Amy Finkelstein, a professor of economics at M.I.T. and one of the authors of the Oregon study, described this finding as their least surprising result. “There’s overwhelming evidence from our study and others that when you cover people with health insurance, they use more health care,” she said.

But what about prevention? In certain situations, early spending on someone’s health will stop an expensive disease in its tracks, reducing future spending. Giving people health insurance often enables them to get just this sort of preventive care — and Obamacare requires insurers to offer most preventive services without charging any co-payments. You might expect health spending to jump initially, then slow in future years as people benefit from new prevention. But research shows that even preventive care rarely ends up saving money.

Here’s why: For the individual patient whose heart attack is prevented by a cholesterol screening, to give one example, that blood test is a cost-saver. But to prevent one heart attack, the health care system has to test hundreds of healthy people — and give about a hundred of them cholesterol-lowering drugs for at least five years. Added together, those prevention measures cost more than is saved on the one heart attack treatment. (My colleagues Aaron E. Carroll and Austin Frakt have written a helpful article on this concept, known in medicine as the “number needed to treat.”)

Joshua T. Cohen, the deputy director of the Center for Evaluation of Value and Risk in Health at Tufts Medical Center, said: “We’ve all heard it before: ‘An ounce of prevention is worth a pound of cure.’ It doesn’t really play out when you analyze the numbers, and the reason for that is that you have to give a lot of people those ounces of prevention to end up with one person who’s going to get that pound of cure.”

There’s also the unavoidable fact that every time you prevent people from dying from one disease, they are likely to live longer and incur future medical expenses. The patient who benefits from the cholesterol screening may go on to develop cancer, arthritis, Alzheimer’s or some other costly illness.

There are some prevention measures that actually do seem to save money. Counseling on contraception is one because the costs of prenatal care, delivery and pediatric care associated with an unplanned pregnancy are so substantial. But a lot of the preventive health measures that we tend to value a lot — mammography, screening for diabetes — tend to cost more than they save. Researchers like Mr. Cohen, who study these trade-offs, say we need to think more broadly about the value of what our preventive spending buys, not the narrow question of whether it saves money.

That may be one way of thinking about the law’s benefits for the newly insured, who had limited access to the health care system. For many, their new coverage means they can get free checkups and preventive disease screenings, and can more easily pay for big health care needs, like prescription drugs and operations. Early evidence suggests they are facing less financial stress as a result of their coverage.

Some are now managing chronic and potentially life-threatening health conditions. But those benefits don’t mean we should be surprised by the actuaries’ recent findings on spending growth. More people in the health care system means more dollars spent on health care. The increase doesn’t mean that Obamacare is leading to runaway costs, or that it’s failing to reform the health care system to make it more efficient. But it does mean that we can’t think of the coverage expansion as free.