One potential downside to steady regimented fasting _when combined_ with caloric restriction, is a decrease in your basal metabolic rate (BMR).
Our bodies are very good at adapting to "the new normal" and will optimize itself to function on the fuel you give it. This is true in both directions. If you eat less, your body will learn to live on less, and if you eat more, eventually your body will ramp up to try and burn more energy to try and reach an equilibrium.
When fasting, it's important to keep your caloric intake up during brief eating windows. Otherwise, your body will adjust, and when you stop fasting, the slightly increased caloric intake will cause you to gain weight before your body readjusts to the new normal. Many fasting theorists suggest that fasting at random intervals is the best strategy for weight loss without reducing BMR. If your body is used to burning 3000 calories a day just to keep you alive, it can't ramp down those cells immediately so it needs to instead convert stored fat to energy and burn that instead.
These aren't fasting specific, but oftentimes in the western world, we begin fasting as a means to an end (weight loss) and often combine it with other dietary changes, like eating less/healthier.
The other downside is literal starvation or the development of an eating disorder, but those are pretty obvious.
I saw a low-budget documentary on YouTube[0] a couple of weeks ago talking about "fasting retreats" in Russia and now parts of Europe that are seeing great results with patients water fasting for 2+ weeks. They're treating everything from Diabetes to arthritis and dementia. Pretty cool stuff.
Personally, I definitely have seen the light and will likely continue fasting regularly in some form the rest of my life. Since I just turned 26 last month, that's going to be a while! I'm currently half way through a 72 hour fast and feeling great. Just had a cup of coffee and some electrolyte spiked water this morning. I still have about 50 pounds to lose but I'm hopefully for my future now and I'm a lot healthier.
