Celebrities have been known to follow all kinds of weird diet trends over the years. And one of the latest trends among celebrities is the "one meal a day" (or "OMAD") diet.
Many OMAD supporters claim that it helps them manage their weight better and stay 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 for certain days or eating your meals only during a specific time window, people who follow OMAD eat all of their daily calories in one large meal.
But while OMAD proponents say that dieting improves many aspects of health, we actually know very little about the effect of eating just one meal a day on the body — let alone whether it's safe.
Fasting and health
Evidence supporting the use of OMAD is limited. There are very few studies that have actually analyzed OMAD itself - and most of those have been conducted on animals.
As such, most claims about OMAD are anecdotal. Or they are based on the assumption that if other forms of fasting can benefit health, OMAD will, too.
Some evidence suggests that a form of intermittent fasting known as the "5:2 diet" (where a person eats normally five days a week, then 800 calories or fewer two days a week) may help people manage their weight better. However, it is not better than other diet approaches.
Research has also found that time-restricted eating (where you eat all the calories in a day within a set period of time) can help people better manage their weight. It has other health benefits such as lowering blood pressure.
One review study also found that several different types of fasting (including intermittent fasting and bi-day fasting) can improve several aspects of metabolism.
These include improving blood sugar and cholesterol levels, reducing levels of inflammation 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 to date 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 duration of the study. For half of the study, participants ate these calories at one meal, before switching to eating their daily calories divided into three meals a day.
Each meal pattern was followed for only 11 days - not very long at all. One meal was eaten between 5 pm and 7 pm. Only 11 participants completed the study.
And when the participants ate just one meal a day, they saw a greater reduction in body weight and fat mass. However, the participants also had a greater decrease in lean mass and bone density when eating just one meal per day. This may lead to decreased muscle function and an increased risk of bone fractures if the diet is followed for a longer period.
Animal studies looking at the effects of OMAD have had conflicting results, with research showing that rats 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 is 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 included only thin young adults). It will also be important for studies to look at the effect of OMAD over a longer period of time, and for these trials to be conducted in a real-world setting.
It would also be interesting to see whether meal timing could improve results further and whether the nutritional profile of the meal made a difference.
Other negatives
If someone only eats one meal a day, it will be very difficult for them to meet all their nutritional requirements, especially for energy, protein, fiber, and essential vitamins and minerals. Not getting enough of these important nutrients can lead to a loss of muscle mass, a risk of constipation, and poor gut health.
And someone after Omad will need to make sure they get a good serving of protein, plenty of vegetables, nuts and seeds, some fruit and whole grains during their single daily meal to meet these nutritional requirements.
They'll also need a good serving of dairy products to make sure they meet calcium and iodine requirements - or a supplement or substitute if they're vegetarian.
This is not a diet that we would recommend for children, any woman who is pregnant, hoping to become pregnant or breastfeeding, and certainly not for someone who may be at risk of developing an eating disorder.
It is also important to note that while this diet may work for celebrities, they also have access to nutritionists and high-quality diets and nutritional supplements when needed. And for most of us, this type of diet may be unsustainable — and potentially harmful in the long run.
The report was prepared by Amanda Avery, Lecturer in Nutrition, University of Nottingham.
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