The latest health obsession: your guts.

Everyone from The New York Times to her royal Goop-ness Gwenyth Paltrow has written about the importance of good bacteria in your intestines and stomach. But what is good bacteria? What happens in a healthy gut?

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Here's what you need to know.

First of all, by good bacteria, most sources are referring to microbes, a mixture of bacteria, viruses and fungi. While that may sound gross, microbes are fundamental for a healthy body and mind.

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Microbes live not only in our guts, but in our fluids, cavities and skin. Some microbes cause disease, but the majority live peacefully within our bodies. They need us, and we need them.

Your gut and your diet

A healthy microbiome is what fights off obesity, allergies and disease. The bacteria in your gut breaks down the food in your colon making itself stronger and producing a very useful byproduct in the process.

"These byproducts are essential components of chemicals that affect mood, appetite, metabolism, inflammation and the immune system," writes The Guardian.

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A gut-friendly diet is full of many different kinds of healthy foods, including prebiotics (foods that nourish the microbiome like artichokes, leeks, and onions), probiotics (foods that contain the actual microbes, like yogurt and kefir), and synbiotics (fermented foods that contain both probiotics and prebiotics, like sauerkraut and miso).

Your gut and your physical health

Once you have a health micriobiome, your body is better able to fight against disease and conditions, even those which you are genetically predisposed to.

According to The Guardian, scientists are currently studying the connection between bacterial groups and inflammatory arthritis. Both genetics and environmental factors play a role in whether or not you will develop rheumatoid arthritis—but it's a lot easier to change your (microbiotic) environment.

Because tests for rheumatoid arthritis can have a positive effect years before any symptoms appear, changing your diet can help delay the onset of the condition.

Your gut and your mental health

Multiple sclerosis researchers have found a link between MS and the bacteria in the guts of those suffering from the disease. "The gut is well-positioned for an important role in the development of autoimmune disease, including MS.,” said neurology professor Ilana Katz Sand to Scientific American.

A person's depression levels are also believed to be affected by gut health. “We now know that good brain health depends on good gut health," says Professor John Cryan of University College Cork. "The gut microbiome affects every aspect of brain functioning and human behavior.”

Cryan tells The Guardian, “There’s no downside to recommending a diverse Mediterranean-style diet that includes lots of fibre and cuts down on emulsifiers, processed foods and artificial sweeteners.” For now, he says, it's better to get your probiotics from your food as opposed to from a pill.

A lot more study is needed on the subject before anything is anywhere near conclusive, but for the moment.

According the scientists, it looks like Gwyneth is actually onto something.