More often than not, finding the motivation to exercise is plain difficult. It’s hard work getting out for a run, lifting weights, and pushing your body to the limit. But there’s no better motivation than knowing that with each push-up and leg press, you’re ensuring a healthier, longer life. Scientists have struggled for some time to figure out how beneficial exercise is in the long-term, but now, a small new study on identical twins lends us some clues.

Researchers from Finland’s University of Jyväskylä investigated how being physically active affected the bodies of 10 twin pairs, from their brains’ structures to their body fat percentages. Among each pair of identical twins, one had stopped being physically active for about three years prior to the beginning of the study, while the other had remained active. Both had been active during their childhood but had gone down different fitness paths once leaving their childhood home.

Studying twins gave the researchers a unique opportunity to investigate the health effects of exercise without confounding factors like childhood environment — assuming each pair grew up in the same household — and diet. Both had similar diets throughout their lives and during the study period. But most importantly, each of them started with the same genetic makeup, putting them at the same base level for the ways in which they respond to exercise, The New York Times reported.

For the study, the researchers sifted through the country’s FinnTwin16 database, which included twins’ answers to questionnaires about their health and medical history from the age of 16, as well as follow-ups. They narrowed their search down to the 10 pairs, all of whom were aged 32 to 36. Each of them went through tests to measure body fat composition, insulin sensitivity, endurance, and brain structure.

They found physical activity, and a lack of it, brought drastic changes to the twins’ bodies. Among those who exercised regularly, body fat percentages were lower, endurance levels were higher, and gray matter in their brains was more abundant — especially in the regions responsible for motor control and coordination, the striatum and prefrontal cortex. Meanwhile, those who lived mainly sedentary lives for three years were on the verge of insulin resistance, indicating a trajectory toward metabolic disorder and possibly even type 2 diabetes.

Despite its small number of participants, the study shows how consistent physical activity might have a substantial effect on our bodies after only a short time. Obesity affects over a third of the U.S. population, putting millions of people at risk for heart disease, cancer, and diabetes, all of which are leading causes of preventable death — heart disease is number one. By taking charge of our lives and exercising regularly, we put ourselves in a position to live longer, happier, and healthier.

Source: Rottensteiner M, Leskinen T, Niskanen E, et al. Physical activity, fitness, glucose homeostasis, and brain morphology in twins. Medicine & Science in Sports & Exercise. 2015.