Carb Fasting
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The evidence, study by study

15 studies, laid out the same way: design, who was studied, what it found, and where it falls short. For how we weigh them, see how we research.

Area Study Design Year
Mechanism Biyikoglu et al., European Journal of Nutrition Randomized crossover (acute, controlled feeding) 2025
Low-carb Gardner et al., American Journal of Clinical Nutrition Randomized crossover, two 12-week diet phases 2022
Low-carb Goldenberg et al., BMJ Systematic review & meta-analysis of RCTs 2021
Low-carb Zheng et al., International Journal of Obesity Systematic review & meta-analysis (41 RCTs, ~3,806 adults) 2025
Low-carb Athinarayanan et al., Frontiers in Endocrinology 2-year non-randomized clinical trial (continuous remote care) 2019
Fasting Kalam et al., Obesity Science & Practice Single-arm pilot study 2019
Fasting Sun et al., eClinicalMedicine Umbrella review of systematic reviews/meta-analyses of RCTs 2024
Medication Wilding et al., New England Journal of Medicine 68-week randomized controlled trial 2021
Medication Wilding et al., Diabetes, Obesity and Metabolism Off-treatment extension of the STEP 1 trial 2022
Medication Xiong et al., Complementary Therapies in Clinical Practice Dose-response meta-analysis of RCTs 2020
Biomarker Battelino et al., Diabetes Care International consensus recommendations 2019
Biomarker Matthews et al., Diabetologia Methodology paper 1985
Safety Kirkpatrick et al., Journal of Clinical Lipidology Scientific statement / evidence review 2019
Safety Acharya et al., Diseases Systematic review & meta-analysis 2021
Safety Murdoch et al., British Journal of General Practice Practical clinical guide 2019

Mechanism

Carbohydrate restriction elicits metabolic effects similar to fasting

Biyikoglu et al. · European Journal of Nutrition · 2025

Design
Randomized crossover (acute, controlled feeding)
Who
Healthy adults
Found
Acutely restricting carbohydrate produced post-meal metabolic effects (lower glucose and insulin, a tilt toward fat oxidation) similar to fasting, the core premise behind carb fasting.
Limits
Acute, mechanistic study of post-meal metabolism, not a long-term weight-loss or outcomes trial.

DOI 10.1007/s00394-025-03646-5 · PMID 40111529

Low-carb

Keto-Med: ketogenic vs Mediterranean-plus diet (randomized crossover)

Gardner et al. · American Journal of Clinical Nutrition · 2022

Design
Randomized crossover, two 12-week diet phases
Who
Adults with prediabetes or type 2 diabetes
Found
HbA1c was not different between the diets; triglycerides fell more on keto; LDL-cholesterol rose on keto and fell on the Mediterranean phase; fiber and some nutrients were lower on keto.
Limits
Short diet phases; the LDL response varied between individuals.

DOI 10.1093/ajcn/nqac154 · PMID 35641199

Low-carbohydrate diets for type 2 diabetes remission

Goldenberg et al. · BMJ · 2021

Design
Systematic review & meta-analysis of RCTs
Who
Adults with type 2 diabetes
Found
Low-carbohydrate diets improved blood sugar and supported type 2 diabetes remission at around 6 months versus control.
Limits
Benefits attenuated by 12 months; substantial heterogeneity between trials.

DOI 10.1136/bmj.m4743 · PMID 33441384

Low-carbohydrate diets and metabolic syndrome

Zheng et al. · International Journal of Obesity · 2025

Design
Systematic review & meta-analysis (41 RCTs, ~3,806 adults)
Who
Adults with metabolic syndrome
Found
Low-carbohydrate diets (≥12 weeks) improved BMI, waist circumference, blood pressure, HbA1c, triglycerides, and HDL-cholesterol versus controls.
Limits
Heterogeneity across trials; medium-term follow-up.

DOI 10.1038/s41366-025-01822-5 · PMID 40579564

Two-year nutritional ketosis for type 2 diabetes (Virta)

Athinarayanan et al. · Frontiers in Endocrinology · 2019

Design
2-year non-randomized clinical trial (continuous remote care)
Who
Adults with type 2 diabetes
Found
A continuous-care intervention with nutritional ketosis sustained glycemic improvement and reduced diabetes medication use over two years.
Limits
Non-randomized, selected participants; the remote-care model is a confound.

