"Keep in mind that our bodies are able to absorb more iron from food if the meal is also rich in vitamin C," Sheth said. "If you're having spinach, you might have tomatoes or a citrus dressing with it to increase absorption."
Omega-3 fatty acids probably represent the greatest nutritional challenge for vegans, the two nutritionists said. Thought to be critical for cognitive function and healthy cardiovascular function, omega-3s appear in large amounts only in fatty fish such as salmon -- a dietary no-no for vegans.
Some plant sources -- flaxseeds, soybeans, pumpkin seeds and walnuts, for example -- contain a type of omega-3 fatty acid, but it's not the same type found in fish and has not been proven to have the same level of health benefits, Giancoli said.
"There's some concern that vegans may be missing out," she said.
Finally, vegans need to keep in mind that it's just as easy for them to indulge in an unhealthy diet as it is for omnivores, Sheth said.
She recommends that her vegan clients follow the federal government's "My Plate" guidelines for eating, the same as everyone else should. "You're basically just replacing the protein source," Sheth said. "Otherwise, it's the same meal."
The U.S. Department of Agriculture has more on vegetarian eating.
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