Have you ever gained (or lost) a lot of weight overnight? You wake up one morning, weigh yourself and... gasp! ... you're suddenly 1 - 5 pounds heavier (or lighter). Or, say you're doing well and losing weight, but suddenly over a short time (like a few days to a week) you've gained several pounds in spite of eating exactly the same?

What's going on? The answer is that your body has suddenly gained or lost a bunch of water.

How do we know a sudden large gain is not fat gain? Consider these two facts:

One pound of actual body fat is equal to 3500 calories
Except in unusual circumstances, only dietary fat can be stored as body fat.

So to gain even just ONE pound of pure fat in one day, you would have to eat 3500 calories of fat in excess of your total daily expenditure. Depending on your maintenance level calories, that may mean consuming a total of 5000 - 6000 total calories in one day, with 3500 of it being fat calories. To gain 2 pounds of actual fat, you'd need to eat on the order of 8500 - 9000 calories with 7000 calories of that as fat. Not very likely!

Okay, so some of that could be fat, if you know for a fact you overate and you ate enough dietary fat. Let's say you ate a lot of fatty foods and ended up with 1000 total excess calories which is a lot but quite doable. In that case you could end up with maybe 1/3 pound of actual body fat. So, reality is that your body fat stores can't change significantly overnight.

The primary (or maybe only) reason for big changes in weight is related to water fluctuations.

This is because water is heavy and your body is 70% water. Just 2 cups of water weighs 1 pound, so even very little water fluctuations will show up on the scale.

Water levels change easily and frequently due to a number of things including:

carbohydrate stores
salt intake
less sleep
more stress (rising cortisol levels)
bowel movements
fat loss itself
menstruation.
change in medication
drinking less water

Carbohydrates and Glycogen

Your body breaks down ingested carbohydrates into glucose. If there is excess glucose beyond immediate needs, it stores it in glucose chains called glycogen, primarily in the liver and muscles. In short, our bodies store excess carbohydrates as glycogen.

Here's what's interesting: The body requires 3 - 4 grams of water to store 1 gram of glycogen. That means roughly 130 grams of stored glycogen equals 1 pound of weight.

Stated another way: 1 gram of carbs is roughly 4 calories, so storing 130 grams of glycogen equals about 520 calories.

So if you were to eat around 520 excess calories one day (above maintenance calories) as easily digestible carbohydrate, your body weight could increase by 1 pound as a result of glycogen stores.

Just to be clear -- this weight gain is in the form of glycogen stores. It's not fat. And it can only happen if you are eating in excess of your maintenance level calories (eg, more than your body's energy needs).

This has obvious effects with low-carb dieting:

As soon as you cut or drastically reduce carbohydrates from your diet, your body will start flushing that extra water from your system and you will notice it right away. In the first week of a strict low-carb diet, you could easily lose 2 - 6 pounds of water weight, and a bit more in the week after that.
Conversely, as soon as you begin eating carbs after a strict low-carb diet is over, you will regain the same water weight - which could mean gaining 2 - 6 pounds in the first week if you are.

Another factor is glycogen depletion with exercise. As you exercise, your body is burning off glycogen as well as fat. Each gram of glycogen that is burned off through exercise means a potential loss of 3.5 grams of now unnecessary water. After 90 minutes of steady exercise, assuming no carb intake, glycogen stores may be depleted -- representing around 2.5 pounds of water weight loss.

Other Water-Related Weight Changes

Hydration levels. Simply drinking more or less water or sweating (heat, exercise) can affect water levels. Remember a mere 2 cups of water affects the scale by 1 pound.

Consuming extra salt causes your body to retain more water to maintain the proper balance of salt in your body. Eating a large amount of salt one day may cause your weight to appear higher the next day than it would otherwise. Salt in proper amounts isn't bad; in fact your body needs sodium (and potassium) especially on strict diets. Just realize that sudden fluctuations in salt intake will naturally cause your body to gain or lose water.

Women can experience extra water retention as they approach menstruation. For some women, the extra water can cause their weight to rise by as much as one-half to one pound per day over the course of about ten days. So even if you were losing a pound a day before that, it could easily appear that your weight loss was suddenly slowing to just half a pound a day or even completely stalling. If you were only losing half a pound a day, it could appear as if you were gaining weight. Luckily, you are still burning fat, and the extra water quickly leaves as menstruation begins.

The contents of your stomach and intestines can hold a considerable amount weight. Much of this is due to liquid (water) content which means it weighs a lot. A bowel movement can mean 1-2 pounds (or more). On a lower calorie diet your bowel movements may come less often due to the decrease in incoming food. As waste slowly builds up over the course of a few days, it affects your weight by as much as a pound or more. You may notice a significant drop in weight the day after your bowel movement, followed by a few days of slower weight loss.

As your body breaks down fat tissue, it will often "fill the gaps" temporarily with water, making it look like you aren't losing any weight for several days in a row. Then suddenly the water gets flushed out and you drop a pound or more in one day. This kind of water retention is more pronounced in women, especially during their menstrual cycle. Men often lose in a more steady fashion.

Stress raises the level of the stress hormone cortisol. One effect of cortisol is water retention. So worrying and stressing out about your weight loss, or lack thereof, will only make things worse. You'll still burn fat, but it will be harder to tell. Cortisol-mediated water retention can change when calories or diet changes.

Best Course of Action

What should you do when your weight changes without any explanation, and it just doesn't make any sense? Or if you've been doing well and nothing has significantly changed in your routine, yet your weight isn't changing or is even gradually rising?

More often than not you should just wait it out and things will eventually turn around. Because it's probably caused by increased water retention. And because gaining fat overnight or even 1 - 2 pounds in a week isn't likely to happen unless you have really gone off track. If you've been eating in a reasonably controlled way, any weight gain is likely water gain.

Try to remove your emotions from the situation. Try to be patient. Keep doing what you're doing for another week or two before taking drastic action.

Side note

There is a water-weight-gain condition called edema that can cause swelling, typically in the legs. Edema may be caused by a high-sodium diet, sitting or standing too long, hormones, medications, or other medical issues. If it occurs, ask your doctor about it. The kind of water weight fluctuations we are referring to on this page don't typically cause puffiness or swelling but rather the more invisible water fluctuations.