One Meal A Day (OMAD) Diet: Safety, Benefits, Risks (2024)

The One Meal a Day diet is a type of fasting. Fasting is defined as abstaining from eating food and calorie-containing beverages for a period of time.

There are two mainstream types of intermittent fasting diets––time restricted feeding (TRF) and alternate day fasting (ADF). Examples include fasting for 16 hours followed by an eight-hour eating period and fasting on alternate days. Intermittent fasting diets have one common feature—taking periodic breaks from eating.

With OMAD, only one meal is eaten for the whole day, explains L.J. Amaral, a registered clinical and research dietitian at Cedars-Sinai in Los Angeles specializing in cancer nutrition research and ketogenic and fasting-mimicking diets.

The one meal eaten on the OMAD diet is usually consumed around dinner time, says Amaral.

“You fast all day for up to 22 to 23 hours,” notes Ella Davar, a registered dietitian and health counselor based in New York City. However, non-calorie beverages such as water and coffee are allowed throughout the day, she adds.

“There are not necessarily calorie restrictions, but most people try to fit all of their [nutritional] needs in this one meal,” explains Amaral.

“Intermittent fasting has been shown to help individuals lose weight,” says Joan Salge Blake, EdD, a nutrition professor at Boston University and host of the nutrition and health podcast, SpotOn!. “However, OMAD is an extreme version. Weight loss will likely occur as calories will be restricted,” she notes, explaining it can be difficult to fit all your calorie and nutrient needs into one meal.

There are no guidelines on the type of food to eat and no food restrictions with the OMAD diet. As such, Dr. Salge Blake says “the meal can contain any food, which can make it unhealthy.” For example, she adds that a person “could feast on cookies and cakes” in one meal, making it a sugar-rich meal.

Studies specific to one meal a day or reduced meal frequency are limited. However, intermittent fasting in general has been shown to offer potential health benefits.

Weight Loss

“Fasting is an effective way to help you lose fat,” notes Davar. Indeed, time restricted eating (TRE) was noted to induce a loss of fat mass, as well as a 3% average weight loss, in a 2020 Nutrients study examining 23 clinical trials on the health effects of TRE (where the eating period is restricted to 10 hours or less with a fasting period of 14 hours or more)^[1]Adafer R, Messaadi W, Meddahi M et al. Food Timing, Circadian Rhythm and Chrononutrition: A Systematic Review of Time-Restricted Eating's Effects on Human Health. Nutrients. 2020;12(12):3770. . Interestingly, the fat loss was observed without any restriction on calorie intake.

Specific to reduced meal frequency, a 2017 Journal of Nutrition study with over 50,000 participants concluded that an overnight fast of 18 to 19 hours with a five to six-hour eating window that included two meals, ideally breakfast and lunch, may be a helpful strategy for weight control^[2]Kahleova H, Lloren JI, Mashchak A, et al. Meal Frequency and Timing Are Associated with Changes in Body Mass Index in Adventist Health Study 2. J Nutr. 2017;147(9):1722-1728. .

Beneficial Metabolic Effects

In the aforementioned Nutrients review, the following beneficial metabolic effects were observed with TRE, notably with a fasting period of 16 hours. Researchers also observed that these effects may be generated independently of calorie intake. The research suggests that nutrition impacts health not only through diet quality and quantity but also through the timing of meals.

• Decreased blood sugar level
• Improved insulin resistance
• Lower triglyceride level
• Increase in HDL (good) cholesterol
• Decreased systolic blood pressure

Anti-Cancer Strategy

Fasting may also help with cancer treatment, notes Amaral. However, while research in this area is promising, it’s still in its infancy. In a 2020 International Journal of Molecular Sciences review, researchers explain that fasting before and after chemotherapy can help with the effectiveness and tolerability of chemotherapy by encouraging the survival of normal cells while eradicating cancer cells. The authors concluded that large randomized control trials are needed to establish safety and efficacy^[3]Deligiorgi MV, Liapi C, Trafalis DT. How Far Are We from Prescribing Fasting as Anticancer Medicine?. Int J Mol Sci. 2020;21(23):9175. .

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While experts agree that there are benefits to intermittent fasting, they also note the risks of the OMAD diet and fasting in general.

The OMAD diet’s severe restriction “may lead to disordered eating and make it extremely challenging to meet daily nutrient needs for vitamins, minerals and fiber,” cautions Dr. Salge Blake.

“Fasting for more than 14 hours may cause some people to experience a higher stress response, irritability and lower blood sugar,” notes Davar.

“Low blood sugar can include symptoms such as dizziness, headaches, shakiness, elevated heart rate and intolerance to cold,” explains Amaral who goes on to say that the OMAD diet could also lead to dehydration.

Additional risks include:

• Faster weight loss than is deemed safe by experts
• Not enough calories (less than 1,200 calories)
• OMAD diet does not teach healthy eating habits
• The diet is difficult to sustain, particularly in social situations
• One meal a day is contraindicated in people with diabetes or who are on certain medications that must be taken with food during the day

Intermittent fasting in general can be beneficial for those who are living with metabolic syndrome, those with borderline diabetes, as well as people trying to lose weight, says Amaral.

Specific to the OMAD diet, Davar notes that “it may be beneficial to incorporate for one to two days a week, to see if it’s something that your body responds well to.” It may also be helpful for busy people with demanding hours or someone who’s already experienced intermittent fasting and sees the benefits, she notes. However, she recommends starting with less restrictive forms of intermittent fasting such as fasting for 14 to 16 hours with two to three meals. Always be sure to speak with a registered dietitian or medical professional before starting such a diet.

While Dr. Salge Blake agrees with the general benefits of less restrictive intermittent fasting, she does not recommend the OMAD diet specifically to anyone. “There are other ways to improve [health markers] and lose weight without such a severe restriction,” she says, going on to note that she favors meal timing. “Eating more of your calories in the day and tapering them as the day goes on can impact your weight and other [health] markers,” Dr. Salge Blake continues, citing a 2013 Obesity Society randomized study^[4]Jakubowicz, D, Barnea, M, Wainstein, J et al. High Caloric intake at breakfast vs. dinner differentially influences weight loss of overweight and obese women. Obesity. 2013; 21(12). . The benefits of meal timing are also illustrated in a newer 2021 Nutrients review^[5]Kim JY. Optimal Diet Strategies for Weight Loss and Weight Loss Maintenance. J Obes Metab Syndr. 2021;30(1):20-31. .

Dr. Salge Blake advises against intermittent fasting diets for the following groups of people.

• Growing individuals such as children and teens
• People who are pregnant or breastfeeding
• Older adults
• People with a history of disordered eating
• People with diabetes

Since fasting has the potential to affect blood sugar and blood pressure, Amaral does not recommend intermittent fasting for people on the following medications without medical supervision.

• High doses of insulin
• Certain medications that lower blood sugar
• Steroid medications
• Medications that lower blood pressure

The OMAD diet, like other intermittent fasting diets, may help with weight loss. But since it allows only one meal in a day, it can make meeting nutrient needs difficult and potentially cause adverse health effects. It can also be challenging for people who enjoy the social aspects of food such as family mealtimes and lunchtime with coworkers, adds Amaral.

Less restrictive forms of intermittent fasting with a greater eating period allowing for more than one meal or meal timing would be more feasible compared to the OMAD diet. Consult with your doctor before starting any new diet. A registered dietitian can also provide you with individualized nutrition advice, adds Dr. Salge Blake.