Fad Diets: The 5:2 Diet

Intermittent fasting, or restricting calorie intake to a certain number of hours per day, has become pretty popular lately. A version of intermittent fasting called the 5:2 diet entails restricting calories (“fasting”) for two days a week, while eating a normal amount on the other five days.

Seems reasonable, right? Just count calories two days a week and, on those days, aim for a total of either 500 kcal (women) or 600 kcal (men), roughly a quarter of the caloric needs for healthy adults. The other five days, eat normally.

But…why?

According to the official 5:2 diet website, the purpose of this diet is…weight loss! Supposedly, this eating plan promotes gradual and sustainable weight loss as well as improved blood pressure, cholesterol levels, and insulin sensitivity.

What does science say?

Very few studies have been published on the 5:2 diet. While PubMed.gov lists over 250 articles on “intermittent fasting” and almost 70 studies on “alternate day fasting” (more of a 4:3 diet), only one human study has been published specifically on the 5:2 diet. This study compared the impact of a 5:2 diet (restricting calories two days a week) with a “continuous” calorie-restricted diet (i.e. consuming about 1500 kcal/day or 75% of caloric needs every day) on weight loss and risk markers of metabolic disease in 107 overweight women ages 30 to 45 over a six-month period. While most subjects adhered to the diets, the research team found that the 5:2 diet was no easier for participants to follow than the continuous calorie restriction diet, and the groups on both diets lost similar amounts of weight (~5%) and had similarly improved insulin sensitivity.

Since it’s not a good idea to draw conclusions from a single study, it’s safe to say that the verdict is still out on the effects of a 5:2 diet.

While the claims of benefits from this diet are surprisingly moderate, in comparison to a lot of other trendy diets out there, it’s good to keep a few things in mind:

1. Caloric deficits over time are almost always required anyways for healthy people to lose weight.

From the standpoint of scientific evidence, though, we simply do not know whether being at a deficit for two days a week instead of continuously is any easier or more effective.

2. Maintaining a healthy weight is beneficial for blood pressure, cholesterol levels, and insulin sensitivity, 5:2 diet or not.

These benefits are not unique to weight loss from a 5:2 diet. Weight management decreases your risk for high blood pressure, high cholesterol, and insulin resistance, no matter what kind of eating plan you prefer. 

3. The gradual weight loss (about 1lb/week) promised by the 5:2 diet is also the rate recommended for sustainable weight loss.

Both the 2015-2020 DGA and the CDC recommend a continuous caloric deficit, or decreasing daily caloric intake by 500 to 1000 kcal per day, as a means to healthy weight loss.

So, did you try it?

Full disclosure: this post was supposed to be a Try It Tuesday where I would “test drive” the 5:2 diet for one week. I even had an eating plan for my 500 kcal days all mapped out!

But when I started thinking about all the things I wanted to accomplish on those two days, I seriously doubted my ability to get it all done on such a small amount of energy. For me, this level of restriction was too much, even for just two days. If I were looking to lose weight, decreasing my calorie intake every day would be a better option.

While there is little scientific evidence supporting the efficacy of this diet, its approach to weight loss as a gradual process is reasonable and realistic. Sustainable weight loss occurs over time by burning more calories than you consume.

Julie Hess, IFIC 's Sylvia Rowe Fellow contributed to this article.

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