Previous studies have shown that high blood levels of antioxidants, of which carotenoids are one form, may be a marker of good health.
A commonly-known carotenoid is beta-carotene, found in high levels in orange fruit and green, leafy vegetables.
Antioxidants help keep other molecules in the body from producing free radicals, which can damage cells and contribute to disease.
"Individuals with greater optimism tended to have greater levels of carotenoids such as beta-carotene," said Julia Boehm, of the Harvard School of Public Health, who led the study, the journal Psychosomatic Medicine reports.
"This is the first study of its kind to report a relationship between optimism and healthier levels of carotenoid concentrations," she added.
One theory is that antioxidants might have a de-stressing effect, according to the Daily Mail.
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The current study evaluated blood concentrations of nine different antioxidants, including carotenoids such as beta-carotene and vitamin E in nearly 1,000 American men and women aged between 25 to 74 and 74 years.
Participants filled out a questionnaire about their life attitudes and provided blood samples to the researchers.
People who ate two or fewer servings of fruits and vegetables a day were significantly less optimistic than people who ate three or more servings a day.
They also measured the degree of optimism in the same group.
Researchers found that people who were more optimistic had up to a 13 percent increase in carotenoid concentrations in their blood compared with people who were less optimistic.
The researchers believe that higher levels of fruit and vegetable consumption among more optimistic people may at least partially explain the results.