Whey protein side effects: diarrhoea and bloating explained

The complaints people report most after whey are bloating, gas and diarrhoea. Most come from 3 different causes, each with a different fix. This page helps you work out which one it is likely to be. If symptoms are severe or don't go away, see a doctor.

1. Lactose in whey

Whey comes from milk, so it carries some lactose, the sugar in milk, more or less depending on the type. The U.S. National Institutes of Health (NIDDK) lists the symptoms of lactose intolerance as bloating, diarrhoea, gas, nausea and stomach pain. Whey concentrate keeps more lactose than isolate. If a glass of milk does the same to you, try whey isolate and read the label. More in lactose intolerance or milk allergy: can you take whey?.

2. Sugar-alcohol sweeteners

Some products are sweetened with sorbitol, maltitol, xylitol or erythritol. A 2019 article in Canadian Family Physician explains that these have a laxative effect: sorbitol causes gas, bloating and cramps depending on the dose, from about 5–20 g a day, and more than 20 g a day can cause diarrhoea. People differ in how sensitive they are. Look for these names in the ingredients; several scoops plus sugar-free snacks add up over a day.

3. Cow's milk allergy

This is more than an upset stomach. Mayo Clinic lists milk allergy symptoms that start within minutes to hours, such as hives, wheezing and vomiting, and later ones such as loose stools that may contain blood and abdominal cramps. Milk allergy can cause anaphylaxis, a medical emergency. People allergic to milk protein must avoid all whey, isolate included; the alternative is plant protein.

Narrowing it down

Check whether it happens only with whey or with ordinary milk too; if milk does it as well, cause 1 is likely
If you mix it with milk, try water for a few days to tell the milk apart from the whey
Read the ingredients for names ending in -itol
With hives, trouble breathing or vomiting, stop and see a doctor straight away

Kidneys

People with kidney disease, or anyone told to limit protein, should ask a doctor before starting whey. Sources and details are in how many scoops of whey a day.