article we had a middle aged female who was hopped up on alcohol, pizza, wings, laughter, and a lack of sleep. This concoction, as you now know, can be a recipe for short term weight gain that had nothing to do with gaining fat.

All too often unnecessary stress is caused by a sudden increase in scale weight, usually accompanied by an “oh shit!” moment. But seeing now that weight fluctuation can be caused by a shift in fluid balance, you can rest easy knowing that you probably didn’t gain a spare tire in a day if all other factors surrounding diet, exercise, and health are consistent and in check.

To drive this point home one last time, it may also be beneficial to explain that physiologically speaking, gaining a substantial amount of fat in one day is not physically possible. The caloric surplus necessary to gain one whole pound of fat in a day is tremendous and not attainable in one day by most people. In a week? That’s a different story, one that we’ll save for another time.