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Carla Ladeira at home in Vancouver, Nov. 19, 2013. She was diagnosed with osteoporosis at age 41 and is focusing on getting her requirements met from food sources instead of supplements.Rafal Gerszak/The Globe and Mail

Carla Ladeira has osteoporosis, a bone disease resulting in tiny fractures in her spine.

To strengthen her bones, a specialist in Vancouver prescribed high doses of calcium supplements as well as magnesium and vitamin D. For at least three months, Ladeira followed the recommendation – until she came across scientific research showing that calcium supplements only made bones more brittle.

Ladeira, the 41-year-old founder of a senior-care service, said she was so dismayed by the contradictory information that she ignored her doctor's advice and stopped taking calcium and vitamin pills. "I guess I just developed mistrust for all the supplements," she said.

As study after study crushes widely held assumptions about what our bodies need to stay healthy, findings from nutritional science have left many of us more confused than ever.

A century of nutrition research has persuaded us to think of food as a vehicle for macronutrients such as vitamins and minerals. Recently, however, scientists have acknowledged that the mechanisms involved in how we metabolize and absorb nutrients from foods are much more complicated than previously thought.

Torn between simplistic advice ("eat your vitamins!") and heavy marketing from dietary-supplement manufacturers, many people no longer know what to swallow.

Nutrition science's latest fall from grace involves vitamin D, hyped just three years ago as preventive medicine for everything from cancer to bone loss to childhood diabetes. The so-called sunshine wonder drug lost its halo in October when a large review published in the Lancet found that vitamin D did little to improve bone loss. "Most healthy adults do not need vitamin D supplements," lead author Dr. Ian Reid concluded.

Other studies have debunked health benefits associated with supplements ranging from fish oils to vitamins A through E.

Media reports on the latest nutrition findings have left many people wondering, "How could top scientists have got it so wrong?"

There is a simple explanation, according to scientists working in an emerging field known as nutrigenomics, the study of how small variations in genes make large differences in how individuals absorb and metabolize nutrients from food.

The majority of nutrition studies still do not take genetics into account, according to Dr. Steven Zeisel, director of the nutrition research institute at the University of North Carolina. The result is one-size-fits-all dietary guidelines and public-health strategies for disease prevention that do not accurately reflect the diversity of our population.

Zeisel is known for his discovery that women require the nutrient choline for healthy liver function and fetal development. About half of Caucasian women have a genetic misspelling that interferes with their body's production of choline, and must consume food sources of the nutrient, including soy lecithin and eggs. But young women without the genetic misspelling do not require a choline-rich diet. Thus, telling all women to eat choline may not make sense, because depending on their genetic makeup, "either they need it, or they don't," he said.

According to Zeisel, the new era of individualized nutrition will require an integrated research approach from experts in genetics, epigenetics (the study of heritable changes in gene activity that are not caused by changes in DNA) and the microbiome – the makeup of microbes in our bodies that metabolize much of what we eat.

"I think people who are at the cutting edge of nutrition research all realize that this is where we have to go," he said.

Nutrigenomics emerged around the time the Human Genome Project was completed in 2003 as scientists began to identify important differences in gene-diet interactions and how our bodies respond to different foods. The field has not been without controversy since then, especially as it expanded from the laboratory into the marketplace. Companies began marketing kits offering dietary advice based on genetic tests that were not properly tested or based on solid scientific evidence.

Personalized nutrition products got a bad reputation when the first genetic-testing kits hit the market more than five years ago, said Dr. Ahmed El-Sohemy, an associate professor of nutritional sciences at the University of Toronto. El-Sohemy added that he was one of their biggest critics because at the time, it was premature to give nutrition advice based on genetic testing.

But since then, the science has become more conclusive, especially concerning five genetic markers associated with health indicators such as blood levels of vitamin C. (Last year, he launched his own personalized nutrition product: a $350 test that uses a saliva sample to detect genetic markers associated with potential risks and benefits of consuming coffee and nutrients such as sodium and folate.)

All Canadians are advised to cut back on sodium, for example, but "about 30 per cent of the population really don't have to," he said, citing a study published in the journal Hypertension.

Despite recent advances, however, El-Sohemy acknowledged that nutrigenomics is still in its infancy. Much more research is needed before dietitians can provide more comprehensive personalized nutrition advice, he added.

In the meantime, the best nutrition advice is probably a diet that is higher in plant-based foods, lower in animal products and lower in calories, Zeisel said. As for specific diets and nutritional supplements, he said, "You have to realize it's not that different from all the other forms of gambling."

Instead of taking supplements, Carla Ladeira tries to meet her calcium needs by eating dairy products, dark leafy greens and bone broths, a dietary mainstay in Japan, where osteoporosis is less common.

After two years of eating this way, she hasn't gotten any new fractures. "I do feel like I've gotten stronger," she said.