DOI 10.3389/fendo.2019.00348 · PMID 31231311

Fasting

Alternate-day fasting combined with a low-carbohydrate diet

Kalam et al. · Obesity Science & Practice · 2019

Design
Single-arm pilot study
Who
Adults with obesity
Found
Pairing alternate-day fasting with low-carb eating produced weight loss and metabolic improvements over the study period.
Limits
Small, single-arm pilot with no control group.

DOI 10.1002/osp4.367 · PMID 31890243

Intermittent fasting and health outcomes (umbrella review)

Sun et al. · eClinicalMedicine · 2024

Design
Umbrella review of systematic reviews/meta-analyses of RCTs
Who
Adults
Found
Intermittent fasting showed benefits for weight and some metabolic markers, but frequently performed similarly to simple calorie restriction.
Limits
Quality varied across the included reviews.

DOI 10.1016/j.eclinm.2024.102519 · PMID 38500840

Medication

STEP 1: once-weekly semaglutide for weight management

Wilding et al. · New England Journal of Medicine · 2021

Design
68-week randomized controlled trial
Who
Adults with overweight or obesity (without diabetes)
Found
Once-weekly semaglutide 2.4 mg produced about 14.9% mean weight loss over 68 weeks versus 2.4% with placebo.
Limits
On-treatment effect of a medication; not a diet study.

DOI 10.1056/NEJMoa2032183 · PMID 33567185

Weight regain after withdrawal of semaglutide (STEP 1 extension)

Wilding et al. · Diabetes, Obesity and Metabolism · 2022

Design
Off-treatment extension of the STEP 1 trial
Who
A subset of STEP 1 participants
Found
One year after stopping semaglutide (and lifestyle support), participants regained about two-thirds of their prior weight loss.
Limits
Observational extension in a participant subset.

DOI 10.1111/dom.14725 · PMID 35441470

Berberine and obesity indices (dose-response meta-analysis)

Xiong et al. · Complementary Therapies in Clinical Practice · 2020

Design
Dose-response meta-analysis of RCTs
Who
Adults
Found
Berberine did not significantly change body weight (−0.11 kg, not significant), but produced small significant reductions in BMI (about 0.29 kg/m²) and waist circumference (about 2.75 cm), modest, and far smaller than GLP-1 drug effects.
Limits
Supplement-trial heterogeneity; the body-weight effect was not significant and the others are small.

DOI 10.1016/j.ctcp.2020.101113 · PMID 32379652

Biomarker

Clinical targets for continuous glucose monitoring (time in range)

Battelino et al. · Diabetes Care · 2019

Design
International consensus recommendations
Who
People using CGM
Found
Defined standardized continuous-glucose-monitoring targets, including "time in range" (70–180 mg/dL), for interpreting CGM data.
Limits
Consensus guidance, not an outcomes trial.

DOI 10.2337/dci19-0028 · PMID 31177185

Homeostasis model assessment (HOMA-IR)

Matthews et al. · Diabetologia · 1985

Design
Methodology paper
Who
Adults
Found
Introduced HOMA-IR, a way to estimate insulin resistance from fasting glucose and insulin, still widely used as a research proxy.
Limits
A model/estimate, less standardized clinically than glucose or HbA1c.

DOI 10.1007/BF00280883 · PMID 3899825

Safety

Low-carb/keto diets and lipids (National Lipid Association statement)

Kirkpatrick et al. · Journal of Clinical Lipidology · 2019

Design
Scientific statement / evidence review
Who
General adult population
Found
Low-carb and ketogenic diets can lower triglycerides and raise HDL, but LDL-cholesterol rises in some people, so lipids should be monitored.
Limits
Narrative scientific statement, not a single trial.

DOI 10.1016/j.jacl.2019.08.003 · PMID 31611148

Kidney stones on the ketogenic diet (systematic review)

Acharya et al. · Diseases · 2021

Design
Systematic review & meta-analysis
Who
People on ketogenic diets (mostly therapeutic)
Found
Pooled kidney-stone incidence on ketogenic diets was about 5.9% (around 7.9% in adults) over ~3.7 years of follow-up.
Limits
Mostly therapeutic ketogenic diets, more restrictive than carb fasting.

DOI 10.3390/diseases9020039 · PMID 34070285

Adapting diabetes medication for low-carb eating

Murdoch et al. · British Journal of General Practice · 2019

Design
Practical clinical guide
Who
Adults with type 2 diabetes on medication
Found
Provides a practical protocol for lowering diabetes medications (especially insulin and sulfonylureas) when starting low-carb, to avoid hypoglycemia.
Limits
Practical guidance for clinicians, not a trial.

DOI 10.3399/bjgp19X704525 · PMID 31249097

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