Tip · Fasting

OMAD – One Meal A Day

Q: Is OMAD better than 16:8?

That has never been compared directly. What exists is a controlled study on one meal a day in 15 healthy people over eight weeks, in which fat mass fell but fasting glucose, LDL cholesterol and blood pressure rose at the same time. The most recent systematic review on meal frequency rates the evidence as very uncertain and finds no difference in weight.

Q: Why is my fasting glucose higher on OMAD?

That is exactly what was measured in the controlled study: 95.9 versus 85.4 mg/dl after eight weeks of one meal a day. It is an adaptation to the long fasting phase, not a disease – the value came straight back down after returning to three meals. If you monitor these values anyway, you should still know that OMAD pushes them up.

Q: Does cholesterol really rise on OMAD?

In the only controlled study, yes, and clearly: total cholesterol 216.5 versus 191.0 mg/dl, LDL 136.2 versus 113.3 mg/dl, each with identical calorie and nutrient intake. HDL also rose, from 56.7 to 61.9 mg/dl. Whether the effect persists over months has not been studied – the study lasted eight weeks.

Q: Will I lose muscle with OMAD?

There are no reliable figures on this for OMAD specifically. In the controlled study, fat mass fell while weight stayed constant, which points to a rather favorable change in body composition. For intermittent fasting as a whole, however, a meta-analysis of 28 randomized trials with 1,833 participants found a significant loss of fat-free mass in younger and older adults.

Q: Can I drink coffee during the fasting phase?

In the study there were no calories outside the one meal, and black coffee, tea and water contain practically none. One point belongs here, though: caffeine acutely lowers insulin sensitivity. With fasting glucose already elevated on OMAD, that is relevant, especially if you measure while fasting.

Q: Who is OMAD not for?

For pregnant and breastfeeding women, children, underweight people and people with a history of eating disorders. Also for anyone taking glucose-lowering medication without the dose being adjusted by a physician. And for people with already elevated LDL cholesterol, elevated blood pressure or borderline fasting glucose – exactly these three values went up in the study.

One meal a day. Besides weight loss, the few controlled studies also found unfavorable values for fasting glucose and blood lipids.

One meal a day, nothing for the remaining 20 to 23 hours. OMAD is here because it is the only popular form of fasting for which the controlled study explicitly found unfavorable values – and that with identical calorie intake.

In short

Body composition changes favorably: fat mass fell significantly, although participants stayed within 2 kg of their starting weight throughout the six months. At the same time, several values moved in the unfavorable direction – fasting glucose 95.9 versus 85.4 mg/dl, LDL cholesterol 136.2 versus 113.3 mg/dl, plus higher blood pressure and more hunger. The entire controlled evidence is one crossover study with 15 participants in two analyses, two periods of eight weeks. The effect on glucose tolerance disappeared immediately after returning to three meals – an adaptation, not damage. On meal frequency as a whole, the most recent review rates the evidence as very uncertain.

What is behind it

The only controlled human study cleanly isolated the timing effect because it kept calories constant. 15 healthy, normal-weight middle-aged people went through two periods of eight weeks each in a randomized crossover. In both, they received exactly the amount of energy that maintained their weight – once spread over three meals, once in a single one.

The result is split in two. Fat mass fell significantly, although weight stayed within 2 kg of baseline over six months – so there was no weight loss, but a shift in body composition. Hunger rose significantly and persisted over eight weeks. Cortisol fell, which the authors themselves interpret cautiously: it was measured at a fixed time of day.

Why the metabolic values rise

If the entire daily energy arrives in one meal, the body has to process a very large glucose and fat load in a short time, and the 20 to 23 hours in between are an extended fasting phase. Fat loss benefits from the long fasting phase; glucose regulation suffers from the combination. This fits the known starvation insulin resistance: after just one to three days without food, insulin sensitivity falls measurably – an adaptive process meant to reserve glucose for the brain. With OMAD it repeats daily.

Then there is the time of day. In the study, the one meal was eaten between 4 and 8 p.m., that is, at the time of day with physiologically poorer glucose tolerance. In practice it is the same, because most people eat in the evening.

Adaptation, not damage

The most important sentence for putting this into context is in the glucose analysis: the effect on glucose tolerance reversed quickly after returning to three meals; there was no lasting effect on glucose metabolism. Fasting insulin, glucagon, leptin, ghrelin, adiponectin, resistin and BDNF were not significantly changed.

No recommendation on meal frequency as a whole can currently be justified. The most recent systematic review analyzes 17 reports on 16 studies; 15 of them had a high risk of bias. Result: very low-certainty evidence according to GRADE and no difference in weight. The honest state of knowledge is not “OMAD is harmful” but “the data do not support any statement”.

