Diet 101: The Fast Diet

This diet became all the rage after it aired on BBC during the 2012 London Olympics. But is fasting the healthiest way to lose weight, stay healthy and live longer?
cottage cheese
This diet became all the rage after it aired on the BBC during the 2012 London Olympics, and The Fast Diet book has become a best-seller. But is frequent fasting the healthiest way to lose weight, stay healthy and live longer?

The authors of The Fast Diet are Dr. Michael Mosley and Mimi Spencer. Dr. Mosley was trained as a doctor in London, and over the past 25 years he has made numerous history and science documentaries for the BBC (British Broadcasting Station). Mimi Spencer has spent the past 25 years writing about fashion, food and body shape for numerous national newspapers and magazines.

The theory behind the diet is that our ancestors lived through periods of feast or famine and that's how our bodies were programmed. In today's society, we snack too often, which the authors blame on a snacking industry that over-promotes its goodies.

By following a two-day-a-week fast (dubbed the 5:2 approach) consisting of 500 to 600 calories on each day, followers of the diet are promised weight loss, a longer life, a decreased risk of cancer and heart disease and an overall healthier life. The creators also promise 10 pounds of fat loss over 10 weeks, plus the ability to maintain the lost weight over a long period of time.

The Plan

The diet consists of fasting on two non-consecutive days each week. On each of the fasting days, women consume 500 calories and men consume 600 calories. When to eat depends on what works best for you; Dr. Mosley splits his calories 50:50 between breakfast and dinner while Mimi Spencer splits them between two meals and several snacks.

During fasting days, it's recommended that dieters choose foods with a low glycemic index and low glycemic load to help them feel more satiated. These include vegetables, legumes, nuts, seeds and some fruit. Broth-based soups and miso are recommended over creamy ones. On the non-fasting days there are no guidelines; dieters can eat whatever they'd like.

Although there's no set eating plan, the authors provides tips to help make fasting days easier, such as fasting with a friend, preparing fast-day food in advance, staying hydrated with a favorite no-calorie drink and reading the food labels for the appropriate portion sizes. Supplements aren't recommended, as adherents of the plan should be choosing a variety of foods on fast days. Dieters are also encouraged to continue with their regular exercise regimen on these days. Those who reach their target weight are encouraged to adopt the Maintenance Model, which consists of only one fast each week.

Sample Fast Day

  • Cottage cheese
  • 1 pear
  • 1 fresh fig

Total: 142 calories

  • Salmon and tuna sashimi with soy sauce, wasabi and pickled ginger
  • 1 tangerine

Total: 352 calories

Daily total: 494 calories
The Costs

The Fast Diet book costs about $24 with the cookbook costing about the same amount. You can also get information from The Fast Diet website where you can find forums, meal plans, tips and inspiration. Other costs are ... the lack of enjoyment of food twice a week! In addition, you'll be pretty hungry on 500 to 600 calories.

The Good
  • No food is off-limits.
  • Exercise is encouraged.
The Not-So Good
  • The few calories you'll be consuming twice a week will make it impossible to meet your nutritional needs.
  • Eating so few calories will make you feel hungry, and you'll probably experience uncomfortable side effects, like headaches.
  • The feasting-then-famine days disrupt metabolism and are probably not the best way to shed pounds.
  • This diet resembles a quick fix or crash diet.
  • It's tough to follow for the long haul.
  • It's a tough plan to follow if you have a family and will be doing most of the cooking.
  • It's a tough plan to follow during parties or social events (such as business dinners).
  • There is a lot of controversial scientific information and studies that this plan is based on.

The Bottom Line: Part of living life is learning to enjoy food in moderation every day, not just 5 days a week. Although this may seem like an easy way to lose weight, it's not. Plus, the safety and long-term consequences of losing weight by fasting are questionable.

TELL US: Have you tried the British Fasting Diet?

Toby Amidor, MS, RD, CDN, is a registered dietitian and consultant who specializes in food safety and culinary nutrition. See Toby's full bio »

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