Intermittent fasting (IF) is not a universal panacea, regardless of whether you’re not eating anything at all for a few days each week/month or just restricting your feeding window to a few hours per day.
Some protocols, eg, 20h fasting every second day, significantly improve insulin sensitivity in adipose tissue (Halberg et al., 2005). This is expected to make fat gain easier, and while this wasn’t meant to be a study on body composition per se…
After just a few weeks, things weren’t changing in a good way (NS).
Exhibit B. And in this study where they fasted every other day, fat-free mass also declined (but at least so did fat mass) (Heilbronn et al., 2005):
From the abstract, “Subjects lost 2.5 ± 0.5% of their initial body weight (P < 0.001) and 4 ± 1% of their initial fat mass (P < 0.001). Hunger increased on the first day of fasting and remained elevated (P < 0.001).”
HEY DID YOU SEE THAT RECENT ARTICLE BY TAUBES?
Further, “As expected, a significant increase was found in feelings of hunger (from 37 ± 5 to 56 ± 4 mm; P < 0.001), and a significant decrease was noted in feelings of fullness (from 43 ± 3 to 23 ± 4 mm;P < 0.001).”
Exhibit C. Reduced feeding window (3 meals per day vs. 1 four-hour feeding window) (Carlson et al., 2008).
Skipping breakfast and ingesting all of their calories in a four-hour feeding window in the evening: “Subjects consuming a single large daily meal exhibit elevated fasting glucose levels, and impaired morning glucose tolerance associated with a delayed insulin response, during a 2 month diet period compared to those consuming 3 meals/day.”
To summarize: loss of muscle mass (Halberg and Heilbronn), increased hunger (Heilbronn), and diabetogenic (Carlson). These aren’t good things; this way of eating is definitely not for everyone.
Can some people benefit from intermittent fasting? Sure, but best choose your fasting/feeding schedule wisely; keep in line with circadian rhythms as meal timing is an important zeitgeber. That means eating when the sun is up; frequency is up to you.
I’m not 100% against intermittent fasting and I don’t deny that some IF protocols may work [for some people], but IF is not a universal panacea or necessary to be healthy, and can even be harmful in some contexts.