What does intermittent fasting do to your insulin sensitivity?
Intermittent fasting improves insulin sensitivity in most people, but watch out for possible muscle loss and hormonal effects, especially if you combine it with intensive exercise.
All three common forms of intermittent fasting (alternate-day fasting, the 5:2 schedule, and time-restricted eating) consistently lower fasting insulin levels and make the body more sensitive to insulin. Many studies combined found clear reductions, but warned that the majority of that evidence was of low quality. The trend is visible in people with a healthy weight as well as those with excess weight, although the study periods are often short.
The most striking result comes from a crossover controlled study in men with prediabetes: early time-restricted eating (all meals before 15:00) improved insulin sensitivity and the functioning of the insulin-producing cells in the pancreas, without the participants losing any weight. This suggests that meal timing itself plays a role, independent of calorie reduction. That said, this is one study in a specific group of men, and it is not certain that this effect occurs in everyone.
An important caveat: in trained men who ate all their calories within an eight-hour window for eight weeks, not only insulin but also testosterone (the main male sex hormone) and a substance that drives muscle building declined. That is a drawback to weigh up if you combine intermittent fasting with exercise. Research also points to loss of muscle mass, not just fat.
A 2023 review highlights that intermittent fasting deserves extra attention in certain age groups and that more systematic research into its safety is needed. For most healthy adults it does not appear to lead to more disordered eating or daytime energy problems, but a direct comparison with ordinary calorie restriction is still largely missing.
The evidence is based on a review combining more than 130 studies, several narrative reviews, and one crossover controlled study. The combined review rated most of the evidence as low to very low quality (GRADE). The crossover study is the strongest individual study involving people included here.
Which foods improve your insulin sensitivity?
Diet can clearly improve your insulin sensitivity. The Mediterranean diet has the strongest evidence base, but the timing and order of your meals also play a demonstrable role.
How do you measure insulin resistance?
Insulin resistance is measured most accurately with the clamp method. In practice, a calculation based on blood fat and fasting blood sugar is a good approximation.
What are the earliest signs of insulin resistance?
Early signs of insulin resistance are measurable long before a diabetes diagnosis: elevated fasting insulin, abdominal fat, abnormal blood fats, and excess fat within the muscles. If you have risk factors, have your fasting insulin measured, because standard blood sugar tests respond too late.
Does fermented food help with insulin resistance?
Fermented food, especially yoghurt, kefir and probiotic supplements containing Bifidobacterium, lowers blood sugar and insulin resistance in people with diabetes or prediabetes. Early research suggests this effect can be enhanced further by combining probiotics with fibre-rich prebiotic substances.
Does intermittent fasting help improve your insulin levels?
Fasting probably improves your insulin levels, but most studies are small and short-term. Eating early in the day appears to be most effective; be aware of possible muscle loss if you are older or physically vulnerable.
What are the downsides of intermittent fasting?
For most healthy people, intermittent fasting produces similar weight loss to ordinary dietary advice. For vulnerable groups, such as people with eating disorders or serious heart problems, there are indications of specific risks.