DEAR DR. BLONZ: Sometime earlier in the year, I began adding a tablespoon of flaxseed to my morning granola. After about one week, I began to experience symptoms of vertigo. One day it was so bad that I could not even get out of bed in the morning. I have never experienced symptoms of vertigo before, so I thought it might be the flaxseed that was responsible. I stopped taking it, and the symptoms disappeared. Have you ever heard of such a case? -- F.D., Hayward, California

DEAR F.D.: Seeds are often sources of substances associated with food allergies, but reactions to flax are not common. Such an occurrence, however, is not out of the realm of possibility. The experience you relate certainly does cast suspicion in that direction. But your query highlights a situation we all must acknowledge: namely, that untoward reactions from foods do not always present with a clear connection between the cause and the effect.

When we experience something unexpected after eating, it is reasonable to consider the last thing consumed as a likely suspect, especially if it was something new. If it seems to recur whenever it’s eaten, we start to think we are on solid footing. What happens, however, if there is little basis for the connection, and our health professionals are unable to offer guidance?

If we are confident in our decision, we might act on our assumptions and cross the suspected offender off our “to eat” list. We might, however, be persuaded by the guidance of health professionals and begin to look elsewhere to explain the reaction. Or, we might simply chalk up the reaction as one of the many unexplained events experienced in life.

The bottom line here is that medical science is not always able to tell who will react to what. And while it may be a single substance you are reacting to, it may also be the dose that determines the reaction. For some, any amount of intake can cause problems, while others will only react if they take in more than their “threshold” of sensitivity. To further complicate the issue, there may be other foods or medications that change that threshold. Have a set amount of that certain food alone, no problem -- but have that same amount along with another food, or in a particular situation, and you experience the reaction.

There can be quite a detective game afoot, which is why it makes sense to keep a log of all foods consumed to help identify any patterns.

Research studies tend to say what is likely to happen to the “average” individual under a controlled set of circumstances. Such information can provide valuable insight, but regardless of what a study might tell you, we are all unique. Arm yourself with the required knowledge, then collect information about the reaction and the circumstances associated with its occurrence. Then, consult with a trusted health professional to exercise due diligence to chart the path forward.

Science is only as good as the latest study, so as new information and experiences become available, be prepared to reexamine your choices and make the appropriate changes. While it is essential to control items that make us ill, it is best to avoid dubious assumptions that continually shrink the circle of foods we can enjoy.

Send questions to: “On Nutrition,” Ed Blonz, c/o Andrews McMeel Syndication, 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 pernicious anemia, for which I receive B12 injections. Why is it that intrinsic factor cannot be synthesized and introduced into the body by injection or pills? It seems as though that would be better. -- T.D., Anderson, South Carolina

DEAR T.D.: As you know, but other readers may not, two separate components are needed for vitamin B12 to be absorbed. The first is the vitamin itself, and the second is a protein manufactured by the body that is called the “intrinsic factor.” That factor links with vitamin B12 and provides the escort that carries it through the absorptive surface of the intestines. Without its intrinsic factor, little, if any, vitamin B12 would ever get into your system.

Pernicious anemia, which only occurs in about 2% of individuals, is when the stomach produces an insufficient amount of that intrinsic factor protein. When this is the case, dietary sources of vitamin B12 do not get absorbed. We cannot get the same effect from taking intrinsic factor as a dietary supplement, as it will get broken down like other nutritional proteins.

To date, the most workable treatment for pernicious anemia is the periodic injection of the preformed vitamin B12. The strategy should be worked out with your health professional, who can help identify factors or medications that may be involved. I encourage you to discuss whether your situation is one where alternatives might be available, such as elevated oral doses of vitamin B12, or ones administered as a sublingual tablet, nasal gel or spray.

Check out the additional discussion at the National Institutes of Health at b.link/pernicious.

DEAR DR. BLONZ: A lady in my office says that she was told the calcium in milk is the wrong kind for our bodies, and is not easily absorbed. Since she is pregnant, I’d like to be sure this is correct, so she’s not ignoring something important. Is there any truth to this? -- M.M., Phoenix, Arizona

DEAR M.M.: No. The idea that the calcium in milk is the “wrong kind” is without merit.

There are many types and dietary sources of calcium. Dairy products represent an excellent and convenient source. One advantage is that the body absorbs a higher percentage of dairy calcium than calcium carbonate, which is a common form of calcium found in dietary supplements.

A key consideration here is that while calcium is essential -- especially for a mother and her developing child -- dairy products are not.

Send questions to: “On Nutrition,” Ed Blonz, c/o Andrews McMeel Syndication, 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’m a 69-year-old male, vegan, with an active cycling routine. For most of my adult life, I’ve been a blood donor without any problems. About two years ago, I began consistently failing the initial screen due to low blood iron. Eventually, one of the technicians asked if I drank tea. I had, in fact, begun drinking tea daily around the time I began failing the blood screen. A few weeks ago, after abstaining from tea for a few days, I passed the screen and successfully donated.

My online research seems inconclusive about whether tea depletes iron. I would have thought that low blood iron would seriously affect my cycling performance, but I haven’t seen definite signs of that. To what extent does tea deplete blood iron? Will drinking a cup or two of tea per day affect endurance and athletic performance? -- S.H., Tulsa, Oklahoma

DEAR S.H.: My compliments to you, and all who are blood donors; it is a generous pay-it-forward way to give of ourselves, and help others in a way unlike any other.

Donation centers don’t typically measure blood iron directly; what they screen for is the level of hemoglobin, the iron-containing “metalloprotein” in red blood cells designed to carry oxygen. Blood accounts for about 10% of body weight, about 1.2 to 1.5 gallons, and the amount removed during the donation should only have a minimal, short-term impact on the oxygen-carrying ability of a donor’s blood. We have extra blood stored in the spleen, so the blood volume is rapidly replenished. For more on donations, see b.link/donor.

Kudos also on your healthful approach, and to your detective work in having tea on the suspect list. Teas made from black tea leaves, and to a lesser extent, oolong, then green, contain tannins that can bind with minerals such as iron and affect their absorption. This is not tea depleting body iron; it is the tannins affecting absorption, so dietary iron is less able to make it in. (I realize this might seem odd, but for physiological reasons, the digestive tract is often considered to be outside of the body; it’s only after a substance is absorbed that it is considered “in.”) Think of the tannin effect as a physical phenomenon within your digestive system, where the tannins are binding with the mineral to form a compound that is too large to be absorbed.

Your experience with a brief cessation of tea drinking and with your cycling would suggest that this may only be a minor issue. You could counter the effect by avoiding drinking high-tannin teas with iron-rich meals. (There is a list of vegetarian sources of iron at b.link/zr38h.)

I do recommend you relate these experiences to your health professional to help decide if a more informative blood test would be indicated.

Finally, tea has several healthful benefits, so enjoy. There are teas with low, or no tannins, and certain herbal teas are tannin-free.

Send questions to: “On Nutrition,” Ed Blonz, c/o Andrews McMeel Syndication, 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.