You may want to think twice before reaching for that fat-free yogurt.

Dr. Walter Willett, the world's most-cited nutritionist, says the fat-free diets that emerged decades ago remain among the biggest inaccuracies when it comes to healthy eating.

"Often sugar is substituting for fat, and many times the new-low fat product is worse than the original full-fat version," Willett told CTV's Canada AM on Monday.

Willett, the 2013 recipient of the $50,000 Bloomberg Manulife Prize to promote active health, says more research is needed on what a healthy diet looks like. But he added that certain dieting myths, such as the role of fats, are in immediate need of debunking.

"What we've learned is not all fat is bad," he said. "There are good fats, like the unsaturated fats, and bad fats, like trans-fat and saturated fats.  So if you're removing mostly good fat and replacing it with refined starch or sugar, that's not going to be good on balance for our long-term well-being."

The amount of calcium needed for good bone health is another dieting myth that's pervasive in North America, Willett, the author of the best-selling book “Eat, Drink and Be Healthy,” said.

While researchers still don't have all the answers when it comes to the effect of calcium intake on bones, Willett said he hasn't seen much evidence that shows that drinking milk helps to prevent fractures.

"In the U.S. we're told to have at least three glasses of day of milk product, and we just don’t see any suggestions of lowering the risk of fracture when people consume those high levels of milk."

Canada's Food Guide recommends 500 millilitres, or two cups, of milk a day, which is in line with the World Health Organization’s recommendation.

"We have to realize if we look around the world, most of the world does not drink milk," he said. "It's actually the high-drinking milk countries that have the highest rate of fractures."