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Work out regularly? Eat dairy, which contains casein, the optimal protein to prevent muscle breakdown, and whey, which helps promote muscle growth. Whole milk, yogurt, and cheese are high in saturated fat and cholesterol, so opt for 1 percent or skim, and low-fat or non-fat yogurt. Look for reduced-fat varieties of cow and goat cheeses, and stick to one ounce – about the size of two nine-volt batteries. Choose nonfat Greek-style yogurt, which has 18 grams and a consistently high dose of probiotics to help promote digestion and gut health. A six-ounce container of plain, low-fat yogurt has only 9 grams of protein and, on average, fewer probiotics. Protein power: Excellent.