One meal a day

What it is

All of the day's food is eaten in one meal, usually in the evening. The trials found had people eat all the calories they needed for stable weight in one meal. The meal fell between 4 and 8 p.m. in one US trial and between 5 and 7 p.m. in one Dutch trial, and the trials compared this with three meals a day. No trial found tested a 1-hour (23:1) window: the windows studied were 4 hours and 2 hours.

What the research says

The evidence is thin: three small crossover trials in healthy normal-weight adults (two reports from one trial, lasting 8 weeks per period, and one 11-day trial) and one observational cohort. When calories stayed the same, one meal a day brought small drops in fat mass. The 2007 trial also found higher hunger, blood pressure, total/LDL/HDL cholesterol and fasting glucose, with worse morning glucose tolerance. A 2022 11-day trial found lower weight and afternoon glucose with no loss of strength or aerobic capacity. In NHANES, eating one meal a day was associated with higher all-cause and cardiovascular mortality.

Evidence grade: Limited evidence.

The schedule

• Pattern: daily
• Fasting hours: 20
• Eating hours: 4

Why people choose it

Normal-weight adults were able to stick to one meal a day for 8 weeks. In short trials that kept calories the same, it lowered fat mass slightly compared with three meals. But it also raised hunger, blood pressure and LDL cholesterol, and it impaired morning glucose tolerance. In a large US cohort, eating one meal a day was associated with higher death rates. No trial tested it for weight loss in people with obesity or against ordinary calorie restriction.

Cautions

• People taking insulin or sulfonylureas: Hypoglycaemia is the immediate risk for people with diabetes on these medicines. (Source 1)
• Pregnant or breastfeeding women, frail older adults, children, people with eating disorders: These groups are listed as people who should not follow intermittent fasting. (Source 1)
• People with high blood pressure or high cholesterol: In the 8-week trial, one meal a day raised blood pressure and LDL cholesterol even though weight stayed the same. (Source 2)
• People with prediabetes or impaired glucose tolerance: One large evening meal raised fasting glucose and impaired morning glucose tolerance in healthy adults. (Source 3)
• Older adults (muscle loss): Lean mass losses have been reported in older participants in fasting trials, possibly because they ate too little protein. Fitting enough protein into one meal was not studied. (Source 4)

What goes wrong

On one meal a day, participants had more hunger, higher blood pressure, and higher total, LDL and HDL cholesterol than on three meals. (Source 2)

• Randomized trial, Certainty not rated.
• Size: 15 completers (10 women, 5 men), per the figure on the PubMed record; pilot randomised crossover.
• Who: Healthy, normal-weight, middle-aged adults.
• How long: Two 8-week periods.
• Result: Significant increases in hunger, blood pressure, and total, LDL and HDL cholesterol. Cortisol decreased.
• Funding: NIH intramural research (PubMed record)

However, when consuming 1 meal/d, subjects had a significant increase in hunger; a significant modification of body composition, including reductions in fat mass; significant increases in blood pressure and in total, LDL-, and HDL-cholesterol concentrations; and a significant decrease in concentrations of cortisol.

People eating one large meal a day had higher morning fasting glucose and impaired morning glucose tolerance with a delayed insulin response. This reversed after they returned to three meals. (Source 3)

• Randomized trial, Certainty not rated.
• Size: Healthy normal-weight adults (same crossover trial as Stote 2007)
• Who: Healthy, normal-weight, middle-aged men and women.
• How long: Two 8-week periods.
• Result: Higher fasting glucose, greater and more sustained glucose rises on OGTT, delayed insulin response.
• Funding: NIH intramural research (PubMed record)

Limit of this finding: Tested with a morning glucose tolerance test after an evening-only meal pattern.

Subjects consuming a single large daily meal exhibit elevated fasting glucose levels and impaired morning glucose tolerance associated with a delayed insulin response during a 2-month diet period compared with those consuming 3 meals per day.

In US adults aged 40 and over, eating one meal a day was associated with higher total and cardiovascular mortality than eating three meals. (Source 5)

• Cohort study, Certainty not rated.
• Size: 24,011 adults; 4,175 deaths including 878 cardiovascular deaths.
• Who: US adults aged 40 years or older, NHANES 1999-2014.
• How long: 185,398 person-years of follow-up.
• Result: HR 1.30 (95% CI 1.03 to 1.64) all-cause; HR 1.83 (95% CI 1.26 to 2.65) cardiovascular mortality.
• Funding: no funding to disclose (publisher page)

Limit of this finding: Observational. Eating pattern came from 24-hour recall, and people who ate one meal a day may differ in other ways. It shows an association only.

In this large, prospective study of US adults aged 40 years or older, eating one meal per day was associated with an increased risk of all-cause and CVD mortality

Ghrelin, a hunger hormone, rose on the one-meal-a-day regimen. (Source 3)

• Randomized trial, Certainty not rated.
• Size: Healthy normal-weight adults.
• Who: Healthy, normal-weight, middle-aged adults.
• How long: Two 8-week periods.
• Result: Ghrelin elevated.
• Funding: NIH intramural research (PubMed record)

Limit of this finding: The same abstract reports that fasting ghrelin levels were not significantly affected by meal frequency; the rise was in ghrelin responses measured during the study tests.

Levels of ghrelin were elevated in response to the 1-meal-per-day regimen.

