Nutritionists recommend people eat 300 to 500 grams of vegetables in five colors each day to avoid lacking nutrients and getting sick. (Photo/ Xinhua)

June 17 was National Eat Your Vegetables Day in the US and also marked the reveal of shocking nutrition news: 84 percent of Americans eat fewer vegetables than the recommended daily minimum from the United States Department of Agriculture (USDA), according to news portal prnewswire.com. A similar survey done by Life Times in 2013 also showed that 95.3 percent of those surveyed in China eat fewer than five colors of vegetables a day. The China Nutrition Society recommends people eat 300 to 500 grams of vegetables a day in five different colors.

"The lack of vegetables could easily cause humans to miss out on nutrients like vitamin A, vitamin C and dietary fiber, putting people at risk for constipation, anemia, obesity, and an overall decline of the immune system," said Li Yuanyuan, a Beijing-based nutritionist focusing on vegetable consumption.

Shu Ran, a 32-year-old yoga teacher in Beijing, started maintaining her own good vegetable eating habits more than five years ago when she began studying nutrition. Her regimen means she changes the types of vegetables she consumes every few days, but still manages to take in a variety each day. One day of vegetables could look something like this: rape, purple cabbage, tomatoes and black fungus.

"I was not very healthy before and often came down with colds or other minor illnesses," Shu said. "After I started eating enough vegetables, I have felt better, have a better figure, and rarely get sick. Everyday I am in good spirits and have a good appetite."

Li said some people have the misunderstanding that they should focus on eating one specific color of vegetables.

"In order to get a balanced and comprehensive intake of nutrients, we should have a rainbow structure in which every color of vegetables, such as green, red, yellow, purple and black, has different healthy functions. For example, green vegetables can prevent cancer, improve the growth of bones and teeth and protect your eyes. Red keeps your urinary system healthy and improves memory. Purple (including blue and black) can improve blood circulation and brain function," Li said.

Doctors in the US even recommend some vegetables, such as spinach and beetroot, as "prescriptions" to combat obesity in low income families in a program called The Fruit and Vegetable Prescription Program (FVRx) which began in 2010, according to health news portal mercola.com.

Generally speaking, vegetables can be classified into two categories based on their colors and nutrients: dark and light. People can not simply judge whether a vegetable is light or dark by its color. Eggplants and cucumbers look dark in color, but they are not dark vegetables. Only those that are dark inside and out can be classified as dark vegetables and they have more nutrients than light vegetables, Li said.

"Dark vegetables should account for more than a half of your daily consumption, and the rest should be light vegetables, such as light green and white. Of dark vegetables, dark green should account for more than one-fourth of a diet, and the rest should contain purple, red or yellow vegetables," Li said.

Li warned that using the wrong cooking methods can easily turn healthy vegetables into unhealthy junk food.

"Eating vegetables raw is the best, but if your stomach can not stand it, you can fry them with light fire and oil or steam them to keep the most nutrients. Deep-frying or grilling are not recommended because they can cause the loss of nutrients or produce hazardous substances," Li said, adding that people can try substituting different types of vegetables with the same nutritional content to see what they like.