The gut bacteria in people eating lots of fiber can provide up to 12% of their daily energy, while those on a typical Western diet get only 2%–5% from their gut bugs — so what you eat changes how much energy you get from your microbiome.

What you eat—especially fiber and complex carbs—mostly decides how much gut fermentation happens, and eating more of these carbs can boost fermentation by up to five times compared to a standard Western diet. The types of gut bacteria you have don’t really change the overall amount.

Your gut bacteria don’t change how much waste they produce overall, but they do change what kinds of waste chemicals—like butyrate and lactate—are made, and this mix is different for everyone depending on their unique gut bug lineup.

What you eat overall—like veggies, meat, or carbs—is probably what’s really changing your gut bacteria, not the sugar-free sweeteners in your diet.

Humans can't digest plant fiber like cows do—we don't have the right enzymes. Instead, our gut bacteria do a little bit of breaking it down, which suggests our ancestors relied more on meat than plants for energy.