At least some of the connection between optimism and blood lipids in the new study appeared to result from the optimists' tendency to have a healthy body weight and a "prudent" diet, according to the researchers.
"It is one additional piece of evidence suggesting that our psychological health and physical health are intertwined, and that viewing the world optimistically may have some tangible benefits for our health," said lead author Julia Boehm, a research fellow at the Harvard School of Public Health in Boston.
Previous research by Boehm and her colleagues had shown a link between optimism and lowered heart attack risk. So they decided to look at whether there was an independent connection between optimistic or pessimistic outlooks and cholesterol, which is known to play a role in heart attack risk.
The group analyzed data from the Midlife in the United States study, which included phone interviews and lab tests for 990 people aged 40 to 70.
Based on the interviews, participants' levels of optimism were rated on a scale from 6 to 30 depending on their agreement or disagreement with statements like, "In uncertain times I usually expect the best.
According to results published in The American Journal of Cardiology, people with higher optimism scores also had more high-density lipoproteins (HDL), the desirable form of cholesterol that is believed to protect against heart disease. They also had lower levels of triglycerides, the fatty molecules involved in hardening of the arteries.
There was no connection between optimism and total cholesterol levels, or to low-density lipoproteins (LDL), the "bad cholesterol."
For every increase of 5 points on the optimism scale, however, HDL in the blood increased by 1 milligram per deciliter.
That same HDL increase would translate to a three percent reduction in the risk for heart disease, experts said. For comparison, regular exercise can decrease heart disease risk by six percent.
"Honestly I'm not surprised, this is what I expect," said Dr. Franz Messerli, a cardiologist at St. Luke's-Roosevelt Hospital in New York, who was not involved in the study.
It's still impossible to say whether optimism causes a change in cholesterol, or cholesterol influences outlook, or both are subject to some third variable, according to Messerli.
"In the case we have here, we really don't know what is the chicken and what is the egg," he told Reuters Health.
Boehm's group did try to account for other influences, and when they factored-in lifestyles - including diet and alcohol consumption, and body weight - the link between optimism and blood fats became weaker.
That suggests the optimists' tendency to have healthier lifestyles and weight may explain "in part" the differences in their blood lipids, the researchers conclude.
"If you're a forward looking, positive individual, that attitude appears to have broad, far reaching consequences in areas of your life," said Dr. Hilary Tindle, who studies mind-body medicine at the University of Pittsburgh but wasn't involved with the new research.
Conversely, risk of heart attack and stroke goes up in depressed people, Messerli points out. "But nobody has shown the opposite, that all of a sudden if you go from a pessimist to an optimist your risk goes down."
Because the current study only found a link to blood lipids, but not to heart disease or cardiovascular "events," more research is needed, the experts agreed.
The time when attitude adjustments could be used in pursuit of physical health is still a long way off, Messerli said.