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Start by staying away from foods with added sugar. Bacteria just love sugar, and bacteria cause acne. Also, a number of studies have indicated that low-glycemic index (GI) foods can reduce the severity of acne.  Low-GI foods are those foods that release sugars into your blood more slowly. The lowest GI foods are:  Bran cereals, natural muesli, rolled oats Whole wheat, pumpernickel, whole grain breads Most vegetables except beetroots, pumpkin, and parsnips Nuts Most fruit except watermelon and dates.  Mango, banana, papaya, pineapple, raisins, and figs have medium GI. Legumes and beans Yogurt Whole grains are low- to medium-GI. The lowest GI are found in brown rice, barley, and whole grain pasta. Studies have shown a small link between dairy and acne. If you eat a lot of dairy and have forehead acne, try reducing the amount of dairy in your diet. Vitamin A helps boost your immune system and is a natural antioxidant. Vitamin D helps boost the immune system, helps reduce inflammation, is anti-microbial, and helps reduce oil production. The best way to increase these vitamins is through food.  Foods high in Vitamin A include vegetables, like sweet potato, spinach and other dark leafy greens, carrots, pumpkin, broccoli, red peppers, summer squash; fruit such as cantaloupe, mangoes, and apricots; legumes, meat, liver, and fish.  Foods high in Vitamin D include fish, like salmon, tuna, mackerel, and cod liver oil; eggs, mushrooms; and oysters. Many foods are also fortified with vitamin D.  You can also get Vitamin D through natural sunlight, as this triggers your body to produce the vitamin. Spend about 10 – 20 minutes outside without sunscreen each day. If your skin is darker, spend more time in the sun. Otherwise, be sure to practice sun safety by wearing a broad spectrum, SPF 30 sunscreen, a wide-brimmed hat, and covering up as much as possible. You can also take a D3 supplement. Omega-3 fatty acids are believed to help control oil-producing molecules. You can find omega-3s in food. Seeds and nuts, like flaxseeds and flaxseed oil, chia seeds, butternuts, walnuts, are good sources. Fish and fish oils, found in salmon, sardines, mackerel, whitefish, and shad, also also very effective. Avocados are also a great source. You can also take a supplement.
Reduce sugar. Reduce dairy. Get more Vitamin A and D. Eat foods high in omega-3s.