Celebrities have popularized all sorts of outlandish diet trends over the years. One of the latest trends among celebrities is the "one meal a day" diet (or "OMAD").
Fans of OMAD include Bruce Springsteen and Coldplay frontman Chris Martin. Many proponents of OMAD claim it helps them better manage their weight and keep fit.
OMAD is essentially a more extreme version of other types of fasting diets, such as intermittent fasting and time-restricted eating.
The main difference is that instead of only fasting certain days or only eating your meals during a specific time window, people following OMAD eat all their day's calories in one single, large meal.
But while supporters of OMAD say that following the diet improves many aspects of health, we actually know very little about what effect eating just one meal a day has on the body – let alone if it's safe.
Fasting and health
Evidence supporting the use of OMAD is limited. Very few studies have actually looked at OMAD itself – and most of those that have were conducted in animals.
As such, most of the claims that OMAD works are anecdotal. Or they are based on the assumption that if other forms of fasting can benefit health, then OMAD will too.
Research into fasting diets is still emerging. Some evidence indicates that one form of intermittent fasting known as the "5:2 diet" (where a person eats normally five days a week, then 800 calories or less two days a week) may help people better manage their weight. However, it's no better than other diet approaches.
Research has also found that time-restricted eating (where you eat all your day's calories within a specific window of time) can help people better manage their weight. And it has other health benefits such as lowering blood pressure.
One review study also found that many different types of fasting (including intermittent fasting and fasting every other day) can improve several aspects of metabolism.
These include improving blood sugar and cholesterol levels, reducing inflammation levels and helping people better regulate their appetite. This, in turn, may help reduce a person's risk of obesity, type 2 diabetes, and cardiovascular disease.
Just one meal
One study so far has looked at the effect of OMAD in humans. In this study, participants were given the same number of calories to eat each day for the study's duration.
For half of the study, participants ate these calories in a single meal, before switching and eating their daily calories split into three meals per day.
Each meal pattern was only followed for an 11-day period – not very long at all. The one meal was taken between 5 pm and 7 pm. Only 11 participants completed the study.
When participants ate just one meal per day, they saw a greater reduction in their body weight and fat mass. However, participants also had greater reductions in lean mass and bone density when eating just one meal a day.
This could lead to reduced muscle function and greater risk of bone fractures if the diet was to be followed for a longer period.
Animal studies looking at the effects of OMAD have shown conflicting results, with research showing mice who ate one large meal a day actually gained more weight compared to those who ate multiple meals.
While these results may indicate that OMAD could have benefits for some aspects of health, there's still a lot we don't know about it.
It will be important for future studies to investigate the effect of OMAD in a larger number of participants and in other groups of people (as this study only included lean, young adults).
It will also be important for studies to look at the effect of OMAD over a longer period of time, and to conduct these trials in a real-life setting.
It will also be interesting to know whether the timing of the meal can further improve the results and if the nutritional profile of the meal makes a difference.
If someone is just having one meal a day then it's going to be quite difficult for them to meet all their nutritional requirements, especially for energy, protein, fibre and the key vitamins and minerals.
Someone following Omad will need to ensure they get a good serving of protein and plenty of vegetables, nuts, seeds and some fruit and wholegrains during their single daily meal to meet these nutritional requirements.
They will also need a good serving of dairy to make sure that they meet their calcium and iodine requirements – or a supplement or alternative if they're plant-based.
This is not a diet that we would recommend for children, anyone who is pregnant, hoping to become pregnant or breastfeeding and definitely not for a person who may be at risk of an eating disorder.
It's also important to note that while this diet might work for celebrities, they also have access to nutritionists, high-quality diets, and supplements where needed.
For most of us, this kind of diet could be unsustainable – and potentially harmful in the long run.
An earlier version of this article was published in April 2023.