What is well supported

Most robust is what was measured with calories held constant. Total cholesterol 216.5 ± 5.3 mg/dl with one meal a day versus 191.0 ± 5.3 mg/dl with three meals (p = 0.001), LDL 136.2 ± 4.0 versus 113.3 ± 4.0 mg/dl (p = 0.001), HDL 61.9 ± 1.8 versus 56.7 ± 1.8 mg/dl (p = 0.01). Morning fasting glucose 95.9 ± 1.7 versus 85.4 ± 1.7 mg/dl (p = 0.0002). Plus higher liver values (SGPT 40.0 versus 33.0 IU/l, p = 0.01) and lower hemoglobin (13.2 versus 13.6 g/dl, p < 0.0001). On the plus side is the significantly reduced fat mass at the same weight.

What the studies show

The only controlled OMAD study

Randomized crossover design, two periods of eight weeks each with eleven weeks without a diet in between, 15 healthy normal-weight middle-aged people (10 women, 5 men). Both periods with an exactly weight-maintaining amount of energy, the only difference being the distribution over one or three meals – so the study measures the pure timing effect without a deficit. Fat mass fell significantly, hunger rose significantly, blood pressure as well as total, LDL and HDL cholesterol rose significantly, cortisol fell.

The same study, analysis of glucose regulation

The same 15 participants, the one meal between 4 and 8 p.m. Morning fasting glucose with one meal a day was 95.9 versus 85.4 mg/dl (p = 0.0002) – in the range that would clinically be called impaired fasting glucose. In the glucose tolerance test, glucose rose more and stayed elevated longer; the insulin response was delayed only as a trend. Crucially: the effect disappeared immediately after returning to three meals.

Systematic review on meal frequency

Review with meta-analysis of 17 reports on 16 randomized trials, at most three versus at least four meals a day, at least two weeks’ duration, primary endpoint weight. 15 of 16 studies had a high risk of bias, and five could not be included in the meta-analysis at all because of insufficient reporting. Result: very low-certainty evidence according to GRADE, no difference between high and low meal frequency.

Where the data stop

The entire controlled evidence consists of one study with 15 participants in two analyses. There is no randomized trial lasting more than eight weeks, none with clinical endpoints and none with more than 15 participants. The participants were healthy, normal-weight and middle-aged, the calories constant – this cannot be transferred directly to the most common real-world use, OMAD for weight loss in overweight people. Whether the rise in cholesterol persists or normalizes like glucose tolerance has not been studied; a case report of a 42-year-old man with class III obesity describes around 10 % weight loss after three months and better values for HbA1c, ALT and blood pressure – with paradoxically increased LDL. On autophagy under OMAD there is no measurement in humans, and on loss of lean mass no data of its own.

How to do it

What the study actually did: one meal a day between 4 and 8 p.m., with the full weight-maintaining amount of energy, that is, without a calorie deficit. Eight weeks in a row, then eleven weeks’ break, then the comparison period with three meals. The food was provided. From this it follows: the only studied version of OMAD is one meal in the late afternoon or evening with the full daily energy. Everything else – morning instead of evening, with a deficit, with snacks in between, over months – has not been studied. The advice to switch only after months of 16:8 is also a rule of thumb. In practical terms, one meal has to cover the entire need for protein, micronutrients and fiber; in the study, the investigators ensured this.

Safety

The documented changes in healthy people were not dangerous, but they went in the unfavorable direction: higher fasting glucose, poorer glucose tolerance, higher LDL, higher blood pressure, higher liver values, lower hemoglobin. Anyone who already has elevated blood lipids, elevated blood pressure or borderline fasting glucose shifts exactly the values with OMAD that are already unfavorable – these should be measured before and during a trial. Anyone taking glucose-lowering medication changes the basis of their own dosing with a single meal a day; with intermittent fasting, the risk of hypoglycemia does not rise only if the medication is adjusted, and for type 1 diabetes and gestational diabetes the data are explicitly insufficient. OMAD is not suitable for pregnant and breastfeeding women, children, underweight people and people with a history of eating disorders. One more point of its own: hunger rose significantly and persisted over eight weeks – a pattern that can encourage binge eating.

BK-Score Thin human evidence

Human evidence		4
Mechanism		4
Safety data		6
Hype gap		3
Track record of use		7

There is exactly one controlled study on one meal a day – 15 participants, published in two analyses. It was designed to maintain weight and found less fat mass at the same weight, but unfavorable changes in fasting glucose and blood lipids that disappeared immediately after returning to three meals. The data do not support a recommendation, neither for nor against.

What is rated is the state of knowledge, not the effect. “Safety data 9” means well studied – not harmless.
Subjective assessment by Biohacking Kompakt based on published scoring rules – not a scientific rating and not a medical recommendation. Rules and all ratings (German)

Frequently asked questions about OMAD – One Meal A Day

Is OMAD better than 16:8?