NYU professor of Nutrition, Food Studies, and Public Health Marion Nestle attends a panel at The International Culinary Center on November 15, 2011 in New York City.

NYU professor of Nutrition, Food Studies, and Public Health Marion Nestle attends a panel at The International Culinary Center on November 15, 2011 in New York City. (Neilson Barnard, Getty Images for The International Culinary Center)

Anyone can claim to be a nutritionist, which is why finding a good one can be a challenge. Licensure and credentials are an important place to start, but they don't necessarily ensure quality, some experts say.

"Nutrition is a controversial and changing area. Keeping up to date requires a lot of work," said Dr. Stephen Devries, executive director of the nonprofit Gaples Institute for Integrative Cardiology in Deerfield. The best professionals, he said, have undergone a rigorous formal training program and continuously update their nutrition knowledge.

Red flags that your nutrition professional may be unqualified include over-the-top promises such as a guarantee of permanent or quick weight loss or the idea that diet can cure cancer. Also be wary of practitioners who sell products and who have a one-size-fits-all philosophy. If they can't support their claims with scientific research instead of anecdotes, consider one who can.

Marion Nestle, who has spent her career telling people how to eat, is not a registered dietitian but holds a New York State license as a nutritionist/dietitian. She has a master's in public health nutrition and a doctorate in molecular biology.

Nestle suggests approaching anyone's advice, including her own, with skepticism. She likes nutritionists with "critical thinking skills, those who understand how and why people eat the way they do, focus on food rather than nutrients, and are able to read current research as well as interpret and apply it in context."

"I know people trained in dietetics who can do that," said Nestle, a professor in the department of nutrition, food studies and public health at New York University. "But I know even more nutritionists who are not dietitians who also do that. Would licensing straighten all this out?  Only if it sets academic standards that all nutritionists must meet."