The appropriately-dubbed "Fast Diet" (also referred to as the "5:2 Diet" or the "Feast and Famine Diet") claims to help you drop pounds while allowing you to stuff your face, guilt-free, as long as you only do that five days a week and then force yourself to function on a mere 500-600 calories for the other two days. Generally, on a usual day of fasting, followers of the diet eat two very small meals, each containing around 250-300 calories, based on gender (women are allowed roughly 500 calories on a fasting day and men get roughly 600 calories).
It's suggested that you focus on getting more plant-based foods and protein on fast days, and that you plan your fast days to be non-consecutive. Proponents of the diet claim that after you fast for several hours, your body starts to alter the type of fuel you burn, shutting off your body's ability to store fat. People who follow this diet claim it actually helps them reduce their appetites.
The creator of the diet, Dr. Michael Mosley, believes in the diet so much that he's written a best-selling book about it, simply titled "The Fast Diet". Dr. Mosley says that following the diet helped him lose almost 20 pounds, reduce his body fat by eight percent and significantly lower his cholesterol and blood sugar levels in nine weeks. However, nutrition experts warn that these benefits are only seen if people eat healthy during those five non-fasting days. If you consume too many calories during non-fasting days, your average daily caloric intake won't budge, and neither will the bulge.
What Does the Research Say?
Preliminary research has suggested that intermittent fasting may aid in weight loss, decrease the risk of certain cancers, lengthen lifespan, cut the risk of developing diabetes, increase energy and help the brain function better. However, these conclusions were drawn from results of animal studies (on rats) and not from studies on humans. Health experts in Britain, where the diet is all the rage, warn there are large gaps in the scientific evidence. Researchers claim they're going to begin studying the effects of intermittent fasting in humans soon.
To date, there have only been a few controlled studies done on humans. One study of alternate-day (intermittent) fast dieting done on humans was conducted at the University of Illinois, which found that fasting was correlated with reductions in bad cholesterol (LDL) and triglycerides and decreased blood pressure. Results showed that participants in that study did not overindulge drastically on non-fasting days, consuming only an additional 10 percent. No long-term studies (more than a year) have been conducted yet.
IT'S NOT JUST THE "FAST DIET" THAT MAY NOT WORK... ALL DIETS MAY NOT WORK. FIND OUT WHY!
Could This Diet Be Dangerous?
Experts in the fields of nutrition and health worry that intermittent fasting diets can lead to nutrient deficiencies (particularly of vitamins and minerals) and possible disordered eating patterns, especially for those individuals who take the diet to dangerous extremes. Experts agree that this type of diet will discourage people from having a healthy relationship with food and making the kinds of lifestyle changes that lead to long-term weight loss.
Additionally, the "Fast Diet" creator himself, Dr. Mosley, issues a warning to avoid this diet if you're pregnant, under 20 years old, are underweight (BMI of less than 18.5) or have a history of an eating disorder. Other health experts offer the additional warning to steer clear of this diet if you've had iron or calcium deficiency, are diabetic, or taking certain meds.
The Bottom Line
There have only been a couple studies performed on humans regarding this fasting diet--all other studies were done using animals. Most of the supposed health benefits of the "Fast Diet" are anecdotal in nature, simply based off of followers' personal experiences and not from sound, scientific research. Until this diet is studied extensively in humans and found to promote health without any adverse effects, health experts can't recommend it.
This controversial diet encourages erratic eating behaviors and it centers around the notion that we should ignore our own internal hunger cues. Additionally, this eating pattern doesn't seem like one that could be maintained long-term, especially if you plan to keep friends and family around, which would prove hard to do if you're crabby all the time because of the extremely low amount of calories you must stick to for several days. I don't know about you, but I'd rather not feel completely miserable for two full days just to lose weight. It hardly seems like a diet I'd want to stick to for the rest of my life.
Lastly--and perhaps most importantly--this diet permits you to eat dangerously high amounts of heart-clogging saturated fat if you feast on foods such as cheeseburgers, fries, cake--you name it--because technically nothing it off-limits.
Kari Hartel, RD, LD is a Registered, Licensed Dietitian and freelance writer based out of St. Louis, MO. Kari is passionate about nutrition education and the prevention of chronic disease through a healthy diet and active lifestyle. Kari holds a Bachelor of Science in Dietetics from Southeast Missouri State University and is committed to helping people lead healthy lives. She completed a yearlong dietetic internship at OSF St. Francis Medical Center in Peoria, IL, where she worked with a multitude of clients and patients with complicated diagnoses. She planned, marketed, and implemented nutrition education programs and cooking demonstrations for the general public as well as for special populations, including patients with cancer, heart disease, diabetes, Alzheimer's disease, obesity, and school-aged children. If you would be interested in working with Kari one-on-one, sign-up for FitDay Dietitians.