Fasting Evidence

Metabolic

Fasting and your blood sugar, insulin and lipids

Markers improve. The hard question is whether they improve more than they would from the same weight loss achieved any other way.

Updated 3 min read 9 citations Evidence strength 4/5

What the highest-quality reviews say

The Cochrane review on intermittent fasting for adults with overweight or obesity [1] and the BMJ systematic review of fasting strategies [2] are the two syntheses to read if you read only two. Both find real effects on weight and associated cardiometabolic markers, and both are careful about the size and certainty of those effects.

The recurring caveat: markers improve roughly in proportion to weight lost. Trials comparing intermittent fasting with continuous energy restriction in type 2 diabetes found comparable outcomes [4], which is what you would expect if the mechanism is the deficit.

Glycaemic improvement

The diabetes trials

The EARLY trial is the most clinically interesting: a 5:2 intermittent fasting meal-replacement diet tested for glycaemic control in adults with diabetes, with HbA1c as the endpoint [6]. Meal replacement removes much of the adherence problem, which may be why the result was positive.

A randomised comparison of 16:8 against 14:10 and a control group reported on both weight reduction and metabolic parameters [8], reinforcing that window length is not the active ingredient.

Age matters

A review specifically examining risks and benefits of intermittent fasting for the ageing cardiovascular system [3] is worth reading if you are over 60. Older adults have less lean mass to lose, more medication interactions, and a higher chance that a period of reduced intake produces problems that would be trivial at 30. A Cell Metabolism trial measuring brain responses to intermittent fasting versus a healthy-living diet in older adults [7] is one of the few studies designed for this population.

What does not hold up

  • "Fasting reverses insulin resistance independently of weight loss." Not established. Trials that matched weight change do not show a clear independent effect.
  • "Fasting fixes your cholesterol." Lipids move with weight and diet composition. Fasting per se has no established lipid-specific effect.
  • "Fasting lowers inflammation." Inflammatory markers fall with fat loss. Attributing that to the fasting pattern requires a control that most trials lack.
  • "You will become fat-adapted and burn fat all day." Substrate use shifts during a fast and shifts back after eating. Twenty-four-hour fat oxidation is governed by energy balance.

Common questions

Will fasting reverse my prediabetes?
Weight loss substantially reduces progression to type 2 diabetes, and fasting is one way to achieve weight loss. There is no evidence it beats other routes to the same loss.
Do my fasting glucose readings going up mean it is not working?
Fasting glucose can rise transiently during extended fasts as the liver maintains supply. It is not by itself a sign of harm, but discuss any unexpected readings with a clinician.
Is fasting good for cholesterol?
Indirectly, through weight loss and often through improved diet quality. No fasting-specific lipid effect is established.
Can I fast with type 2 diabetes?
Trials have done it safely under supervision, including in insulin-treated patients. Doing it without adjusting medication is the dangerous part — see safety.

References

Every citation below links to the original peer-reviewed record on PubMed or via DOI. Nothing here is a substitute for medical advice.

  1. Intermittent fasting for adults with overweight or obesity Garegnani LI, Oltra G, Ivaldi D, et al. · The Cochrane database of systematic reviews · 2026 · Meta-analysis DOIPubMed 41692034Full text
  2. Intermittent fasting strategies and their effects on body weight and other cardiometabolic risk factors: systematic review and network meta-analysis of randomised clinical trials Semnani-Azad Z, Khan TA, Chiavaroli L, et al. · BMJ (Clinical research ed.) · 2025 · Meta-analysis DOIPubMed 40533200Full text
  3. Risks and Benefits of Intermittent Fasting for the Aging Cardiovascular System Ozcan M, Abdellatif M, Javaheri A, et al. · The Canadian journal of cardiology · 2024 · Systematic review DOIPubMed 38354947
  4. Intermittent fasting versus continuous energy-restricted diet for patients with type 2 diabetes mellitus and metabolic syndrome for glycemic control: A systematic review and meta-analysis of randomized controlled trials Wang X, Li Q, Liu Y, et al. · Diabetes research and clinical practice · 2021 · Meta-analysis DOIPubMed 34391831
  5. Intermittent fasting and weight loss: Systematic review Welton S, Minty R, O'Driscoll T, et al. · Canadian family physician Medecin de famille canadien · 2020 · Systematic review PubMed 32060194Full text
  6. A 5:2 Intermittent Fasting Meal Replacement Diet and Glycemic Control for Adults With Diabetes: The EARLY Randomized Clinical Trial Guo L, Xi Y, Jin W, et al. · JAMA network open · 2024 · Randomised controlled trial DOIPubMed 38904963Full text
  7. Brain responses to intermittent fasting and the healthy living diet in older adults Kapogiannis D, Manolopoulos A, Mullins R, et al. · Cell metabolism · 2024 · Randomised controlled trial DOIPubMed 38901423Full text
  8. Effect of intermittent fasting 16:8 and 14:10 compared with control-group on weight reduction and metabolic outcomes in obesity with type 2 diabetes patients: A randomized controlled trial Sukkriang N, Buranapin S · Journal of diabetes investigation · 2024 · Randomised controlled trial DOIPubMed 38932663Full text
  9. [Intermittent fasting and human metabolic health] Carvajal V, Marín A, Gihardo D, et al. · Revista medica de Chile · 2023 · Review DOIPubMed 37906749