If we all begin the same exercise routine tomorrow, some of us will become much fitter, others will get a little more in shape and a few of us may actually lose fitness. Individual responses to exercise can vary that wildly and, until now, unpredictably. But a fascinating new study of more than 650 men and women suggests that the levels of certain proteins in our bloodstream might foretell whether and how we will respond to various exercise regimens.

The study needs replication and expansion, but it represents a meaningful start toward a blood test to indicate the best types of exercise for each of us and whether we can expect to gain more or less benefit from the same workout as our spouse, offspring, or other training partners or rivals.

Exercise response is a topic that probably should be discussed more often and openly than it is. We know exercise is wonderful for our health. Countless studies show that people who exercise tend to live longer, more happily and with less risk of many diseases than sedentary people.

But those findings refer to broad averages. Parse the study data closely and you can find a dizzying gamut of reactions, from outsized health and fitness gains in some people to none in others. (The same is true of responses to weight loss programs.)

Disobligingly, little about our bodies and lives currently predicts how we will respond to exercise, including our genetics. Identical twins, with identical DNA, can react quite differently to workouts, studies show, as can people who are equally lean, obese or aerobically fit at the start of a new exercise program. Some, for mysterious reasons, wind up fitter and healthier afterward than others.

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These enigmas intrigued researchers from Harvard University, Beth Israel Deaconess Medical Centre in Boston and other institutions. The scientists had long been interested in how exercise alters the molecular environment inside the body, as well as how those changes influence health and how diverse the alterations can be.

Now, for the new study, published in May in Nature Metabolism, they decided to see if certain molecules in people’s blood might be related to how their physiologies react to workouts. To find out, they turned first to the valuable trove of data produced during the large-scale Heritage study, which had delved into exercise and health in parents and their adult offspring. The Heritage study included precise, laboratory testing of people’s aerobic fitness, as well as blood draws, followed by 20 weeks of moderate aerobic exercise, and more testing.

The researchers now pulled records for 654 of the men and women who had participated in Heritage, covering a range of ages and ethnicity, and began looking deeply into their blood. They focused on the varieties of large, complex protein molecules created in tissues throughout the body that, when released into the bloodstream, flow to and jump-start biological processes elsewhere, affecting how well our bodies work.