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How dietary supplements can lead to nutrient loss

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Be wary of taking supplements, as the way nutrients interact with each other can have bad results.

QUESTIONYou wrote recently about muscle cramps, for which I take a magnesium supplement (400-500mg) an hour before bed. But, if magnesium blocks calcium, how do I balance that out? With plenty of dairy foods?

ANSWER: “The art of simplicity is a puzzle of complexity,” said US clergyman Douglas Horton. He clearly wasn’t referring to nutrient interactions, but there’s no better way to describe the Ministry of Health’s guideline statements for healthy eating.

At best, they could be called boring and simplistic: “Enjoy a variety of nutritious foods, including: plenty of vegetables and fruit; grain foods, mostly whole grain and those naturally high in fibre; some milk and milk products, mostly low- and reduced-fat; some legumes, nuts, seeds, fish and other seafood, eggs or poultry, or red meat with the fat removed.”

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However, also on the ministry’s website is a comprehensive 317-page document, Nutrient Reference Values for Australia and New Zealand: Including Recommended Dietary Intakes. It explains the complex topic of nutrients in depth: how much of them we need, from what sources and how they interact with each other.

Altering nutrient intake with dietary supplements, or through taking prescription drugs, can harm our health in unforeseen ways due to these interactions.

For example, zinc and iron act against each other by competing during absorption in the intestine. Researchers have found that taking zinc supplementation can lower the iron levels of young women.

On the other hand, vitamin D helps with the absorption of calcium, and vitamin C improves non-haem iron (found in vegetables, cereals, beans and lentils) absorption.

Magnesium is no different. It interacts with other minerals, compounds and even medicines. For example, the phytic acid in tea and coffee, along with nuts, seeds and grains, can inhibit the absorption of magnesium and other minerals such as iron and zinc. This is why it is good to separate tea drinking from main meals.

Certain medications also affect magnesium uptake. For example, H2 blockers, proton-pump inhibitors and antacids all reduce magnesium levels, along with some antihistamines, blood-pressure and other drugs. This is particularly important for older adults, as magnesium absorption declines with age, reducing by more than 30% in the elderly.

In the case of magnesium and calcium, an interesting relationship exists. Studies in the 1960s revealed that healthy adults with low-magnesium and low-calcium intakes had a negative calcium balance, meaning they were losing more calcium than they were absorbing. However, this negative calcium balance was reduced and reversed when magnesium intake improved.

Why? Magnesium is involved in hundreds of intricate cellular reactions and some of these convert vitamin D into its active form, which helps calcium absorption.

Our bodies are far smarter than we might imagine. The proportion of calcium we absorb increases as calcium intake decreases and vice versa. The same goes for iron absorption.

If we eat a balanced diet, we can trust a healthy body to sort out all the details when it comes to nutrient absorption and use.

Eating a range of calcium-rich foods such as milk, yogurt, cheese and buttermilk is always a good idea. Leafy green vegetables, tofu, tinned fish with bones, nuts and seeds also contain calcium.

If you’re going to take magnesium supplements for cramp – despite the evidence for their effectiveness being inconclusive – check with your doctor first if you are taking any drugs. The safe upper limit of supplemental magnesium a day is 350mg for healthy adults. Above this level you may experience side effects or toxicity.

This article was first published in the February 23, 2019 issue of the New Zealand Listener.