These popular foods are often claimed to benefit your gut – but what's the truth?

Probiotic supplements , including spirulina, might be helpful, but it hasn't been proven that the bacteria reach the gut intact. Some supplements have other well-established health benefits, but they tend to be expensive. Most probiotic supplements contain a limited array of microbes compared to what you can get from a good diet. Even if they do have health benefits, they are no substitute for eating a balanced diet.

Fermented foods include sauerkraut, kimchi, miso, kombucha and many pickles. We can’t be certain the bacteria they contain reach the gut, but in countries where this type of food is eaten frequently people appear to have better gut health and less bowel disease. However, other factors could be responsible. Fermented foods can be cheap and easy to make at home, so eat them if you enjoy them. Mass-produced pickles use vinegar instead of traditional methods of fermentation, so don't have the same benefits.

Raw milk . The variety of microbes found in raw milk is very similar to pasteurised milk – there’s just much more of them in raw milk. There is a strong correlation between drinking raw milk in childhood and a reduced incidence of allergies. This might be because of the high numbers of microbes in raw milk, but we can't be sure. Children who drink raw milk often live on farms, which also bring microbial advantages. However, raw or unpasteurised milk may contain harmful bacteria that can cause food poisoning.

Sourdough breads have fermented slowly using a wide range of bacteria and fungi found naturally in the air and ingredients. Commercial yeast, used in most breads, is a single strain that causes bread to rise much faster. It is not known if the additional microbes in sourdough survive cooking. One study found that the bacteria don't need to be alive to provide health benefits, but this is not conclusive. Many people claim they find sourdough easier to digest than other bread, but it is likely that the lengthy fermentation process is most beneficial. This is because microbes have had more time to break down the protein strands that might otherwise cause digestive problems.

Traditionally produced cheese can contain a huge array of probiotics (from the natural bacteria used in the production of the cheese). Some studies have found that these can benefit health, but more research is required. We cannot be sure the bacteria in some cheeses survive digestion for long enough to be beneficial. However, it is possible that other properties of cheese help preserve bacteria during digestion. Mass-manufactured cheeses don’t have this potential benefit because of the way they are made.