The Harvard Medical School's Dr. Wesley Boyd, an author of the study, finds it ironic that these firms would invest nearly $2 billion in companies that sell food often linked to obesity and cardiovascular disease.
"The insurance industry, so far as it seeks to make a profit, it does so in an amoral way," Boyd said.
Boyd said health insurers should be held to higher corporate standards.
Among big investors in fast food companies are life and disability insurers, like Prudential Financial, Northwestern Mutual and Massachusetts Mutual.
According to the study, Northwestern Mutual owns $422.2 million in fast-food stock, with $318.1 million invested in McDonald's. Massachusetts Mutual owns $366.5 million of fast-food stock, including $267.2 in McDonald's.
Holland-based ING, an investment firm that also offers life and disability insurance, has total fast-food holdings of $406.1 million, including $12.3 million in Jack in the Box, $311 million in McDonald's, and $82.1 million in Yum! Brands, which owns Pizza Hut, KFC and Taco Bell.
New Jersey-based Prudential Financial Inc. sells life insurance and long-term disability coverage. With total fast-food holdings of $355.5 million, Prudential Financial owns $197.2 of stock in McDonald's and also has significant stakes in Burger King, Jack-in-the-Box and Yum! Brands.
The authors write that the recent passage of health care reform will likely expand the reach of the insurance industry, arguing that if insurers are to play a greater role in health care that they should be held to a higher standard of corporate responsibility.
"There's a ton of irony in it," said Boyd, a psychiatry professor. "In order to generate profits, they will invest in any area they need to … to make money, even if what they invest in, in this case fast food, is an industry that is known to cause people to get sick and to die early."