Canadians eager to boost their health by consuming megadoses of vitamins C, E and D may be doing more harm than good, a fifth estate investigation into the vitamins and supplements industry reveals.

"When people walk into the dietary supplement or vitamin store, they think that everything is just perfectly safe," says Dr. Paul Offit, chief of the division of infectious diseases at Children's Hospital of Philadelphia and a bestselling author.

But he is alarmed at the daily high doses people are taking.

"There are studies that show that if you take a megavitamin you actually can hurt yourself. You actually can increase your risk of cancer, increase your risk of heart disease," he says.

fifth estate documentary that airs tonight on CBC-TV shows that a common popular daily megadose of vitamin C — 1,000 milligrams — is equal to what you'd get from seven or eight entire cantaloupes.

"You're not meant to eat eight cantaloupes. It's a dangerous thing to do," Offit says.

Dr. Paul Offit

Dr. Paul Offit, chief of the division of infectious diseases at Children’s Hospital of Philadelphia and a best-selling author, is alarmed at the daily high doses of vitamins and dietary supplements people are taking. (CBC)

The risks are even higher with large daily doses of vitamin E. You can get all the vitamin E you need in just 30 almonds. But many vitamin E capsules contain more than 50 times that amount — a dangerous level particularly for men, Offit says.

"If you take large quantities of vitamin E as a supplement, you clearly and definitely increase your risk of prostate cancer," he says.

'Many people are taking too much vitamin D'

Vitamins, minerals and herbal supplements are big business in Canada. Consumers spend $1.4 billion on them every year and three out of four Canadians take them.

But for some pills — such as vitamin D, the so-called "sunshine vitamin" so popular among Canadians — there is little scientific evidence that more is better.

"Many people are taking too much vitamin D," says Brigham and Women's Hospital researcher Dr. Joanne Manson.

Manson is conducting the largest randomized trial of vitamin D in the world — collecting blood samples from 25,000 people to compare disease rates between those who take vitamin D supplements and those who don't. She expects to have the results in two years.

Dr. Joanne Manson

Brigham and Women's Hospital researcher Dr. Joanne Manson says many people 'are taking too much vitamin D.' (CBC)

"When I heard that various groups are recommending 10,000 [international units] a day, or even 5,000 IUs a day routinely, I really want to say 'Show me the data, show me the evidence,' " she told the fifth estate.

Health Canada recommends a daily dose of just 600 IUs.

Both Health Canada and the American Institute of Medicine recommend avoiding consuming more than 4,000 IUs daily because, according to Manson, "that could be associated with adverse events: calcium in the urine, which can be associated with kidney stones, high blood calcium, calcium in the arteries, vascular calcification, as well as soft tissue calcification."

"There are now studies that show ... that those who have high as well as low blood levels of vitamin D have higher risk of cardiovascular disease," she says.

"So we can't assume that more is necessarily better."

The fifth estate designed a "pill converter" to illustrate what you are consuming when you take certain doses of vitamins.

For example, 1,000 micrograms of B12 is the same as drinking 260 litres of soy milk. And 100 milligrams of B6 is almost the same as consuming 200 potatoes.

Watch "Magic Pills" on the fifth estate tonight on CBC-TV.

Try the fifth estate's pill converter. Find out what you're eating when you take those vitamins