Why the scale drops
Diarrhea moves food and fluid through the gut faster, so the body absorbs less water and loses more. The scale drops because you're short on water, not because you burned fat.
Once you're eating and drinking normally again, that weight comes back within days. That's a good thing: it means you're rehydrated.
Here's the math. A pint of water weighs about 1 lb. If you weigh 160 lb and lose 3 lb during 2 days of diarrhea, that's roughly 3 pints, or about 6 cups, of fluid. By a rough rule of thumb, losing 3 lb of fat would take a deficit of about 10,000 calories, far more than a short illness creates.
Signs of dehydration
- Thirst, dry mouth and dark yellow urine, or urinating less
- Dizziness or lightheadedness, especially when standing
- Unusual tiredness or a fast heartbeat
Sip water through the day and add an oral rehydration drink if you're losing a lot. Your daily needs are in the water intake calculator.
Most healthy adults can rehydrate with water, broth, fruit juice or sports drinks. Oral rehydration solutions from the pharmacy have a better balance of salt and sugar, and they're especially useful for children and anyone losing a lot of fluid.
A simple check: urine that's pale yellow usually means you're getting enough fluid.
What to eat while recovering
- Fluids first: water, broth and oral rehydration drinks, sipped often
- Small, simple meals: rice, bananas, toast, crackers, eggs or plain chicken as you feel ready
- Go easy on: alcohol, caffeine, very fatty or spicy foods, and lots of sugar or sugar alcohols
- Dairy: some people tolerate yogurt better than milk for a few days
Common causes
Most short bouts come from viruses or food poisoning. Medicines can play a part too, including antibiotics, metformin and GLP-1 medicines. Lactose intolerance and other digestive conditions cause ongoing symptoms in some people. If diarrhea keeps coming back, a clinician can help find the cause.
Most people can go back to their usual foods within a few days, as stools firm up and appetite returns.
Probiotics may shorten some bouts slightly, but the evidence is mixed, so they're optional rather than essential.
On a GLP-1 medicine
Diarrhea is a common side effect of GLP-1 medicines, especially after a dose increase. Dehydration can strain the kidneys, so it matters. See diarrhea on GLP-1 medicines and staying hydrated on a GLP-1.
For many people it settles within a few days of a dose change. Smaller meals, eating slowly and going easy on greasy foods may help. Keep a note of when it happens, so your prescriber can decide whether your dose schedule needs a look.
Vomiting along with diarrhea, or severe belly pain that won't go away, needs prompt care.
Why it isn't a weight-loss method
Some people notice the lower number and wonder if it's a shortcut. It isn't. The weight returns as soon as you rehydrate, and repeated fluid loss can throw off your sodium and potassium levels and strain your kidneys.
Laxatives and "detox" teas work the same way: they move water, not fat. Our post on laxatives and weight loss explains the risks. If you've felt pulled toward using them to control your weight, you're not alone, and a clinician can help without judgment.
For lasting fat loss, a modest calorie deficit works with your body instead of against it.
When diarrhea keeps coming back
Diarrhea that lasts 4 weeks or longer is called chronic. With chronic diarrhea, weight loss can be more than water, because the gut may not absorb nutrients well. Common causes include:
- Celiac disease
- Inflammatory bowel disease, such as Crohn's disease or ulcerative colitis
- Irritable bowel syndrome
- Food intolerances, such as lactose intolerance
- Some medicines
A clinician can sort these out with a few questions and tests. A simple log of symptoms, foods and medicines makes that visit faster.
When to get care
- Diarrhea lasting more than 2 days (adults)
- Signs of dehydration that don't improve with fluids, or you can't keep fluids down
- Blood or black, tarry stools
- Severe belly pain, or a high fever
- Diarrhea in a baby or young child, older adult or someone with a weak immune system
If you're not sure whether it's serious, call your clinician's office or a nurse line. It's always okay to ask.
Run your numbers
Keep reading
Common questions
Is weight lost from diarrhea fat?
No. It's mostly water and gut contents, and it returns once you rehydrate.
How much weight can you lose from diarrhea?
A few pounds over a couple of days is common, almost all fluid.
When is diarrhea serious?
If it lasts more than 2 days in adults, comes with blood, high fever, severe pain or dehydration, get medical care.