Can you gain weight in one day?

Substantial weight gain that is visibly noticeable by friends and family happens gradually, over weeks or months. It is almost impossible for you to gain significant amounts of weight overnight. Keep in mind that we are talking about weight in the form of stored fat and not water weight, which we will discuss later on in this article.

In order to function, your body requires a certain amount of calories to be consumed everyday. A calorie is a unit of energy and all foods have different amounts of calories. Fruits and vegetables tend to be low in calories as they are mainly made up of water whereas nuts and seeds are high in calories because they are mainly made up of fat.

You can calculate the number of calories you should be consuming using the calorie calculator here. So for example, a lady who is 40 years old, weighs 60 kg, is 160 cm tall and exercises 3 – 5 days a week will need to consume around 1920 calories to maintain her current weight.

If she consumed more than 1920 calories, she would theoretically start to gain weight. But here’s the thing – she would need to eat a considerable amount of food to gain that weight. In order to gain a pound of fat, she would need to eat a large number of calories on top of her 1920 recommended amount.

This is not an easy task. Take a look at our article about how to eat 3000 calories. You will quickly realise that the sheer amount of food is way too much for most people to consume, on top of what they usually eat.

Continuing to eat over your calorie allowance every day for weeks or months will eventually lead to noticeable weight gain. But gaining a significant amount of weight in a day is almost impossible.

If that’s the case, you might be asking yourself, why am I seeing such a noticeable difference when I climb on the weighing scale today? There are a few factors that can affect the number that you see on the scale.

Water retention

Your body contains a lot of water, more than 60%, however this amount is constantly fluctuating. When you are dehydrated, your body clings on to its current water supplies for emergency. Dehydration can be a result of consuming too much salt or sugar, not drinking enough water, drinking alcohol or menstruation, to name a few.

Excess sodium consumption forces the body to retain water. The amount of sodium you need to consume varies from person to person depending on age, gender and current health. However it is recommended that you limit sodium to less than 2300 mg a day. This is equivalent to around 1 teaspoon of table salt. Most people tend to get much more than this amount, sometimes without even realising it. Many foods, even though they don’t taste salty, can be high in sodium. This is generally the case with processed and junk foods.

Alcohol is a diuretic, causing you to use the bathroom much more often than you usually would. You end up losing a lot of water when you urinate and this in turn leads to dehydration. This is why it is important to limit the amount of alcohol you drink and to also make sure you drink enough water at the same time.

Women can also retain large amounts of water before menstruation because of hormone changes. This can cause various parts of the body to swell up.

It may seem strange but the best way to treat water retention is to drink enough water and make sure you are sufficiently hydrated. Drinking adequate amounts of water will help flush out sodium and excess fluids from your system.

Food you have eaten

Everything that you eat has mass. Weighing yourself at different times of the day can result in changes in the numbers that you see. For example if you weigh yourself after waking up and having a bowel movement on one day and then weigh yourself in the evening on the following day, it will seem like you have gained weight. This is why it is better to weigh yourself at approximately the same time everyday, after you have had a bowel movement.

What you eat can also make a difference. Your body stores glycogen, a source of energy, in your liver and muscles. Cutting back on carbohydrates causes these stores to be used up, causing you to lose weight. However the opposite is also true – when you start eating carbohydrates again, your glycogen stores are replenished and this can lead to weight gain. Glycogen carries water with it, so this depletion and replenishment of stores can lead to large changes on the weighing scale. Again, it is important to remember that this weight gain is not in the form of fat.


Certain medication can lead to rapid weight gain. If you have recently started taking a new type of medication, speak to your doctor and let them know your concerns.

Problems with monitoring weight daily

Your body is constantly changing, which is why you should not take too much notice if you have gained any weight within a day. A better option would be to monitor progress once a week; you can do this by weighing yourself at the same time each day, after you have visited the bathroom, and then calculating your average weight at the end of the week. This helps to balance out discrepancies.

Also, a lot of people tend to rely on weighing scales alone, in order to determine whether they have lost or gained weight. However this isn’t always the best approach because your weighing scale cannot differentiate between water weight, fat weight and muscle weight, all of which have mass.

There are some modern digital scales that can measure changes in fat and muscle mass, however they should not be the only thing you rely on as they are not always accurate. You should also use a measuring tape and calipers to determine how much fat you have lost or gained.


It is very difficult to gain large amounts of weight in a day and pretty much impossible for most people. Significant weight gain takes time. If you have noticed a drastic change whilst stepping on the weighing scale, this is most likely water weight and not fat. Think about any dietary changes you may have made recently that could have caused you to retain water.

Always speak to your doctor if sudden changes in your body don’t go away.

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