system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: Why are some fermentations considered healthy (kimchi, kombucha etc) and others are not?
gpt
Firstly, why is fermented food healthy. Food is fermented by tiny creatures (bacteria, fungi and yeast). These creatures need vitamins and nutrients to be healthy like us. Some of these creatures can create vitamins which people can only get via food. Therefore fermented foods with these creatures still present tend to be more healthy.

These creatures also need energy to do their work. They extract this energy from the food. This causes the food to contain fewer calories than the original food. This is why yogurt and buttermilk have less sugar than regular milk.

But why are some fermented foods unhealthy? In some fermented foods and drinks, the healthy nutrients are overpowered by unhealthy stuff like alcohol. These substances are bad for you in large amounts. Also, some of these bad things damage your body when they are broken down. Alcohol gets broken down to a thing called 'acetaldehyde' which can cause cancer.

Some creatures may also use more healthy vitamins and nutrients than they produce, thus reducing the healthiness of the food.

Whether a food or drink is healthy depends on which creatures are used in the fermentation and how many healthy substances are produced as opposed to unhealthy substances. Foods with a lot of alcohol tend to be pretty bad, but in the end most fermentation is done for an improved taste and for preserving foods without a fridge.