"I am glad that you alerted me to the danger of "overdosing" with calcium. My wife needs to know this! "
One might argue that you could just as easily overdose on nutrients by consuming whole foods. Possible, sure—but highly improbable. Exceeding the upper intake level for vitamin C would require eating 24 oranges in as many hours—for days and days. (For most vitamins and minerals, upper levels are based upon chronic daily intake, not a few days of overshooting.) For calcium, you’d need to guzzle eight cups of milk. Even then, it’s unlikely you’d suffer side effects. “There’s never been a case of calcium intoxication from food,” says Robert P. Heaney, M.D., a calcium expert and professor of medicine at Creighton University in Omaha, Nebraska. “Farm workers in the 1930s could consume a gallon and a half of milk a day without trouble. Most of what we know about calcium toxicity comes from supplements.”
Occasionally, eating lots of carrots, sweet potatoes and leafy greens, which are rich in beta carotene, can cause the skin to yellow. Beta carotene is the biological precursor to vitamin A, and when intake is high, less is converted and the rest is stored just under the skin. The effect may be alarming (people often think it’s jaundice, a symptom of liver disease), but it’s considered harmless, and goes away when you vary your vegetable choices. Plus, it’s not all that common: “In the 12 years I’ve been a nutritionist I’ve seen two people whose skin turned yellow because they were eating too many carrots,” says Schupp.
In food, nutrients interact with each other in ways they don’t in a high-dose supplement, often putting the brakes on an “overdose” of a nutrient before it can cause any adverse effects. Calcium and iron compete for absorption; so do copper and zinc. Other food components (including oxalates, compounds in leafy greens) bind nutrients, making them unavailable. Such interactions are factored into the recommended intakes, which assume you get most of your nutrients from food.
Recommended intakes for nutrients, for the most part, have been extrapolated from data that show what foods, in what amounts, have been keeping Americans healthy. That’s why it is so important to meet your nutrient needs with food first. Doing so also ensures you reap the benefits of other components (fiber, phytochemicals, healthy fats) and the synergistic effects of nutrients. While some nutrients limit each other’s absorption, others combine to offer nutrition gains greater than the sum of their respective benefits. For example, vitamin E is better absorbed with fat, which is why sources of healthy fats like nuts, seeds and cooking oils are good choices for getting vitamin E.