Eight healthy men were given meals comprised of the same amount of carbohydrates, fats, and protein.
But the calories in their diets varied. Sometimes they got complex carbohydrates, sometimes they got more fructose.
Dr. Jean-Marc Schwartz of Touro University said it was a really good study of how sugar converts to fat in the body.
"Because of that conversion of sugar to fat, you impact your metabolism in a negative way. Because you increase your liver fat, and this could be associated with liver disease, and this could lead to diabetes."
The people who ate a high proportion of fructose also showed fat in their blood, which could cause cardiovascular disease.
But Schwartz said health problems can be normalized when sugar intake is reduced.
The researchers said the study was small but significant because all subjects had the same response.