The disease of obesity might exacerbate cancer, said Dr. Clifford Hudis, the chief executive officer of the American Society of Clinical Oncology.

But, he added, another reason for poor outcomes in obese cancer patients is almost certainly that medical care is compromised. Drug doses are usually based on standard body sizes or surface areas. The definition of a standard size, Dr. Hudis said, is often based on data involving people from decades ago, when the average person was thinner.

For fat people, that might lead to underdosing for some drugs, but it is hard to know without studying specific drug effects in heavier people, and such studies are generally not done. Without that data, if someone does not respond to a cancer drug, it is impossible to know whether the dose was wrong or the patient’s tumor was just resisting the drug.

One of the most frequent medical problems in obese patients is arthritis of the hip or knee. It is so common, in fact, that most patients arriving at orthopedists’ offices in agonizing pain from hip or knee arthritis are obese. But many orthopedists will not offer surgery unless the patients first lose weight, said Dr. Adolph J. Yates Jr., an orthopedics professor at the University of Pittsburgh School of Medicine.

“There are offices that will screen by phone,” Dr. Yates said. “They will ask for weight and height and tell patients before they see them that they can’t help them.”

But how well grounded are those weight limits?

“There is a perception among some surgeons that it is more difficult, and certainly some felt it was an added risk,” to operate on very obese people, Dr. Yates said. He was a member of a committee that reviewed the risks and benefits of joint replacement in obese patients for the American Association of Hip and Knee Surgeons. The group concluded that heavy patients should first be counseled to lose weight because a lower weight reduces stress on the joints and can alleviate pain without surgery.