Still unsure about whether to buy skim milk or regular? Here’s a guide to help you know when to choose low- or full-fat foods.

Three Times To Go Low:
1. You’ve got cancer in your family. A study published in the Journal of the National Cancer Institute shows that women who limited their fat intake to 24 percent of their total daily calories were 40 percent less likely to develop ovarian cancer than women who got approximately 39 percent of their daily calories from fat.

2. You’ve got arthritis. According to researchers from Scotland’s Centre for Rheumatic Diseases, replacing saturated fats with fruits, veggies, and beans significantly reduces pain associated with rheumatoid arthritis.

3. You’ve got insomnia. Getting more than 30 percent of your total calories from fat can throw off your body’s internal clock. Researchers from Northwestern University in Evanston, Illinois, found that a high-fat diet can alter your daily sleep-wake cycle and lead to binge eating and late-night refrigerator raids.

Three Times To Go Full:
1. You need to improve your “good�?cholesterol. A study published in the American Journal of Clinical Nutrition found moderate fat consumption (think heart-healthy fats like olive oil and avocados) had favorable effects on HDL, the good cholesterol, which lowers your risk of cardiovascular disease.

2. You’d like to build more muscle. After 24 weeks of eating three full-fat dairy foods a day, participants of a study published in Obesity Research had gained noticeably more lean muscle mass. Those who ate more than three servings of full-fat foods also had significant decreases in blood pressure.

3. You’re trying to have a baby. Research from the National Center for Biotechnology Information at the U.S. National Library of Medicine shows that not getting the daily recommended amounts of fat can reduce a woman’s likelihood of conceiving.