Getting enough calcium from your diet can be difficult -- especially if you're calorie- and cholesterol-conscious, or if you're allergic to milk or lactose intolerant.

Taking calcium supplements is an easy and safe way to ensure a proper amount of calcium in your diet, says Dr. Morris Notelovitz, author of "Stand Tall! Every Woman's Guide to Preventing Osteoporosis" (Bantam Books, $8.95). "As far as we know, it doesn't seem to matter which compound you take as long as you get the proper amount of calcium," Notelovitz writes. In his medical practice at the Center for Climacteric Studies in Gainesville, Fla., Notelovitz most often recommends calcium carbonate as the best form of the supplement.

Most calcium supplements on the market contain either calcium carbonate, calcium lactate or calcium gluconate. In each case, the pill is a combination of elemental calcium -- which the body needs -- and other ingredients.

Calcium carbonate tablets contain 40 percent elemental calcium, which is the highest amount of calcium per tablet. Calcium phosphate provides 32 percent, calcium lactate 13 percent, and calcium gluconate 9 percent.

The National Institutes of Health's Consensus Conference on Osteoporosis based its recommendation on elemental calcium alone, so it is important to read the label to find the true dosage.

The only significant differences between the various formulations are the number of tablets that have to be taken each day, the price, and some special considerations, such as:

*Calcium lactate should be avoided by individuals who are lactose intolerant.

*Calcium chloride, which is used in pickling, can irritate the stomach lining and is a bad choice as a calcium supplement.

*Calcium carbonate, while often the best choice, can cause constipation and gas at high doses.

A recent study completed at Creighton University School of Medicine in Omaha found that calcium absorption from calcium carbonate was impaired in patients with little or no stomach acid, a common condition in people over 60. The investigators did find, however, that normal absorption could be achieved by taking the calcium carbonate with meals.

According to Dr. Valery T. Miller, one of the founders of the Menopause Care Center and medical director of menopause research at George Washington University, women are aware of their need for calcium, but either haven't started taking calcium or are just making errors in the dosage.

"Women are unsure of which preparation to buy," says Miller. "And those who have started taking calcium supplements are taking inadequate quantities because of misleading labels, which do not specify amounts of elemental calcium." Miller recommends standard or name brands that require only two tablets a day.

TUMS, Norcliff Thayer's well-known antacid tablet, is gaining unexpected popularity as the "in" calcium supplement. It is calcium carbonate, with each regular 500 milligram tablet containing 200 milligrams of calcium. It's also sodium-free and the least expensive, but requires five tablets a day to equal 1,000 milligrams of elemental calcium.

Bonnie Leibman, director of nutrition at the Center for Science in the Public Interest, warns consumers to look for actual scientific evidence about the superiority of calcium supplements. "When in doubt," Leibman says, "don't believe the inflated claims."

While there is no single best supplement, medical experts agree that there are some general rules, including:

*Avoid supplements made with bone meal and dolomite, which may be contaminated with lead.

*Avoid supplements with vitamin D unless they are recommended by your physician. Too much vitamin D can be toxic and most people already receive enough vitamin D from fortified milk and sunshine.

*Take your calcium supplements in divided doses, between meals and at bedtime. The bedtime dose is especially important since there is increased calcium loss at night.

*Do not take your calcium with high-fiber foods and foods that contain oxalates, such as spinach, asparagus, beet greens, sorrel, dandelion greens and rhubarb. Fiber and oxalates inhibit the absorption of calcium.

*Avoid antacids that contain aluminum, which inhibits the absorption of calcium in the intestine.

*Avoid "chelated" calcium supplements, which cost more and offer no extra benefit.

*If you have a history of kidney stones or stomach problems, or if you are uncertain about choosing the right calcium supplement, consult your physician.