DEAR DR. BLONZ: Calcium citrate and calcium carbonate are sold as supplements to build strong bones and prevent osteoporosis. I would appreciate your help in understanding the advantages or disadvantages of these and other types of calcium supplements. Are they equal in benefits? -- P.S., San Diego

DEAR P.S.: I receive many questions about calcium supplements, and as you point out, there are no shortage of options. The key is to get the calcium from the digestive tract into the rest of the body; that means that the calcium will have to be in solution so that it can pass through the absorptive surface of the intestines. A pill should dissolve in the stomach within 30 minutes if uncoated, or 45 minutes if coated.

Once the calcium compound breaks down, the next step is for it to meet up with the body's vitamin D-dependent calcium binding protein. This protein attaches itself to calcium and escorts it through the intestinal wall. As is obvious by its name, vitamin D is needed to stimulate the production of this binding protein. Although D does not need to be in the stomach at the same time as the calcium, the body has to be adequately nourished with vitamin D for the process to work effectively. The recommended dietary allowance (RDA) for vitamin D is 600 IUs (International Units) per day for those aged 1 to 70, and 800 IUs per day for those over 70. (The body can make its own vitamin D when it's exposed to sunlight, but for those who use sunscreens -- hopefully all of us! -- consuming foods that contain, or are fortified with, vitamin D is important.)

There are many different calcium compounds, each with varying amounts of elemental calcium per unit weight. Be sure to look at the Nutrition Facts or Supplement Facts label on any product, as this will list the actual amount of calcium per serving. The RDA for calcium is 1,000 milligrams per day for those aged 19-50 years of age. Recommended levels for other age groups can be found at: tinyurl.com/nrem7yj.

The calcium in dairy products is readily absorbed; here are some typical calcium-containing compounds found in supplements:

Calcium carbonate: poorest absorption; most calcium per pill (40 percent by weight), which means fewer pills to take; least expensive; most common form of calcium found in supplements; found in antacids. Can cause gastric acid rebound if taken on an empty stomach.

Calcium gluconate and calcium lactate: better absorption, less calcium per pill (9 percent and 13 percent respectively), so more pills needed to get an effective dose; more expensive; less commonly found in supplements.

Calcium citrate and calcium citrate-malate: best absorption; less elemental calcium per pill than carbonate (calcium citrate is 21 percent calcium by weight), so more pills needed; more expensive; gentler to the stomach.

The choice is yours, as any one of these can work as a calcium supplement. It is generally agreed that it is best to take calcium supplements at mealtime.

Send questions to: "On Nutrition," Ed Blonz, c/o Universal Uclick, 1130 Walnut St., Kansas City, MO, 64106. Send email inquiries to questions@blonz.com. Due to the volume of mail, personal replies cannot be provided.

DEAR DR. BLONZ: I have always wondered about the letters and numbers used to identify vitamins. Why do they skip some letters, and why are there so many different B's? -- S.Y., San Jose, California

DEAR S.Y.: The word "vitamin" was coined in 1912 by Polish scientist Casimir Funk, who was searching for a cure for beriberi -- a paralyzing disease that was common in regions where white rice was the main dietary staple.

Contemporary research had suggested that the rice husk and bran might contain a substance that could combat the dreaded disease. Funk identified the key compound -- now called thiamin -- and dubbed it a "vital amine." This term was eventually shortened to "vitamin," and came to signify an entire class of essential compounds that are needed by the body in trace amounts.

All vitamins can be found in food. They do not provide energy (calories) and they are all "organic," in that they are constructed using the element of carbon. They perform specific metabolic functions in the cell, our basic unit of life, and they cannot be made by the body in amounts sufficient to meet its needs.

The absence of a vitamin from the diet can lead to a failure to thrive and the development of a deficiency disease, though the length of time it takes for problems to develop differs with each vitamin. It is interesting to note that not all animals require the same vitamins. Humans, for example, cannot manufacture their own vitamin C (ascorbic acid), which many animals can; this explains why our dogs and cats don't need this vitamin like we do.

