Intermittent Fasting Outperforms Daily Calorie Restriction for Blood Sugar Control: What the Research Shows

Medical disclaimer: This article summarises published research for informational purposes only. It is not medical advice and is not a substitute for guidance from a qualified health professional. Always consult your doctor before starting any fasting protocol, especially if you have an existing health condition or take medication.

Study at a Glance

What This Study Looked At

Researchers at SAHMRI wanted to know whether combining intermittent fasting with early time-restricted eating (iTRE) — eating only in the morning on certain days — could reduce the risk of type 2 diabetes more effectively than daily calorie restriction. Adults at risk of developing type 2 diabetes (prediabetics) were recruited for a 6-month intervention with a further 12-month follow-up. You can read more about why this approach matters in our article on intermittent fasting and insulin sensitivity and the broader evidence on intermittent fasting benefits for metabolic health.

Who Was Studied

Participant profile: Mean age 58 ± 10 years, mean BMI 34.8 ± 4.7 kg/m², adults at increased risk of type 2 diabetes. Both iTRE and CR groups received nutritional support and counselling during the 6-month intervention period.

How the iTRE protocol worked in this study: On fasting days, participants consumed 30% of their estimated energy requirements between 8am and 12pm only — a 4-hour eating window with a 20-hour fast. On non-fasting days (four days per week), they could eat freely with no caloric restriction. This is a hybrid protocol: part intermittent fasting, part time-restricted eating, with an emphasis on eating early in the day when insulin sensitivity is highest.

What the Researchers Found

Glucose Tolerance — The Primary Outcome

The primary outcome was the 2-hour glucose tolerance test — a standard clinical measure of how effectively the body processes glucose. Results are expressed as the incremental area under the glucose curve (iAUC), where a lower number means better glucose metabolism.

• The iTRE group improved glucose tolerance nearly three times more than the calorie restriction group at the 6-month mark (P = 0.03).
• The calorie restriction group's improvement did not reach statistical significance (its confidence interval crossed zero).
• Standard care showed no meaningful improvement.

Insulin Sensitivity and Lipids

• The iTRE group showed greater insulin sensitivity compared to calorie restriction at 6 months.
• The iTRE group achieved a greater reduction in blood lipids compared to the CR group.
• Both active interventions (iTRE and CR) produced weight loss compared to standard care.

Long-Term Results (Month 18)

• The advantage of iTRE over CR in glucose tolerance was not maintained at the 18-month follow-up.
• This suggests the benefit is real but may require sustained adherence to the iTRE protocol to persist beyond the active intervention period.

What Did Not Change

• No significant differences in adverse events between groups.
• Long-term glucose tolerance advantage (iTRE vs CR) was not sustained at 18 months, suggesting that continued adherence to the fasting protocol matters for maintaining the benefit.

What the Researchers Concluded

The researchers concluded that incorporating meal timing advice — specifically, eating early in the day with prolonged fasting periods — led to greater improvements in postprandial glucose metabolism in adults at increased risk of developing type 2 diabetes, compared to standard daily calorie restriction at 6 months.

What This Means If You Fast

• Timing matters, not just quantity. The iTRE group ate the same total calories as the CR group across the week (both had a roughly 30% deficit overall), yet achieved significantly better glucose results. The difference was when they ate, not just how much.
• Morning eating may be the key mechanism. Insulin sensitivity is highest in the morning and decreases throughout the day. The iTRE protocol placed all eating before noon on fasting days, amplifying the metabolic benefit of the fast.
• Intermittent fasting may be more sustainable than daily restriction. Having completely unrestricted eating days (four per week) may make the protocol easier to maintain long-term for many people compared to a daily reduced-calorie approach.
• The glucose benefits require consistency. The advantage disappeared at 18 months, suggesting that ongoing adherence — not just a 6-month experiment — is needed for lasting metabolic benefit.
• People at diabetes risk stand to benefit substantially. If you have prediabetes, elevated fasting glucose, or a family history of type 2 diabetes, this study suggests that a fasting approach with early eating windows may be particularly valuable. See our guide on intermittent fasting and type 2 diabetes for more context.

Study Limitations

• Open-label design — participants knew which group they were in, which may introduce behavioural bias.
• Nutritional support was provided only to the two active groups, not standard care — so some of the effect may reflect the benefit of any dietary counselling, not just fasting specifically.
• The 18-month loss of iTRE advantage raises the question of whether adherence declined — this wasn't fully reported.
• Participants were predominantly older (mean age 58) and had a high BMI — results may differ in younger or leaner populations.
• The iTRE protocol is demanding on social eating schedules — only eating between 8am and 12pm on fasting days is a significant lifestyle constraint.
• Long-term weight loss differences between iTRE and CR groups are not clearly reported in available data.

Source

Hutchison, A.T., Liu, B., Wood, R.E., et al. (2023). Intermittent fasting plus early time-restricted eating versus calorie restriction and standard care in adults at risk of type 2 diabetes: a randomized controlled trial. Nature Medicine, 29, 963–972. PMID: 37024596

Frequently Asked Questions

What exactly is early time-restricted eating (eTRE)?

Early TRE means concentrating all eating early in the day — in this study, between 8am and 12pm. This takes advantage of the body's natural morning insulin sensitivity and aligns eating with the body's circadian rhythm. Evening meals, by contrast, are processed with lower insulin sensitivity.

Does intermittent fasting really beat calorie restriction for blood sugar?

In this study, at 6 months, yes — meaningfully so. The iTRE group improved 2-hour glucose tolerance by −10.10 mg dl⁻¹ min⁻¹ versus −3.57 mg dl⁻¹ min⁻¹ for daily calorie restriction. The difference was statistically significant (P = 0.03).

Why did the benefit disappear at 18 months?

The study doesn't fully explain this, but the most likely explanation is declining adherence after the 6-month active support period ended. This is consistent with most lifestyle intervention research — benefits fade when the intervention is withdrawn or becomes less consistent.

Who conducted this study and where?

Lead author Amy T. Hutchison and senior author Leonie K. Heilbronn at SAHMRI (South Australian Health and Medical Research Institute) in Adelaide, Australia, with a multi-disciplinary team including endocrinologists and dietitians.

Is this protocol safe for people currently taking diabetes medication?

Not without medical supervision. Both fasting and calorie restriction lower blood glucose, which can cause hypoglycaemia in people taking insulin or certain oral medications. Anyone on diabetes medication should consult their doctor before starting any fasting protocol.

Related Research and Articles

• Can Intermittent Fasting Improve Insulin Sensitivity?
• Does Intermittent Fasting Help with Type 2 Diabetes?
• Intermittent Fasting Benefits: The Complete Science-Backed Guide
• Intermittent Fasting and Metabolism: What Science Says
• What Happens to Your Body Hour by Hour When You Fast

Want the complete guide to fasting? Get Intermittent Fasting in Practice on Amazon — and claim 3 months free on our fasting app at fastinginpractice.com/redeem.