In a randomized, placebo-controlled clinical trial published in Cell, intermittent fasting inhibits human hair growth, and hair follicle regeneration in mice1
In the human trial, they tested the popular 16/8 time-restricted fasting, with an 8-hour eating window per day. They found the people’s hair grew back 18% slower on average in the fasting group than the placebo group.
Not only that, but many regrown hairs became shorter and thinner in diameter.
They found that the practice causes hair follicle stem cells to die due to the release of fatty acids into the niche, disrupting metabolism and increasing levels of reactive oxygen species, while not affecting the apoptosis of other stem cells, e.g. epidermal and immune stem cells.
This is just one paper so it’s not conclusive, but it is a study with multiple trials and a well-explained mechanism in a high impact journal, so it is worth paying attention to.