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There are dozens of diet formulas promising to help you shed excess fat. Cut the wheat, eat for your blood type, combine foods properly and eat like a caveman are just a few popular prescriptions for weight loss, many of which offer frustrated yo-yo dieters renewed hope. But do the theories hold up?

Earlier this month, the blood type diet came under fire by researchers from the University of Toronto. Their findings: Your blood type has nothing to do with how well you'll do on the diet.

The diet, made popular in 1996 by naturopathic physician Peter D'Adamo's best-selling book Eat Right For Your Blood Type, proposes that your blood type (A, B, AB, O) determines your "susceptibility to illness, which foods you should eat, and how you should exercise."

The theory goes that our blood type reveals the dietary habits of our ancestors. Type Os were hunters and gatherers so people with type O blood are advised to eat animal protein (animal foods speed up metabolism) and avoid wheat (gluten causes weight gain in type Os), lentils and kidney beans (both legumes make your muscles "less charged" for exercise).

Type As are said to do best on a vegetarian diet that eliminates meat, dairy, kidney beans and lima beans. Historically, type Bs were nomads so people with Type B blood can eat a more balanced diet that eschews wheat. The plan for Type AB blood is a blend of Type A and Type B dietary advice.

Following your blood type diet promises to help you stay healthy, live longer and achieve your ideal weight. If you eat foods incompatible with your blood type, the author contends your digestion will suffer and your metabolism will slow down.

To put the blood type theory to the test, University of Toronto researchers analyzed the usual diets of 1,455 healthy young adults and calculated a score indicating how closely their diets matched one of the four blood type diets (A, B, AB or O).

Blood tests were used to determine each participant's blood type as well as cardiovascular risk factors such as cholesterol, triglycerides and insulin. Body mass index (BMI), waist circumference and blood pressure were also measured.

Certain blood type diets were found to have beneficial effects. Participants who adhered closely to a Type A diet had lower BMIs, waist circumferences and LDL cholesterol, triglycerides, glucose and insulin levels compared to those whose diets didn't closely match the Type A plan. Eating a diet that closely resembled a Type AB plan was also linked with some of these positive effects.

But here's the catch: blood type had nothing to do with these favourable effects. Participants did well if their usual diets closely matched the Type A and Type AB dietary advice whether their blood type was A, B, AB or O.

It's disappointing (at least to me) that the diet's entire premise (one that sold more than seven million books worldwide) – your blood type dictates what foods you should and shouldn't eat – is false. One the other hand, it's encouraging that the blood type diet (Type A and AB plans) appears to live up to its promise of helping people stay healthy and achieve a healthy weight (provided, of course, you can stick to it for the long term).

Then again, most types of calorie-reduced diets are effective at shedding pounds and improving risk factors for heart disease and diabetes for as long as you follow them.

A 2009 study published in the New England Journal of Medicine assigned one of four different weight-loss diets varying in amounts of protein, carbohydrate and fat to overweight adults. After two years, each diet resulted in meaningful weight loss and improvements in blood cholesterol and insulin levels. (Participants attended regular group classes, which helped keep them motivated to stay on track.)

When it comes to weight loss, your best chance for long-term success is to find a plan that's sustainable – one that's not too restrictive and matches your lifestyle and food preferences.

Weight-loss tips

To determine if a weight-loss plan is right for you, ask the following questions:

Does the program exclude an entire food group? If it does, the diet will be lacking certain nutrients and a supplement may be required. A sound weight-loss program should be balanced and include foods from all four foods groups.

Does the program rely on specially purchased foods? A program based on prepackaged meals requires little effort, but it doesn't teach you how to eat.

Does the program promote a loss of one to three pounds per week? Rapid weight-loss programs can be unsafe and should be medically supervised.

Does the program offer strategies to help you keep the weight off?

Does the diet promote a way of eating that's good for your kids? If it's not healthy for them, it's not healthy for you.

Ask yourself if you are going to feel comfortable eating this way for the rest of your life. Successful weight loss requires long-term change. It won't be effortless, but the rewards will be worth it.

Leslie Beck, a registered dietitian, is based at the Medisys clinic in Toronto. She can be seen every Thursday at noon on CTV News Channel's Direct

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