When it comes to weight loss, it's apparently not as simple as "calories in, calories out" or "eat less, move more."

What you eat matters as much as how much you eat.

Over a 20-year period, Harvard School of Public Health researchers studied data on the eating and exercise habits of 120,877 healthy American men and women in their 30s, 40s and 50s. They found there are striking differences in how various foods and drinks impact whether people gradually get fatter.

Translation: It's not age that's packing on the pounds. It's those french fries you can't resist.

"The conventional wisdom is simply, 'Eat everything in moderation and just reduce total calories' without paying attention to what those calories are made of," study leader Dariush Mozaffarian told The Washington Post. "All foods are not equal, and just eating in moderation is not enough."

Eat french fries every once in a while, wash them down with a sugary soft drink and before you know it, you've gained a pound or two a year. Fast forward 20 years, and that's a lot of weight.

The worst food for weight gain is potatoes--in any form. French fries and potato chips are the worst since they're cooked in fatty oil, but boiled, baked and mashed aren't much better. Why? It's probably their effect on the hormone insulin. That is, eating potatoes causes blood sugar and insulin to spike, which makes you feel less satisfied and could cause you to eat more.

But just as some foods encourage weight gain, there are others that seem to help you remain slim.

Top 10 foods that make you gain weight:
1. French fries
2. Potato chips
3. All potatoes
4. Sugar-sweetened beverages
5. Unprocessed red meats
6. Processed meats
7. Trans fat
8. Sweets or desserts
9. Refined grains
10. Fried foods

Top 8 foods that help you stay slim:
1. Yogurt
2. Nuts
3. Fruits
4. Whole grains
5. Vegetables
6. Diet soda
7. Whole-fat milk
8. Low-fat dairy foods

The study findings were published in the New England Journal of Medicine.

--From the Editors at Netscape