If there’s one halfway healthy thing that most people do, it’s take a multivitamin pill. My word, taking a little pill with decent amounts of the essential micronutrients and minerals keeps you from getting scurvy, right?

Right. Most people don’t eat sufficient fruits and vegetables. Most people don’t get enough vitamin C over the course of a week to keep them from getting low-grade scurvy, unless you’re one of the smart few who toss back an orange juice shot in the morning.

(Just for the record, I imbibe espresso shooters in the morning. I’m not getting up on my high elliptical strider and being healthier-than-thou. We’re all on the same dusty rowing machine, here.)

However, you should consider your special circumstances before you even decide whether or not to swallow that vitamin pill.

There are also some very interesting studies relating the regular use of vitamin pills with an increased risk of cancer. Contrary to the expectations of the researchers, one study linked Vitamin A supplementation with an increased risk of lung cancers in male smokers.

In addition, taking a multivitamin increased the possibility of deaths from prostate cancer in men. Why would that be?

The multivitamin-cancer correlation suggests an interesting hypothesis.

In the past, our ancestors, probably even our relatively recent ancestors in the 1900’s, likely experienced transient malnutrition. In the winters, especially, they had less access to fresh, nutritious produce and almost certainly experienced cyclical vitamin deficiencies.

Thus in the winter, a budding cancer cell with its blazing metabolic furnaces would probably starve to death for the lack of vitamin C and vitamin K, which would manifest itself as only a very mild case of scurvy or a few nosebleeds in an adult and would be rectified when tender spring greens appeared.

Now, with our year-round produce and megavitamin pills, we do not experience these cyclical, transient vitamin deficiencies. We are super-nourished, and thus our cancer cells grow robustly in this rich stew of essential nutrients.

Before you give up your daily multi, however, there are some very important things to consider.

People with the highest levels of vitamin D (available in supplemented milk, pill form, and sunshine,) had lower levels of cancer and osteoporosis.

If you’re a woman of childbearing age, taking a daily multivitamin during any trimester of pregnancy or in the month before pregnancy decreases the risk of neuroblastoma in the infant by 30% to 40%. Neuroblastoma is the most common cancer seen in infants and accounts for about 10% of all pediatric cancers. Not to mention that whole folic acid—neural tube defect thing. Taking a big preggers prenatal multi during pregnancy is very, very likely the best course of action.

Also, non-smokers who do not have heart disease who use multivitamins that include A, C, or E reduced risk of dying from heart disease by 15 to 18%, and heart disease kills far more people than cancer does.

So, for a general rule of a healthy thumb, if you’re a smoker, avoid vitamin A, even if you have to take a handful of single-vitamin pills instead of a general multi.

If you have prostate cancer, stop taking your multi.

If you don’t smoke and you don’t have prostate cancer, a multivitamin is probably the best course of action.

If you want hedge your chances, however, here’s an idea: there’s some very good research that supports the hypothesis that eating 300-500 fewer calories per day extends lifetime and, more importantly, extends robust lifetime. That’s right. Eat less.

Some good research came up lately that showed that mice that ate normally every other day and semi-fasted (eating 15% of normal calories) on the off days had essentially the same life extension and reductions in heart disease, cancer, and inflammation. If you try alternate-day semi-fasting, don’t take a vitamin on those days. Taking a megavitamin on feasting days will nourish your body well.

Fasting is associated with life extension and with reducing the debilitating side effects of chemotherapy.