(Reuters) - American adults have made a little progress in recent years in cutting back on calories from fast food, but children are still consuming too much fat, U.S. health researchers say.

French fries, pizza and similar items accounted for about 11 percent of U.S. adults' caloric intake from 2007 to 2010, on average, down from about 13 percent between 2003 and 2006, the Centers for Disease Control and Prevention said in one of two reports released on Thursday.

Younger adults, black Americans and those who are already obese consumed the highest amounts of such food, which is often high in fat, salt and calories that can doom waistlines.

The CDC found in a separate report that while American children, on average, are consuming fewer calories overall than they used to, the percentage of their calories from artery-clogging saturated fat was still above optimal levels.

Recommended U.S. guidelines suggest that no more than 10 percent of one's daily calories should come from such fat, but American youth took in between 11 percent and 12 percent from 2009 to 2010, data from the CDC's National Center for Health Statistics showed.

Americans' diets and weight is a source of constant scrutiny and research in a country where two-thirds of the population is considered overweight or obese. According to the CDC, 36 percent of U.S. adults, or 78 million, and 17 percent of youth, or 12.5 million, are obese. Another third are overweight.

The slight decline in fast food consumption among adults reflects a growing trend toward healthier options. Many food and beverage companies have revamped their products or created new, healthier options to account for the shift in consumer tastes.

Still, Americans lead the world in calorie consumption. Portion sizes also have increased over the years, coupled with an increasingly sedentary lifestyle, have added up to extra pounds. Complications from obesity include diabetes, heart disease, arthritis and some cancers.

"Previous studies have reported that more frequent fast-food consumption is associated with higher energy and fat intake and lower intake of healthful nutrients," CDC wrote.

Young black adults are especially a concern. Those aged 20 to 39 get more than one-fifth, or 21 percent, of their calories from fast food versus whites and Hispanics in the same age group who get about 15 percent from such foods, CDC found.

Obese and overweight adults also ate more fast food, it added.

Healthy weight is calculated by measuring body mass index, or BMI, using height and weight. For example, a 5-foot, 6-inch (1.7 meter) woman weighing 186 pounds (84 kilograms) would be considered obese as would a 6-foot (1.8 meter) man weighing 221 pounds (100 kilograms).

The CDC also said that American boys aged 2 to 19 took in about 2,100 calories daily during 2009 and 2010, a drop from 2,258 calories in 1999-2000. Girls saw their daily caloric intake fall to 1,755 from 1,831 during the same timeframe.

It is not yet clear how the recent change has affected childhood obesity rates, the agency added.

Among the other findings:

* The consumption of calories from fast food "significantly decreased" with age;

* Fast food consumption was about the same for low-income and higher-income adults;

* More children are eating more protein, except for black girls;

* Carbohydrate consumption is lower among white boys and girls as well as black boys.

(Editing by Doina Chiacu)