Introducing cow’s milk to a baby or toddler can change their poop color and consistency.

This 1-year-old baby has yellow stools that smell bad after introducing cow’s milk. Can you relate?

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Mom’s Question:

We introduced cow’s milk to our one-year-old daughter two weeks ago. She has approximately 150ml of cow’s milk plus 400ml of toddler milk daily. She eats a balanced diet, and I have had no problems in the past.

Since this change, she has had loose (not diarrhea) yellow stools (similar to when they are very young) with a strong bad smell. She also seems very irritable. Any advice, please?

Smelly, Runny, Yellow Poop: Cow’s Milk or Something Else?

Stool Changes Due to Introducing Cow’s Milk

Since introducing cow’s milk is the only change you have made to her diet, I would definitely take it out of her diet again and see if there is an improvement.

It is possible that her loose stools, irritability, and the smell and color of her stools are all related to either lactose intolerance or milk protein allergy.

Her stool’s color is not necessarily an issue unless it is chalky pale yellow. But you describe the color as similar to breastmilk poop, which shouldn’t be a problem. Even if it is related to the cow’s milk, the color alone is probably nothing to worry about.

I haven’t found any research supporting the idea that the poop color can change to yellow when introducing cow’s milk. However, a quick search on Google makes a substantial amount of forum posts with similar questions pop up.

However, diarrhea and loose stools (for any reason) are often relatively yellow simply because they travel faster through the digestive system. So it is possible that those babies that get loose stools from the cow’s milk introduction also simply have more yellowish poop for that reason. Not because of something in the milk but because the baby is sensitive to cow’s milk, gets some degree of diarrhea, and thereby more yellowish stools. I don’t know if this could be the reason, but I would say that is a probable connection between cow’s milk introduction and yellow stools. Does any of you, parents, have lactose intolerance or cow’s milk allergy?

In any case, unless you notice other signs of illness (such as a fever or vomiting), take her off dairy products – either all dairy or start experimenting with lactose-free products. At the same time, schedule an appointment with her healthcare provider to discuss if you should test her for lactose intolerance or milk allergy or if you should simply introduce the milk more slowly.


Of course, your toddler may have loose, foul-smelling stools for reasons other than the cow’s milk introduction. Just because some things happen at the same time, they are not necessarily related.

Giardiasis is a possible cause of all your baby’s symptoms, including yellow stools. This disease is caused by a parasite. It is a very common cause of diarrhea, especially among children. In many cases, no treatment is needed, but if the symptoms persist, there is medicine available.

Giardiasis is diagnosed through a stool sample.

Fat Malabsorption

Another possible reason for yellow stools is fat malabsorption. This is when the body, for some reason, is not able to absorb nutrients (in this case, fat) from the digestive system. Yellow, foul-smelling stools can be one symptom of fat malabsorption. Malabsorption needs to be diagnosed by a doctor and the underlying causes investigated.


To conclude, I can’t say the reason for your baby’s yellow, loose foul-smelling stools. Sensitivity to cow’s milk, malabsorption, or Giardiasis are definitely options to investigate. I think you should have your child examined by a pediatrician.

I’ll include a few links about babies with similar issues below.

Good luck!

Read Next: More Babies With Cow’s Milk Issues

Find comments to this question below.

Research References

  • Azer SA, Sankararaman S. Steatorrhea. [Updated 2022 May 23]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-.
  • Marzuki, Nanis & Akib, Arwin & Boediman, I. (2016). Cow’s milk allergy in patients with diarrhea. Paediatrica Indonesiana. 44. 239. 10.14238/pi44.6.2004.239-42.
  • Charles Patrick Davis, MD, Ph.D. Giardiasis
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This Post Has 2 Comments

  1. Agnes

    My 13-month-old just had a stomach virus and green stools, and her dr said that the green is bile in the stools :( He kept her on a normal diet, just minus the cheese and lactose-free milk (they do make whole milk Lactaid) instead of regular whole milk.

  2. Anonymous

    Hi, my 1 yr old son just started drinking cow’s milk. It seemed fine at first, but I started to notice a color change in his stools and a really bad diaper rash. They are more like dark green or light green stools and are very runny.