A month of daily fasting barely shifts blood sugar or insulin in healthy people

Intermittent fasting is one of the most popular eating patterns in modern nutrition, and Ramadan offers something rare: millions of healthy adults fasting from dawn to dusk for a month, every year, across many countries. Individual studies of what this does to blood sugar, insulin and appetite hormones have pointed in different directions, with some suggesting benefit and others none. Pinning down the true effect matters both for people who fast for religious reasons and for anyone weighing intermittent fasting as a health strategy. This review set out to settle the question in healthy, non-athletic adults.

The researchers searched ten databases from their inception to January 2020 for studies measuring metabolic markers around Ramadan fasting in healthy, non-athletic Muslims. Seventy-two studies qualified, conducted in 22 countries between 1982 and 2020 and covering 3134 participants in total. The markers analysed were fasting glucose, insulin, insulin resistance measured by HOMA-IR, and the hormones leptin and adiponectin. The team pooled the effect sizes across studies and used meta-regression to test whether factors that varied between studies changed the results.

Fasting glucose fell, but only slightly: the pooled effect was small enough to be of little practical consequence for a healthy person. Insulin, insulin resistance, leptin and adiponectin did not change meaningfully in any direction; for each of these, the pooled estimate sat close to zero. In other words, a full month of daily daytime fasting left the metabolic profile of healthy adults essentially where it started. The authors concluded that this style of fasting imposes no adverse metabolic impact and might modestly improve some markers.

This pooled analysis helps resolve a literature that had genuinely conflicted, with single studies claiming improvements, harms or nothing at all. It complicates the more enthusiastic claims made for intermittent fasting by showing that, in healthy people at least, a month of it moves metabolic markers barely at all. Ramadan fasting also excludes fluids during daylight, which distinguishes it from most popular fasting plans, so translation to those protocols is not automatic. Whether the small glucose change persists after the month ends, and what happens in people with metabolic conditions, remain open questions.

If you observe Ramadan, this is reassurance on a large scale: in healthy people the fast does not appear to harm blood sugar control or hormone balance. If you are considering intermittent fasting in hope of a metabolic overhaul, this is a reason to temper expectations, at least for this pattern of fasting in healthy adults. It suggests any meaningful changes probably come from what and how much is eaten rather than from meal timing alone. People with diabetes or other conditions were not studied here, so nothing can be inferred for them.