Ever wonder why some people swear by a low-carb diet while others claim only a low-fat diet works for them? According to fitness trainer and author of The Diet Solution, Isabel De Los Rios, the reason is all about your body’s metabolic type.

The Diet Solution program is a holistic diet and lifestyle plan that combines foods matched to your metabolic type along with a mind-body connection.

“Most diets don’t work because people eat the wrong foods. When you start eating the right kinds of natural foods specific for your metabolic type, you will start to lose weight, enjoy beautiful skin, and feel more energized,” says De Los Rios, who created The Diet Solution after years of helping people lose weight.

“It is a completely natural way of losing weight that works and empowers nonbelievers that it is possible to lose weight,” she says.

This is not a quick fix diet with fast results; instead expect to lose one to two pounds per week, which is the amount recommended by most diet and nutrition experts.

The Diet Solution: The Theory

The Diet Solution is based on the theory that losing weight starts with an understanding of your body’s metabolic type, calorie requirements, and specific amounts of carbs, protein, and fat in your meals.

According to De Los Rios, of The Diet Solution everyone falls into one of three metabolic diet types – protein, carb, or mixed -- and if you eliminate the foods that don't suit your metabolic type, you will lose weight.

"Eating refined carbs will add fat to some people while having little impact on others, so if you avoid the foods according to your type, you will lose weight," De Los Rios says.

The The Diet Solution  also makes assertions beyond weight loss and claims to control blood sugar levels, blood pressure, and cholesterol; improve other health-related conditions, improve skin, and increase energy levels.

The Diet Solution: How It Works

First, you determine your metabolic type by filling out a questionnaire. Next, you calculate your calorie goal. You then follow the six-week meal plans that outline food choices for each meal and snacks.

Customize the meal plans to your type by adding more protein or carb to satisfy your metabolic type. If you are a mixed type, the meal plans require no modification.

Regardless of your metabolic type, The Diet Solution encourages you to eat three main meals and two snacks and take an omega-3 supplement.

Diet Solution plans with portion sizes guide your food choices once you have determined your metabolic type. Dieters are encouraged to keep a food journal to record daily intake of foods.

There's no calorie counting or exercise regimes. Meal plans guide your food choices. Exercise is briefly addressed with a recommendation to include strength and cardiovascular training.

A quick-start guide, daily meal plans, recipes, shopping lists, frequently asked questions, and a journal for tracking success are included in the e-book The Diet Solution (a file you download after paying for it) that comes with a money-back guarantee.

The Diet Solution: What You Can Eat

Foods allowed are limited to whole natural organic foods, mostly healthy meats, fish, vegetables, seeds, nuts, and raw high-fiber fruits.

“Most people do best when they get all their carbs from fruits and vegetables and totally eliminate grains so they are mostly excluded, but there is an option to include them,” De Los Rios says.

She also takes issue with dairy unless it is raw and organic, artificial sweeteners except Stevia, processed foods, sugars, sweets, most breads, pasta and grains, sweetened drinks, fruit juices unless freshly squeezed, and soy, and more.

The Diet Solution: Expert's View

Nutrition experts agree about the importance of good intentions, motivation, and staying positive while dieting. But beyond that, The Diet Solution falls short of evidence for many of its claims.

“It is based upon unsubstantiated, unscientific, and potentially dangerous advice. Therefore I would not recommend the program,” says American Dietetic Association spokeswoman and Washington, D.C.-based nutrition counselor Katherine Tallmadge, MA, RD.

Tallmadge adds that the author is not a nutrition expert qualified or licensed to practice and her sources are not from peer-reviewed scientific journals.

On the positive side, Tallmadge agrees with De Los Rios of The Diet Solution that you need to focus on the source of calories to lose weight. But Tallmadge says it has nothing to do with metabolic typing.

Everyone needs to eat the right balance of carbs, protein, and fat at meals.

“There are so many inaccuracies in the book that are potentially dangerous and misleading," says Tallmadge, who is the author of Diet Simple. "From false statements claiming foods which contain ingredients that can’t be pronounced are ‘toxins’ to suggesting that organic foods foster weight loss – these are simply not true."

Tallmadge encourages her clients to eat more ‘natural’ foods, but her definition of ‘natural’ is not as rigid or inflexible as De Los Rios and allows healthy foods that are frozen, packaged, or not organic.

The Diet Solution: Food for Thought

It is hard to believe that everyone fits into three metabolic diet types, especially when determined by a questionnaire.

The reason you will lose weight on The Diet Solution is because it is low in calories, contains plenty of lean protein, and fiber --not necessarily because of your metabolic type.

Most recommended foods in the Diet Solution are super healthy. But be warned: Not all information in the book is grounded in scientific evidence. For instance, raw organic eggs are not safe to eat, and raw unpasteurized dairy can be a health hazard and is not recommended.

The Diet Solution plan is rigid, very restrictive, costly, potentially deficient in calcium, and may be hard to follow long term.

If you are looking for a well-balanced low-calorie weight loss plan that you can follow long term, this has benefits and drawbacks but over all The Diet Solution has merit.

Kathleen Zelman, MPH, RD, is director of nutrition for WebMD. Her opinions and conclusions are her own.