QUESTION: I'm a slender 30-year-old woman who does not need to lose weight, but does want to eat as healthful a diet as possible. However, whenever I lower my fat intake, I find I get nervous and depressed. Is there a logical explanation for this, or is it simply my imagination?

ANSWER: No, it's not your imagination—your body and brain both need fat to function properly. Studies have linked reduced cholesterol and fat intake with depression and anxiety. Fat is especially important for a woman of child-bearing age, because normal hormone production requires fat and cholesterol. Too many people have switched to low-fat, high-carbohydrate diets in the belief that all fat is bad.

Research shows that it's the kind of fat you eat, and not simply the overall amount, that has potentially harmful effects on one's health. The most dangerous fat, scientists believe, is partially-hydrogenated (hardened) vegetable oil, which is found in margarine and many processed foods. It is called a trans fatty acid, and a high intake of it appears to be a risk factor for heart disease and possibly cancer.

When people add healthy fat—like that found in fish—to their diet, their levels of harmful cholesterol (LDLs) drop, as does their risk of heart disease. The three healthy fats that I recommend are fish oil, olive oil, and flaxseed oil. If you regularly include these in your diet, you will feel satisfied and be on your way to long-term health.