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.

Now, for the new study, which was 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.

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 levels of 147 proteins were strongly associated with people’s baseline fitness, the researchers found.

More intriguing, a separate set of 102 proteins tended to predict people’s physical responses to exercise.

Higher and lower levels of these molecules — few of which overlapped with the proteins related to people’s baseline fitness — prophesied the extent to which someone’s aerobic capacity would increase, if at all, with exercise.

Finally, because aerobic fitness is so strongly linked to longevity, the scientists crosschecked levels of the various fitness-related proteins in the blood of people enrolled in a separate health study that included mortality records, and found that protein signatures implying lower or greater fitness response likewise signified shorter or longer lives.

Scientists will need to study far more people, with far broader disparities in their health, fitness, age and lifestyle, to zero in on which proteins matter most for predicting an individual’s exercise response.