Think twice before reaching for that fat-free cookie or sugar-free ice cream bar as an afternoon snack, Cleveland Clinic reports.

In most cases, you’re better off having the real thing in moderate portions, says Kristin Kirkpatrick, MS, RD, LD, wellness manager at Cleveland Clinic’s Wellness Institute.

We Americans love our sugar and fat, and for good reason. Studies have shown that sugar opens up areas in the brain and can either energize people or make them feel calmer. Similarly, fat leads to feelings of satisfaction and a sense of fullness, Ms. Kirkpatrick says.

Take reduced-fat peanut butter, for example. Manufacturers typically use fewer peanuts in it than in full-fat peanut butter. Instead, they add more sugar, corn syrup or honey. In some cases, they also include a soy protein to compensate for the loss of protein.

Like many other foods featuring healthy fats, Peanut butter in its original form is a wonderful, satisfying product that is packed with protein and can even help reduce heart disease risk factors, Kirkpatrick says. Taking away the main protein source decreases that benefit. And adding more sugar increases the likelihood of rapid increases and drops in blood sugar.

“This roller coaster up and down — if this is the norm for your diet, it can lead to inflammation in the body and chronic disease,” Ms. Kirkpatrick says.