Can you really gain weight in a day?

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Weight gain is a funny thing – one day your clothes are fitting just fine and then bang, nothing seems to fit and a quick check in with the scales confirms you have gained multiple kilos.

How on earth can this happen and in real life terms is it actually possible to gain weight in a single day or even a weekend?

Physiology and specifically the physiology of weight gain is complex. It is not a matter of energy in vs energy out, but rather a tightly regulated balance between glucose and fat stores, total calorie intake, energy output, hormones among many other influencing variables.

Primarily the body regulates the amount of fat it burns (or stores) depending on what other fuels are available. The human body stores roughly a day or two worth of fuel in our liver and muscles in the form of glycogen so that the brain always has a backup source of energy should food energy be restricted.

At any given time human beings are running on a mix of glucose that comes from glycogen stores as well as fats. When our energy output increases, the total number of calories we are burning does also, and as such a higher proportion of both fat and carbohydrate is burnt.

When we are in a calorie excess – for example, when we have had a massive weekend of eating – once our glycogen stores are fully topped up any extra calories consumed as fats will be stored, or more specifically have less need to be burnt, acting to prevent fat loss. In the short term, as glycogen also stores a good amount of water, the scales will subsequently increase by as much as two to three kilos depending on the total amount of carbs being consumed. It is important to know this is not fat gain per say, rather extra fluid weight. This explains the bloated, heavy feeling many of us experience after a big weekend.

What then happens with your overall weight gain largely depends on what happens with your eating (and exercise) following a period of overeating. If you cut back, burn through the extra stored glycogen and start to burn fat again via more activity, no net fat gain will occur. On the other hand, if you keep overeating, especially high fat, high carb food, eventually over a few days you will start to store extra fat more efficiently. This means that striking a balance between periods of overconsumption and reduced calorie eating is a smart way to help to prevent gradual weight gain that tends to occurs when we do not adequately compensate when we have overdone things.

So ultimately no, you cannot gain weight in a day or two. The chances on the scales will simply be reflecting the extra carbs you have consumed and the fluid that accompanies them. On the other hand if you keep eating and do not let your glycogen stores deplete to some degree so you can burn body fat efficiently or, or don’t go for a big run, over time yes you will be gaining small amounts of body fat. In turn this will eventually add up to a couple of extra kilos of body fat and explain the tightness you feeling around your waistband.