Hanrahan points out the doses found in vitamin C products are sometimes more than 20 times the recommended daily intake set by the Ministry of Health. The recommended doses are 40mg for children, 45mg for adults and 60mg for pregnant women.
Vitamin C products are often sold as 250mg tablets (100 for around $10), 500mg tablets (100 for around $25) and 1000mg tablets (100 for around $25).
An orange has about 30mg and half a cup of broccoli has 110mg of vitamin C, and the latest government research indicates most New Zealanders are getting more than double the recommended daily intake through their diets, she says.
Although the supplements industry argues high doses of vitamin C are beneficial, Hanrahan says the science-based nutrition community is unconvinced.
"It's going to flush straight through your body. It's not stored, so you can't stock up on vitamin C now so that you have enough for next week."
Professor Shaun Holt, a medical researcher who focuses on natural health, says there are proven beneficial products on the market, such as fish oil and echinacea, but many are marketed with "shonky research and old wives' tales".
Vitamin C falls into the latter category, he says.
"The evidence is weak. You'd be better off spending your money on echinacea and zinc, which have been shown to reduce the duration of a cold by around half a day."
However, Natural Products NZ executive director Michelle Palmer, who represents most of the supplements industry, points to research showing a daily supplement with 1000mg to 3000mg of vitamin C helps maintain good health and that 6000mg daily can cut down the duration of a cold.
Holt says it is important to look at the entire body of research on a subject, rather than isolated studies. The Cochrane Collaboration, an independent, worldwide network of researchers, had reviewed all published literature on the subject and concluded it was proven to help only high-performance athletes.
"You can get a study to show just about anything, which is why studies need to be repeated and tested. You have to look at the totality of what research is showing," Holt says.
Natural diet a healthier option for Hugh
New mum Nicola Cranfield has always been "mildly obsessed" with keeping up with the latest health buzz - but says having a son with eczema and colic has taught her to be more careful with what she believes.
"Having a child has taught me to question the information that's out there. I'm always asking questions and I'm very cynical," she says.
There are no cures for Hugh's conditions, but both can be managed with careful attention to nutrition, alongside medical treatment.
"I'm keen on whole foods. I'm always thinking 'how can I get more vitamins and minerals into this meal?' I prefer food-based supplements such as cod liver oil, rather than synthetic products made in labs."
Her family's diet includes lots of fermented food, which are high in probiotics for gut health. Cranfield also sprinkles New Zealand seaweed on to meals to ensure her family are getting iodine, zinc, copper, manganese, magnesium, calcium, iron and potassium, Vitamin B12 and fibre.
She has launched Brooklyn Kitchen, a business which supports parents to improve the well being of their families through eating well.
