Why rashes appear when diet or medicine changes

Why what goes in changes what happens to skin

Stool is not a fixed substance. Its acidity and the activity of the enzymes it contains shift with diet, gut bacteria and how fast things are moving through. Two things follow:

• More acidic stool irritates skin directly. It is the reason a rash so often follows a change in diet or a bout of diarrhoea.
• Looser and more frequent stools mean more contact. Even without a change in acidity, six dirty nappies a day instead of two is a great deal more exposure.

There is usually a lag of one to three days between the change and the rash, which is why the connection so often gets missed. It is worth writing down what is new, because by the time the rash appears you will have forgotten.

Starting solids

Nappy rash frequently appears in the weeks around weaning, and it catches parents out because nothing about the nappy routine has changed.

The foods most often behind it are the acidic ones: tomatoes and tomato sauces, citrus, strawberries and other berries, pineapple, and fruit juice. Juice in particular does two things at once, adding acidity and loosening stools.

What to do:

• Introduce one new food at a time, with two or three days before the next. This is standard weaning advice for spotting allergies, and it happens to be exactly what you need to identify a rash trigger too.
• Raise the barrier cream in the week you introduce something new. Prevention is far easier than treatment here.
• Do not permanently remove a food on the strength of one rash. Most babies tolerate it fine a month or two later; the gut is still maturing. Wait a few weeks and try again.
• Speak to your doctor before removing anything significant from your baby's diet, and straight away if a food causes swelling, vomiting, hives or any breathing difficulty. That is a different problem and it is urgent.

The same applies to a formula change, which can alter stool consistency and frequency for a week or two while the gut adjusts.

Antibiotics

This is the strongest and most predictable link of the lot, and it works in two ways.

First, antibiotics loosen stools. More frequent, looser stools mean more contact, more moisture and more enzyme activity against the skin.

Second, and more importantly, they change the balance of organisms on the skin and in the gut. Candida, the yeast that lives harmlessly on skin, is not affected by antibacterial treatment, so when the bacteria that normally keep it in check are reduced it can overgrow. This is why a rash that appears during or shortly after a course of antibiotics is often a yeast rash rather than ordinary irritation.

What that looks like: deeper, beefier red than an ordinary rash, involving the skin creases rather than sparing them, often with a slightly raised border and small separate red spots on otherwise clear skin just outside the main patch.

What to do:

• Never stop a prescribed course of antibiotics because of a rash. Speak to the prescriber. Finishing the course matters.
• Change more often than usual for the length of the course and for a few days after it, including once overnight if your baby is waking anyway.
• Barrier cream at every change, thickly.
• If the rash has the pattern above, ask about an antifungal. Our guide to the types of rash describes it in detail. A barrier cream will not clear a yeast rash, and continuing with it is the most common reason these rashes last for weeks.
• If you want to ask about probiotics, ask the prescriber. Evidence in infants is mixed and it is not something to start on your own while your baby is unwell.

The other changes that do this

• Teething. Teething does not cause nappy rash directly. The extra saliva that gets swallowed can loosen stools, and it is the stool that causes the rash. Worth knowing, because "it is just teething" often means no barrier cream gets applied.
• Diarrhoea and stomach bugs. The most irritating combination there is: frequent, loose, acidic stool against skin that is getting almost no dry time. Expect a rash, and pre-empt it with a thick barrier at every change rather than waiting.
• Other medicines. Anything that loosens stools can do it, and some liquid medicines are sweetened in ways that change stool consistency.
• A change in the breastfeeding parent's diet is sometimes blamed. The evidence is much weaker than for the causes above, and it is worth ruling out everything else first before changing your own diet.
• Travel, heat and illness all change routine, hydration and how long a nappy stays on. A rash on holiday usually has a very ordinary explanation.

What to do in the week you change something

A short protective routine, used for the week around any of the changes above, prevents most of these rashes:

1. Write down what is new, with the date. You will not remember in three days.
2. Change promptly, and check more often than usual.
3. Apply a thicker barrier at every change, and more again at bedtime.
4. Add air time, a few minutes at each change.
5. Look on day three. If a rash has appeared, you already know the most likely cause and can say so at an appointment.