Why You Might Lose Weight When You're Sick

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If you have ever had a stomach bug for more than a few hours, you may have noticed that you drop a few pounds. Not eating and vomiting everything you have eaten for the past day or so will do that. But does that weight actually stay off?

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Causes of Weight Loss

When you have gastroenteritis (more often called the stomach flu), the most common symptoms are vomiting and diarrhea. You may also experience nausea, fever, stomach cramps, and exhaustion. These symptoms can be caused by several different types of viruses, bacteria, or even parasites.

If you have diarrhea and vomiting multiple times, you lose a lot of body fluid in a short amount of time. It's also unlikely that you are able to keep any food or liquids down, so you can't replace what you have lost very easily.

This drastic change in fluid volume in your body can show up on the scale. If your symptoms last for several days, the weight change can be pretty drastic.

Duration of Weight Loss

Since most of the weight that comes off when you are sick is "water weight," it will likely come back when you are feeling better and eating and drinking again. If you're actually trying to lose weight, this is not the way to do it.

Bulimia is a serious eating disorder—and one that is generally not very effective. Even if you do not have bulimia but you think you'll knock a few pounds off if you get a stomach virus, you will likely be disappointed to find that the weight will return shortly after you recover.

This is because when you are vomiting, you aren't losing fat. Your body is trying to get rid of the virus, bacteria, or other germs that are making you sick.

Vomiting and diarrhea are the symptoms of the illness—your body's defense against the germs. It eliminates any food and liquid in your digestive tract in an attempt to kill those germs.

Once you have recovered and can return to eating as you did before, all (or at least most) of the weight will return because you are able to hold food and liquids down again.


The biggest risk and concern with the stomach flu for most people is dehydration. This is more likely to occur in young infants and children as well as older adults.

Signs of dehydration in older children and adults include:

  • Dry or sticky mouth
  • Decreased urination
  • Very dark urine
  • Headaches
  • Muscle cramps
  • Irritability or confusion
  • Dizziness or light-headedness
  • Rapid breathing
  • Fast heart rate
  • Sunken eyes
  • Loss of consciousness

Signs of dehydration in infants and young children include:

  • Decreased number of wet diapers; infants should have one at least 6 wet diapers/day
  • Few or no tears when crying
  • Lethargic, won't play or smile
  • Sunken soft spot on the head
  • Dry mouth
  • Excessively sleepy
  • Wrinkled, dry skin
  • Cool and dry hands and feet

When to Seek Medical Attention

If you are concerned about dehydration, contact your healthcare provider or seek medical attention. In some cases, dehydration can be managed at home if you or your child are able to keep down fluids.

It's important to resume eating and drinking slowly if you have been vomiting, as adding too much too quickly can cause vomiting to return or worsen. If you are unable to hold any amount of fluids down, contact your healthcare provider.

There are medications you may be able to take to stop vomiting. If dehydration is severe enough, you may need IV fluids to recover.

If you are ever with someone who appears to be dehydrated and loses consciousness, seek medical help immediately. Do not try to give an unconscious person anything to drink.

If you have symptoms of a stomach bug that last longer than a week (or persistent vomiting for more than 24 hours), contact your healthcare provider. Most gastroenteritis goes away on its own after a day or two. Diarrhea can last longer but it should be improving over time. In some cases, there could be another cause of your symptoms. You may need additional tests or treatments.


There is no treatment when you get most stomach bugs other than waiting for it to go away. Because they are most often caused by viruses, antibiotics won't work. Even most stomach illnesses that are caused by bacteria, such as salmonella, go away on their own and typically aren't treated with antibiotics.

The best thing you can do is to avoid getting it in the first place. It isn't always easy, especially if someone else in your house has it, but there are steps you can take to try to limit the spread of those germs.

Washing your hands is essential. Wash them as frequently as you can:

  • Before and after you prepare food
  • Before and after you eat
  • After you use the bathroom
  • After you change a diaper
  • After you touch anything that a sick friend or family member has touched

A Word From Verywell

This article does not cover weight loss from intentional vomiting due to eating disorders or weight loss due to chronic or serious illness. These are serious issues and shouldn't be taken lightly.

If you think you may be struggling with an eating disorder, please talk to someone and seek medical attention. If you need information about an eating disorder because you are concerned about yourself or someone else, there are plenty of valuable resources available online and through the medical community.

Losing weight is difficult for many people but there is no quick fix that is truly safe and effective. If you are sick with a stomach bug, focus on taking care of yourself and recovering. The numbers on the scale are not important when your health is on the line.

5 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  2. Santonicola A, Gagliardi M, Guarino MPL, Siniscalchi M, Ciacci C, Iovino P. Eating disorders and gastrointestinal diseases. Nutrients. 2019;11(12). doi:10.3390/nu11123038

  3. Shah MP, Hall AJ. Norovirus illnesses in children and adolescentsInfect Dis Clin North Am. 2018;32(1):103–118.

  4. Churgay CA, Aftab Z. Gastroenteritis in children: part I. diagnosis. American Family Physician. 2012;85(11):1059-1062.

  5. Zollner-Schwetz I, Krause R. Therapy of acute gastroenteritis: role of antibiotics. Clin Microbiol Infect. 2015;21(8):744-9. doi:10.1016/j.cmi.2015.03.002

Additional Reading

By Kristina Duda, RN
Kristina Duda, BSN, RN, CPN, has been working in healthcare since 2002. She specializes in pediatrics and disease and infection prevention.