OMAD (One Meal a Day): What to Know

What OMAD Means

OMAD, one meal a day, means eating all of a day's food in a single sitting and fasting for the rest of the time, roughly a 23-hour fast and a 1-hour eating window. It is the most extreme end of the daily-fasting spectrum covered on this site; see 16:8 intermittent fasting for a less extreme daily schedule with more and newer dedicated research.

What the Research Actually Shows

The Core Trial: Same Calories, Different Timing

The core direct, controlled human-trial evidence on a true one-meal-a-day pattern is a small crossover trial in healthy, normal-weight, middle-aged adults, who ate all of their weight-maintenance calories in either 3 meals a day or 1 meal a day (consumed between 4pm and 8pm), each for 8 weeks, with a washout period between arms (Stote et al., 2007) [1]. This trial was deliberately isocaloric: the same total calories, only the timing changed. That is the single most important framing point for this page: it is evidence about what changing meal frequency alone does, not evidence that real-world, eat-freely-at-one-sitting OMAD causes weight loss. A second, shorter isocaloric crossover trial in lean adults tested the same question over 11 days and found a different direction on body composition and glucose (Meessen et al., 2022) [5]; see "Where the studies disagree" above for how the two compare.

Blood Pressure and Cholesterol

Eating the same calories in one meal instead of three produced significant increases in blood pressure and in total, LDL, and HDL cholesterol, alongside a significant decrease in cortisol (Stote et al., 2007) [1]. Both the LDL and HDL increases are worth stating, not just the LDL rise, since a reader weighing cholesterol risk needs both directions.

Blood Sugar and Insulin Response

In the same trial's glucose-specific analysis, the 1-meal-a-day group showed higher morning fasting glucose, a greater and more sustained glucose rise after an oral glucose tolerance test, and a delayed insulin response compared with the 3-meals-a-day group; the authors describe the impairment as reversible (Carlson et al., 2007) [2]. A single large daily meal places a bigger digestive and post-meal glucose load on the body than smaller, more frequent meals, a general physiological point consistent with this finding, though not a dosing recommendation.

Hunger

Hunger increased significantly on the 1-meal-a-day schedule compared with 3 meals a day at the same total calories (Stote et al., 2007) [1]. Set this expectation plainly rather than assuming OMAD is effortless once a person adjusts.

Why Real-World OMAD Isn't the Same as the Trial Data

The core trial controlled calories precisely so that only meal timing changed, which is exactly what makes it informative about frequency and exactly what makes it a poor stand-in for how most people actually try OMAD: eating freely at a single meal, often ending up eating fewer total calories simply because one sitting can only hold so much food. Whether that real-world pattern produces weight loss, and whether it carries the same blood-pressure, cholesterol, and glucose-tolerance effects found in the isocaloric trial, has not been directly tested.

What OMAD Doesn't Have Direct Evidence For

No citation reviewed for this page directly tests whether a prolonged daily fasting interval like OMAD raises gallbladder or gallstone risk in humans. This is a commonly raised, biologically plausible concern, since longer gaps between meals mean the gallbladder empties less often, but it is not sourced to a specific OMAD trial here; treat it as a mechanistic consideration, not a measured finding. Long-term, controlled outcomes beyond eight weeks, and any trial of real-world ad-lib OMAD for weight loss, are also not covered by the evidence available; a large observational cohort followed US adults for years and found eating one meal a day associated with higher mortality compared with three meals a day, but as an observational association rather than a controlled trial, it does not show that the eating pattern itself caused the difference (Sun et al., 2023) [6].

How OMAD Compares to 16:8

OMAD and 16:8 are both daily-fasting patterns, but 16:8 has a larger, newer, and more varied evidence base (resistance training, liver fat, cognitive performance). OMAD's direct evidence is smaller and older, two small isocaloric crossover trials, one from 2007 to 2011 and one from 2022, plus an observational cohort and a network meta-analysis that both cover broader meal-frequency patterns rather than OMAD specifically; the 2007 trial found caution-worthy signals on blood pressure, cholesterol, and glucose tolerance (Stote et al., 2007 [1]; Carlson et al., 2007 [2]) that the shorter 2022 trial did not replicate in the same direction (Meessen et al., 2022) [5].

Who Should Talk to a Clinician First

People on blood-pressure medication, insulin, or a sulfonylurea should talk to a clinician before trying OMAD specifically: the core trial found increased blood pressure (Stote et al., 2007) [1] and a reversible glucose-tolerance impairment (Carlson et al., 2007) [2] with this pattern, a more specific flag than the general intermittent-fasting caution. People with existing high cholesterol should raise OMAD specifically with a clinician, since the same trial found increases in total, LDL, and HDL cholesterol together (Stote et al., 2007) [1]. Pregnancy, a history of an eating disorder, being under 18, and any chronic condition requiring consistent food or medication timing are reasons to get clinician guidance before starting, as a matter of general clinical caution rather than a specific finding from these trials.

Safety and Cautions

This is general information, not medical advice. The direct evidence behind OMAD is thinner and older than the evidence behind shorter daily-fasting schedules, and it comes from two small crossover trials plus an observational cohort and a network meta-analysis covering broader meal-frequency patterns, not a large or current body of OMAD-specific research. Talk to a clinician before trying OMAD if you are pregnant, under 18, have a history of an eating disorder, manage diabetes or blood pressure with medication, or have existing high cholesterol.