The insurer investing the most in fast food -- $422.2 million -- was Northwestern Mutual, which offers life, disability, and long-term care insurance.
Next highest at $406.1 million was ING, a Dutch investment company selling life and disability insurance.
Massachusetts Mutual, which offers life, disability, and long-term care insurance, and Prudential Financial, which sells life insurance and long-term disability coverage, ranked third and fourth at $366.5 million and $355.5 million, respectively.
At first glance, it would appear that there is an inconsistency with insurance companies that have an interest in protecting health investing in the fast-food industry, although Boyd said that it makes sense financially.
"They're hedging their bets," he said. "First of all, they're making money by directly investing in fast food, and, secondly, they're making money by often charging higher premiums to people who've eaten a little too much fast food and are obese, have diabetes, cardiovascular disease, high blood pressure, etc."
He said that he would like to see insurance companies sell their stakes in the fast-food companies.
"But if they don't divest, at a minimum they could use their position as owners of fast food to insist on higher quality products, lower calories, [and] better information about how many calories are in different foods," he said.
Dr. David Orentlicher of the William S. and Christine S. Hall Center for Law and Health at Indiana University agreed that insurers should be held to a higher standard of corporate responsibility.
"That said," he said, "it is very difficult in a capitalist society to expect people or companies to act against their self-interest. If we want people to act responsibly, we have to create financial incentives for them to do so."
Theodore Marmor, a professor emeritus of public policy and management at Yale School of Management, did not agree that health and life insurers should be held to a different standard than other companies.
"I think this is a foolish approach to improving the health of the public," Marmor said.
"It would be hard to find any corporation that did not have some effect on the public's health. Why are insurers to be held to a higher standard? We have more important worries about health insurers than improving their stock portfolios -- e.g., their behavior as insurers."
ING did not respond to a request for comment.
In a statement, a spokeswoman for Northwestern Mutual disputed the study's figures, saying that the company's stock holdings in the fast-food industry at the end of 2008 totaled less than $257 million. The current total is slightly less at $248 million, or 0.17 percent of a $146.1 billion portfolio, she said.
A spokesman for Massachusetts Mutual said the reported investments were "absolutely incorrect." He said that as of Dec. 31, the company owned about $1.4 million worth of stock in fast-food-related companies, which was "less than one-hundredth of one percent of cash and total invested assets of $86.6 billion."
A Prudential Financial spokesman said, "We can't discuss specific investments within Prudential portfolios or those managed for third parties. That said, we have a fiduciary duty to manage assets in a way that provides the opportunity for consistently strong investment performance to our individual and institutional clients, while managing risk and investing responsibly."