And Americans also overestimate how much they exercise, another barrier to fighting two of the biggest known cancer risks, researchers at the National Cancer Institute said.
If everyone wanted to eat healthily, there would not be enough, Susan Krebs-Smith of the cancer institute told reporters.
Many studies have shown that people who keep a healthy weight, exercise regularly and eat plenty of fresh fruits and vegetables have a lower risk not only of cancer, but heart disease, diabetes and even Alzheimer's.
The administration of President Barack Obama is looking at ways to help Americans eat a healthier diet and exercise more to reduce obesity.
Krebs-Smith and colleagues knew Americans do not come even close to meeting those goals. They checked to see if the U.S. food supply could provide the recommended five servings a day of fresh fruit and vegetables to every American.
It cannot, Krebs-Smith told reporters.
The fruit in the food supply is about half what it needs to be, but we have plenty of calories from fat and added sugars, she said.
The NCI team worked with the U.S. Department of Agriculture to calculate how much food the United States produces, imports and gets to retail outlets.
Fast-food outlets, junk food makers and snack companies are well supplied, they found.
The food supply does supply enough meat and beans, Krebs-Smith said. But only half the vegetables needed for everyone to get what they should are grown or imported.
U.S. habits suggest demand may lie behind these shortages. Our intakes of fruit are low. Our intake of vegetables is low but especially our intake of dark green and orange vegetables and legumes, Krebs-Smith said.
Other studies have shown that Americans underestimate how many empty calories they take in. An average American can eat about 2,000 calories a day, she said -- and once the recommended foods are accounted for, this leaves just 270 discretionary calories a day, or just over 11 percent.
The average American actually gets 38 percent of calories from unneeded sugars and fats.
A second major factor in cancer is a lack of exercise and Dr. Rachel Ballard-Barbash found Americans come up far short there, too.
When asked, anywhere between 30 percent and 40 percent of Americans estimate they get enough exercise.
But a study of 6,329 people who wore a device called an accelerometer showed that in fact, fewer than 5 percent got the recommended minimum of about a half-hour of moderate exercise a day.
The cancer institute says obesity and physical inactivity account for 25 to 30 percent of colon, breast, endometrial, kidney, and esophageal cancers.
In 2002, about 41,000 new cases of cancer in the United States were due to obesity, or about 3.2 percent of all new cancers, the NCI says.
Diet and exercise can also help people survive cancer, Ballard-Barbash said.
Her team looked at breast cancer survivors and found women who ate the healthiest diets and exercised the most had a huge reduction in the risk their cancer would return.
This suggests about an 89 percent reduction in the risk of death over a 1-year follow-up period, she told reporters.
The American Cancer Society estimates that nearly 1.5 million Americans got cancer in 2009 and 560,000 died of it.