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As a lifelong nutritionist, I know that people sincerely want to be healthy and lose weight. But they are confused about how to get there, and a concrete diet plan seems like the answer. Quick weight loss sweetens the pot.

Sometimes a new eating style can be a kick off to better health, but fad diets (or crash diets that promise a speedy slim down) can also be dangerous. They often restrict nutrients to a point of harming your body and distorting your mind. Not only are they harmful in the short and long run, but every fad diet is a missed opportunity to work on lasting change.

Since diets have passionate creators and it's hard not to fall for their zeal, I use these red flags to tell whether a diet is harmful or helpful. I stay away from diets that:

1. Promise quick results with little or no effort

2. Promise weight loss of more than two pounds a week

3. Eliminate one or more food groups

4. Recommend eating a single food over and over

5. Base evidence of effectiveness on testimonies from other dieters

6. Are written or advertised by a celebrity

7. Use supplements or pills as part of the plan

8. Show little or no scientific evidence that the methods used by the diet work

9. Do not include suggestions to consult with your doctor or a registered dietitian

Here is my take on some popular diet plans:

Paleo diet: Mainly meat, fish, vegetables, fruit, roots, and nuts. No grains, legumes, dairy products, salt, refined sugar, or processed oils. Scant scientific evidence. Restrictive, but with meticulous planning, it is an eating style, not a fad diet.

Raw diet: Organic fruits, vegetables and their juices, nuts, seeds, sprouted grains, legumes, plant oils, sea vegetables, and herbs. No animal foods or anything heated above 115 Fahrenheit. Little scientific evidence. Restrictive, monotonous, too extreme.

Atkins diet: High-protein and high-fat foods over carbohydrates (plant-based foods). Eating plan backed by science. Eliminates food groups, but low-carb eating is an eating style, not a fad diet.

Cabbage soup diet: Cabbage soup three times a day for a week to lose 10 pounds. Need I say more?

Master Cleanse diet: Drink nothing but a concoction of tea and lemon juice mixed with maple syrup and cayenne pepper for a week. Promoted by Beyonce, Ashton Kutcher, and others. No scientific backing. Extremely restrictive.

Intermittent fasting: Five days of eating normally, two days of "fasting" on 500 calories. Severely restricted on fasting days. Sketchy scientific evidence. (It's better to follow your hunger/satiety signals.)

By Mary Hartley, R.D. for