Intermittent Fasting For Weight Loss: Does It Work?

“Intermittent fasting is an approach that encourages eating at set times within a day or week,” notes Allison Tepper, a registered dietitian in the Washington D.C. metro area, owner of Tepper Nutrition and an adjunct instructor in the department of health studies at American University. Outside of those set times, food (and sometimes liquids) are restricted.

The exact eating-and-fasting pattern that someone follows with intermittent fasting can vary. “The most common are 16:8 (eating during an eight-hour time period each day, fasting for the other 16) or 5:2 (eating normally [i.e., not a restricted caloric intake] five days a week, and eating only around 500 calories for the other two days),” says Jen Bruning, a registered dietitian nutritionist and spokesperson for the Academy of Nutrition and Dietetics.

However, because intermittent fasting can be so flexible (for instance, an individual can fast for 12 hours and then eat for 12 hours, or fast for 14 hours and then eat for 10), it’s a good idea to speak with a registered dietitian about the best type of intermittent fasting for you and your lifestyle.

When it comes to intermittent fasting and weight loss, results vary, says Bruning. “In some studies, weight loss was observed in participants following intermittent fasting, while in other studies it was not,” she says.

For instance, one 2021 review of existing research on intermittent fasting concluded that not only can this diet lead to clinically significant weight loss, which it defined as a loss of more than 5% from baseline, it may also improve elements of cardiovascular health, including blood pressure, insulin resistance (in individuals with prediabetes and diabetes) and markers of oxidative stress. Further, the study concluded that intermittent fasting is safe, with few reports of adverse effects stemming from the diet.^[1]Varady KA, Cienfuegos S, Ezpeleta M, et. al. Cardiometabolic Benefits of Intermittent Fasting. Annual Review of Nutrition. 2021;41:333-361.

But as Bruning notes, intermittent fasting does not always result in weight loss. “If a person ends up eating fewer calories than their body needs due to the time constraints of the diet, then they may lose weight,” she explains. “However if they eat more than what their body needs [during eating windows], they may gain.”

Indeed, a study from 2017 concludes that alternate-day fasting (meaning consuming approximately 25% of caloric intake needs on days when you’re fasting and then 125% of those needs on alternating days when you’re considered “feasting”) did not lead to better results in terms of losing or maintaining weight in comparison to caloric restriction. Further, the study noted that in the trial, participants’ dropout rate was higher in the alternate-day fasting group compared to the control group and the group that restricted their daily calories, raising questions about how sustainable the practice of intermittent fasting is.^[2]Trepanowski JF, Kroeger CM, Barnosky A, et al. Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults: A Randomized Clinical Trial. JAMA Intern Med. 2017;177(7):930–938.

While a 2018 study also showed that intermittent fasting may not lead to weight loss, it did identify other health benefits. In this study, a small group of men with obesity and prediabetes did not lose weight after intermittent fasting over the course of five weeks, but they did have lower levels of insulin, lower blood pressure and significant improvements in their insulin sensitivity, as well as a notably decreased appetite.^[3]Sutton EF, Beyl R, Early KS, et. al. Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes. Cell Metabolism. 2018;27(6):1212-1221.e3.

Because of the varied results, Bruning notes that intermittent fasting for weight loss may make sense for those “who really struggle with late-night snacking for reasons other than hunger,” as intermittent fasting “could help to curb those tendencies.”

Interestingly, certain studies suggest that the timing of when you are eating versus fasting can impact results. One study published in 2022 found that when it comes to adults living with obesity, eating the same number of calories later in the day as opposed to earlier in the day led to people being hungrier, burning calories at a slower rate and having their fat tissue store more calories, thus increasing their obesity risk.^[4]Vujović N, Piron MJ, Qian J, et. al. Late isocaloric eating increases hunger, decreases energy expenditure, and modifies metabolic pathways in adults with overweight and obesity. Cell Metabolism. 2022;34(10):1486-1498.

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Intermittent fasting may be appropriate for certain populations.

Anyone with a history of disordered eating may want to reconsider this type of diet, notes Tepper. “Any type of diet program, especially one that requires some form of restriction, can impact the relationship with food and increase the risk of developing disordered eating,” she says.

People with certain health conditions or nutritional needs may also want to consider another approach. According to Bruning, some other individuals who may want to speak with their doctor or a registered before trying intermittent fasting include:

• Individuals living with a serious health condition
• Individuals who need to take timed medications with food
• Older adults who find getting enough calories challenging
• Individuals with diabetes who take insulin

Additionally, people who are pregnant or lactating as well as children who are still growing should steer clear of intermittent fasting, says Bruning.

Even if you don’t fall within any of these groups, there are downsides to intermittent fasting worth noting. “For many, eating regularly is a crucial part of their daily routine to encourage adequate energy levels, blood sugar management, adequate nutrient intake and a positive relationship with food,” says Tepper. “Restricting timing of food intake can negatively impact energy levels and concentration, or cause headaches or nausea.”

If you’re interested in learning more about intermittent fasting and how it may work for you, speak with your doctor or a registered dietitian.