Back in 1913, scientists came up with a naming system that was based on two groupings: those that would dissolve in fat were called "fat-soluble A" vitamins, and those that would dissolve in water were referred to as "water-soluble B" vitamins. The naming then proceeded in order of discovery. Many times, a substance thought to be a vitamin was later determined not to be essential. When this happened, that letter ceased being used.

In some cases, the first letter of the compound's function was used in its name. For example, vitamin K comes from the German "koagulation" (coagulation), or the clotting of blood -- a process in which vitamin K plays a role.

The "B" vitamin, originally found in yeast, was originally thought to be only one compound. When it was later found that there were many different compounds, they were given the names vitamin B1, B2 and so forth. This is now referred to as the vitamin B complex.

At present, there is no universally accepted naming system, and letters have apparently lost some of their appeal. Some compounds are called by their chemical name, as opposed to a letter/number designation. Examples include biotin (vitamin B7) and folic acid/folate (vitamin B9).

Send questions to: "On Nutrition," Ed Blonz, c/o Universal Uclick, 1130 Walnut St., Kansas City, MO, 64106. Send email inquiries to questions@blonz.com. Due to the volume of mail, personal replies cannot be provided.

DEAR DR. BLONZ: How real is the danger from the lead content in calcium supplements? One company promotes its products by saying it contains no lead, but it is unclear how much of an issue this is with other (less expensive) supplements. -- F.R., Phoenix

DEAR F.R.: Some natural sources of calcium do carry a risk of lead contamination when they are used in supplements. These include bone meal, oyster shell and dolomite.

When a body is exposed to heavy metals, such as lead, and if the contaminant is coming in at a greater rate than the body can cast it off, the bones become an inadvertent holding location. As a practical example, animals allowed to graze near sources of industrial pollution can end up with higher-than-normal levels of contaminants in their bones. If these bones are then used to make bone meal for dietary supplements, there is a risk of the unwanted elements ending up in the bottle.

The same argument can be made for oyster shell calcium, if the oysters live in contaminated waters. Dolomite, also called dolomitic limestone, is a mined mineral that's composed primarily of calcium carbonate and magnesium carbonate, plus some other elements. If the mineral happens to be in areas that contain elevated amounts of lead, this natural source could also contain a natural contaminant.

These scenarios do not mean that products made from natural sources will automatically be contaminated. But if you choose a calcium supplement sourced from any of these compounds, you should only select a brand that specifically tests and states that their product is low-lead or lead-free.

DEAR DR. BLONZ: I noticed a bread label that read "stone ground wheat bread," but upon closer inspection, I found it to be white bread with molasses coloring. Is this sort of misleading labeling illegal? -- G.T., Hayward, California

DEAR G.T.: Wheat bread is made with wheat flour, and it can be white, whole-grain or a combination. A 100 percent whole-grain wheat bread tends to tout its "100 percent whole wheat" or "whole wheat flour" content. One of those will be the first item on the ingredient statement and there will be no other flours listed.

All wheat flour is made from ground wheat, and "stone ground" says nothing about any "whole-grain" character of the bread. It is somewhat misleading when a bread that is not whole-grain uses colorants to make it appear as though it is. Molasses can be used to provide some nutrients and help make the bread less crumbly. It is not a legal (regulatory) issue as long as the bread is not claiming or implying that it is whole-grain when it's not.

DEAR DR. BLONZ: Do the X-ray security machines in airports negatively affect the vitamins or medications packed in my luggage? Does it destroy their effectiveness? -- T.T., Boston

DEAR T.T.: There is no evidence of negative effects on dietary supplements or medications from the routine use of security machines in airports.

Send questions to: "On Nutrition," Ed Blonz, c/o Universal Uclick, 1130 Walnut St., Kansas City, MO, 64106. Send email inquiries to questions@blonz.com. Due to the volume of mail, personal replies cannot be provided.