Calcium is known best of all as a promoter of healthy and strong bones. But it plays a critical role in performance as well. Too little calcium and you can't relax your muscles. Bad relaxation, makes for less potential energy and so on and ultimately for less than satisfactory contraction. Bad contraction equals bad workouts and no growth. Get the picture? By improving relaxation you avoid cramping, injury and twitching.
Calcium is also very important in fat metabolization, so no cigar for the people who claim milk is fattening. It makes for less fat storage and better use of present macronutrients in the blood. Using the recommended FDA dosage has shown to decrease body-fat significantly in obese people. It's no secret that the RDV (recommended daily values) were established back in the 60's and 70's for a 2000 calorie diet. I don't know many bodybuilders that grow on a 2000 calorie diet. As a matter of fact I know very few who go below that when dieting. Also the amounts are outdated, so instead of the recommended 1000 mg you should be closer to 2000-2500 mg of calcium daily. Milk can help you get a large part of that. And it's safe to use on a diet too, because of the high calcium content and the not so fattening simple sugar lactose that makes up the carbohydrate portion of milk.
And last but not least calcium lowers the blood pressure, may reduce stress and definitely improves the quality of sleep. If that doesn't sell you on milk, I don't know what will. But there is more...
I would just like to add that calcium is the transport means of large number of amino acids and creatine. It has been an issue that combining large amounts of protein with creatine can hinder the effect of last-named nutrient. Though it is best to wait half an hour or so in general, there are exceptions. After a workout and with plenty of carbs the issue is non-existent. Because of the importance of calcium to the bodybuilder it is of utmost importance to respect the calcium need of the body and supply adequate vitamins for its proper functioning. Calcium does not react well to Phosphorus, Sodium, coffee and white flour. All of them will eliminate Calcium. Too much of a certain nutrient (be it protein, fats or carbs) can inhibit absorption. Tetracyline will bind to calcium ions and block the absorption of both substances.
Deficiency: Chronic dietary deficiency can cause osteoporosis. In younger people the consequences will be smaller accrual of bone mass and in older people it may mean a significant decrease in total bone mass. Brittle bones are the result. Hypertension and cancer are two modern diseases that can find calcium deficiency as a cause. Symptoms of deficiency can be, but are not limited to, aching joints, high LDL cholesterol, insomnia, irregular heartbeat, eczema, muscle cramps, convulsions, depressions, delusion, arthritis, rheuma and nervousness.
Toxical effects: Calcium in excess of 2.5 grams daily can cause mineral imbalances (lack of zinc or Iron absorption). Combined with a severe magnesium deficiency, common in bodybuilders, it may lead to kidney stone formation.
Dosage: 1000-1200 mg daily depending on age and sex. You should dose calcium according to phosphorus and magnesium intake. Ca, P and Mg should be stacked in a 2-2-1 relation to each other. Women need less calcium than men because high estrogen levels will deposit more calcium in the bones.
Stacks well with: Vitamins A and D, obviously. Chelation may improve the use of calcium as a supplement.
Sources: Milk, butter, beans, nuts, fruits, seafood and some vegetables.
Note: Many multi-vitamins and protein powders also contain calcium.