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Why Do You Lose Weight When Sick? (4 Scientific Reasons)

You may have noticed you lost some weight when you were sick and you can’t figure out why.

Maybe other people have pointed it out to you and you don’t believe them. It is in fact true that you will lose weight from being sick. But why do you lose weight when sick?

Is it because of your slow metabolism, lack of appetite, or malnutrition?

In this article, we’ll look at 4 reasons why your body weight drops when you’re sick and what you should do about it.

Related Article: Want to create a Calorie Deficit for Weight Loss? Use our Smart Calorie Calculator

Why Do You Lose Weight When Sick?

The reason you lose weight from being sick is because you’re in a calorie deficit due to not eating and drinking enough fluids.

Losing weight from being sick is not a good weight loss strategy. So don’t get clever ideas or even think of trying it as your weight loss plan.

Here are the 4 reasons why you lose weight when sick.

Reason 1: You Lose Water Weight

One of the main reasons why you lose weight when you’re sick is because your body loses a lot of water due to dehydration and you not drinking as much water.

Being ill means you’ll produce more urine, sweat, and vomit which contributes to you losing weight unhealthily.

All this fluid loss can add up to quite a bit of weight loss, especially if it takes you a long time to fully recover from the sickness.

If you have a fever your body will produce sweat to help you cool down. And with that sweat, and dehydration caused by the fever, it will lead to more water loss.

Lastly, when you’re sick you’ll most likely poop out a lot more fluid and this too will make you drop body weight. Believe it or not, this is true.

Reason 2: You Lose Muscle Mass

I spoke with Online Fitness coach, Matt Johnson of Aspire Fitness and this was one of the great points he brought up.

I asked him, “why do you think people lose weight when they’re sick.

And he said “I tend to think it’s an effect of no anabolic response since no training, with a caloric deficit since people tend to eat less when they’re sick. Also, the body uses a lot of energy to heal itself, and building muscle is a lower priority at that time.”

Due to this reason as explained by Matt, the fact that your body is using all its energy to fight an infection it will try and get rid of the virus as quickly as possible to return to its normal state again.

So the body starts “eating” itself by breaking down tissues that contain protein such as muscles. And this causes some weight loss due to unintentional muscle loss.

Be rest assured though, your body is trying to save you and recover from the illness quicker so you can get back to eating regularly and building muscle instead of losing it.

Related Article: How to Lose Weight Fast (3 Steps You Don’t Know About)

Reason 3: You Have Less Appetite

This is a common occurrence with most people who are unwell.

Your stomach doesn’t feel like it can handle much food and even the thought of food becomes unappealing and makes you feel queasy.

This lack of appetite will lead to some drop in body weight. When I spoke to Sean PT Reidy of Reidyfine Fitness, this was one of the points he emphasized.

He said, “Firstly people tend to eat and drink less when they’re ill. When people are ill their body tends to produce more mucus and other fluids that tend to be pissed out and pooped out and people tend to lose weight for that reason.”

If you don’t eat at all when you’re sick, it means your body is in a faster state. Being in a faster state is when you don’t ingest any food or beverage and this is the best position for your body to heal so you recover quicker.

Lastly, in general a lack of appetite due to you feeling unwell means you don’t take in as much food and calories. And this will make you feel like you’re dropping body fat.

But it’s not actually body fat you’re losing, it’s mostly water weight.

And this leads us perfectly onto reason number four…

Reason 4: You’re Exhausted And Don’t Have The Energy To Move

This is probably one of the most obvious reasons on this list.

When you’re sick, unless it’s the sniffles or a common cold; your body is fighting off an infection and that generally means there are signs of inflammation such as fever (which will cause you to lose water weight).

But another reason why people seem to lose weight when they’re sick is because of fatigue.

When you’re sick, your body is working hard to produce the antibodies to heal itself; there isn’t much energy left over for calorie-burning activity and exercise routines.

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How Does Your Immune System Affect Weight Loss?

According to live science, losing weight helps your immune system.

When you have the flu, or a cold, one of the first things to go is your appetite.

This isn’t just because you don’t feel like eating; your body is working hard to fight off the infection and doesn’t have enough energy left over to digest food.

If you’re already overweight or obese, losing weight when you’re sick can help your immune system work better.

There are a few reasons why this is the case. When you’re carrying around extra weight, it puts stress on your body and makes it harder to fight off infections.

Being overweight also increases your risk of developing diabetes or other chronic health conditions, which can make you more susceptible to infections.

So, when you’re sick or trying to recover from an infection, it’s actually a good idea to lose some weight if you can.

This will help your immune system fight off the virus and make sure that all of your body systems are working as well as possible.

If this isn’t enough motivation for you, just remember: a healthy immune system means a healthy body, and a healthy body is more likely to lose weight and keep it off.

Lastly, one thing you might not know is that a healthy lifestyle and good habits are the best ways to keep your immune system strong.

This means eating a healthy diet, getting plenty of exercise daily, and going for regular check-ups with your doctor so that your body is in peak condition when it needs to be.

Related Article: How to Calculate your Calorie Deficit in 2023 (Easy Method)

When To See A Doctor

It’s perfectly normal to shed a few pounds when you’re sick.

How much weight you lose when sick is different for you and I. And your body weight will fluctuate a lot more due to any of the 4 reasons explained in this article.

However, if you’re losing weight rapidly without being unwell, seek professional medical advice from your doctor or general practitioner (GP).

Watch Article Video on our Youtube Channel: Why Do You Lose Weight When Sick?


While losing weight because of an illness might seem like a good idea, it doesn’t mean the weight you lost will stay off.

So getting sick should never be your weight loss strategy because it’s unhealthy and unsustainable.

And how much weight you lose when sick depends on your illness (fever, stomach flu) and how long it goes on for.

While losing body weight is not always a bad thing. But if you’re trying to gain weight, this is something you’ll want to take into account.

If you’re worried, don’t let the thought of losing weight when you’re sick bother you, it happens to everybody.

So there you have it, now you know why you lose weight when you’re sick!

Tell me in the comments – which one of the reasons have you experienced the most when you’ve been unwell?

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