Fad diets are nothing new, but with the breakout success of the 5:2 diet, also called the fast diet, weight-loss hopefuls around the globe are turning to an unusual way of eating. Under the program's guidelines, dieters eat normally for five days of the week and then limit themselves to just 500 calories for women, or 600 calories for men, on the other two days. The plan is based on a concept called intermittent fasting, and supporters report dramatic weight-loss results. But does this diet really work?


The Evidence

The fast-diet trend originated in the UK, and according to that country's National Health Services website, research on fat-loss benefits is lacking. The NHS reports that in one study of 30 obese women, participants did lose weight when fasting one day a week. However, they also followed a low-calorie liquid diet plan on the other six days, and consumed just 120 calories on their fasting day.

Unfortunately, here's where one of the 5:2 diet's flaws becomes apparent: If dieters are allowed to eat whatever they want most days of the week, what's to stop them from consuming too many calories overall? For example, if a woman takes in 3,000 calories per day on her non-fasting days and just 500 calories during fasts, she would average 2,285 calories per day by the week's end--not exactly a recipe for weight loss.

Sustainability

Even if the fasting diet does work for a period of time, sustainability is iffy. It's difficult to eat just 500 calories in a day, and to follow the program for months or years is a lot to ask. The NHS states that people report insomnia, dehydration, fatigue, irritability and even bad breath during intermittent fasts, which many dieters may not be willing to put up with.

The fast diet also falls short in creating lifelong nutritional skills. When confronting weight issues, it's important to learn good eating habits. Nutrition goes well beyond mere calorie counting; eating fruits, vegetables, whole grains and lean proteins, and avoiding processed foods, are all vital factors. But when dieters are allowed to eat whatever they choose, they're likely to pick the same unhealthy fare that led to the weight problem in the first place. Under the 5:2 plan, there's nothing to stop you from eating brownies and cheese puffs all day, so long as you meet calorie requirements two days per week.

Health Benefits

Despite its flaws, there is evidence that the 5:2 plan has its merits--especially when it comes to brain health. According to research from scientists at the National Institute on Aging, fasting two days per week may significantly reduce your chances of developing Alzheimer's or Parkinson's diseases. Apparently, calorie deprivation "shocks" your brain into producing new cells, making it less susceptible to disease. In contrast, overeating may dull brain function.

The bottom line is that intermittent fasting may work for some people, and might not be harmful for healthy young adults. However, it is not a replacement for good eating habits and exercise--which, by the way, also helps preserve brain function. Weight loss still comes down to calories in vs. calories out, no matter when during the week you consume those calories.