Low-fat diets were all the rage in the '90s, and people still seem to want to adhere to the all-or-nothing philosophy of dieting. But moderation pays off when you're devising a healthy plan for eating. Too little fat can be detrimental to many healthful goals. So how little is too little fat? The American Heart Association recommends that 30 percent of a healthy adult's daily calories come from fat. In fact, if you go lower than 20 percent, experts say it can hurt your health by limiting the absorption of some vitamins, like vitamins A, D, E and K.

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A deficiency of essential fatty acids, like those found in flaxseed and fatty fish, has been linked to depression and other mental disorders. Heart disease and risk of some types of cancers increase when fat is severely limited. Research shows that a high intake of omega-3 fatty acids limits prostate tumor growth and inhibits some cancers. Too little fat increases levels of "bad" LDL cholesterol and inhibits HDL, or "good" cholesterol from helping to excrete cholesterol.