What the evidence supports

Normal-weight adults were able to keep to one meal a day for 8 weeks, and it brought modest changes in body composition, including less fat mass. (Source 2)

• Randomized trial, Certainty not rated.
• Size: 15 completers (10 women, 5 men), per the figure on the PubMed record; pilot randomised crossover.
• Who: Healthy, normal-weight, middle-aged adults.
• How long: Two 8-week periods (6 months overall)
• Result: Significant reduction in fat mass on 1 meal/d. Weight stayed within 2 kg of baseline.
• Funding: NIH intramural research (PubMed record)

Limit of this finding: Pilot study with calories held constant (weight maintenance), not a weight-loss trial.

Normal-weight subjects are able to comply with a 1 meal/d diet.

Eating the day's calories in one evening meal for 11 days lowered body mass and fat mass compared with three meals, without harming aerobic capacity or strength. (Source 6)

• Randomized trial, Certainty not rated.
• Size: 13 enrolled, 11 completed (5 men, 6 women)
• Who: Healthy lean adults, mean age 31.
• How long: 11 days per period (randomised crossover)
• Result: Body mass -1.4 vs -0.5 kg (p = 0.03). Fat mass -0.7 vs -0.1 kg (p = 0.049). Aerobic capacity and strength p > 0.05.
• Funding: independent (ZonMW and Dutch Diabetes foundation)

Limit of this finding: Very small and very short. Young lean adults, not over-50s.

Eucaloric meal reduction to a single meal per day lowered total body mass (3 meals/day –0.5 ± 0.3 vs. 1 meal/day –1.4 ± 0.3 kg, p = 0.03), fat mass (3 meals/day –0.1 ± 0.2 vs. 1 meal/day –0.7 ± 0.2, p = 0.049) and increased exercise fatty acid oxidation (p < 0.001) without impairment of aerobic capacity or strength (p > 0.05).

In the 11-day trial, lean mass did not fall during the one-meal-a-day period. (Source 6)

• Randomized trial, Certainty not rated.
• Size: 11 completers.
• Who: Healthy lean adults.
• How long: 11 days.
• Result: No decrease in lean mass observed.
• Funding: independent.

Limit of this finding: Too short to judge muscle effects in older adults.

we did not observe a decrease in lean mass during the one meal period reduction intervention, which may suggest that there was no increased net protein catabolism

What the evidence does not support

One meal a day had no significant effect on heart rate, body temperature or most blood measures compared with three meals. (Source 2)

• Randomized trial, Certainty not rated.
• Size: 15 completers (10 women, 5 men), per the figure on the PubMed record; pilot randomised crossover.
• Who: Healthy, normal-weight, middle-aged adults.
• How long: Two 8-week periods.
• Result: No significant effects on heart rate, body temperature or most blood variables.
• Funding: NIH intramural research (PubMed record)

There were no significant effects of meal frequency on heart rate, body temperature, or most of the blood variables measured.

Fasting insulin, leptin, adiponectin, resistin and BDNF were not significantly changed by eating one meal a day. (Source 3)

• Randomized trial, Certainty not rated.
• Size: Healthy normal-weight adults (same crossover trial as Stote 2007)
• Who: Healthy, normal-weight, middle-aged men and women.
• How long: Two 8-week periods with an 11-week off-diet gap.
• Result: No significant change in these fasting hormones.
• Funding: NIH intramural research (PubMed record)

Fasting levels of insulin, leptin, ghrelin, adiponectin, resistin, and BDNF were not significantly affected by meal frequency.

Where the research disagrees

Effect of one evening meal a day on blood glucose

• Carlson et al. 2007 (8-week crossover), rct: One meal a day raised fasting glucose and impaired morning glucose tolerance. (Source 3)
• Meessen et al. 2022 (11-day crossover), rct: Plasma glucose was lower in the second half of the day during one meal per day. (Source 6)

A common belief, and what the research shows

The belief: Eating once a day is the most effective and healthiest way to fast.

What the research shows: No trial tested one meal a day for weight loss in people with obesity or against daily calorie restriction. In healthy adults eating the same calories, it slightly reduced fat mass. It also raised hunger, blood pressure and LDL cholesterol and impaired morning glucose tolerance. In a US cohort, it was associated with higher mortality.

References

1. Johns Hopkins Medicine (patient guidance; position). An Overview of Intermittent Fasting (The Johns Hopkins Patient Guide to Diabetes). 2020 (modified 2024). Read the source
2. The American Journal of Clinical Nutrition. A controlled trial of reduced meal frequency without caloric restriction in healthy, normal-weight, middle-aged adults. 2007. PMID 17413096, DOI 10.1093/ajcn/85.4.981. Read the source
3. Metabolism 56(12):1729-34. Impact of reduced meal frequency without caloric restriction on glucose regulation in healthy, normal-weight middle-aged men and women. 2007. PMID 17998028, DOI 10.1016/j.metabol.2007.07.018. Read the source
4. Diabetes & Primary Care 26(3) (Harvie M, McDiarmid S). At a glance factsheet: Intermittent fasting for the management of weight and diabetes. 2024. Read the source
5. Journal of the Academy of Nutrition and Dietetics. Meal Skipping and Shorter Meal Intervals Are Associated with Increased Risk of All-Cause and Cardiovascular Disease Mortality among US Adults. 2023. PMID 35964910, DOI 10.1016/j.jand.2022.08.119. Read the source
6. Frontiers in Physiology. Differential Effects of One Meal per Day in the Evening on Metabolic Health and Physical Performance in Lean Individuals. 2022. PMID 35087416, DOI 10.3389/fphys.2021.771944. Read the source