We know that obesity (high body fatness) is a major risk factor for type 2 diabetes. We also know that obesity is very difficult to reverse, and that we need to find ways of preventing type 2 diabetes when fat loss is not feasible. Muscle tissue – the other major body compartment – is likely beneficial for health, but little is known about which aspects of muscle (e.g. whether volume or strength) matter most for the earliest stages of type 2 diabetes, and how these benefits compare with the harms of fat. This project aims to use repeated measures data from ALSPAC offspring and parents to determine which aspects of muscle – whether higher volume based on body scanning, higher strength based on hand grip tests, or higher quality based on a combination of strength and volume – has stronger effects on an extensive set of detailed metabolic traits related to type 2 diabetes susceptibility. It also aims to determine how benefits of muscle compare with harms of fat, and whether muscle interacts with fat in relation to early stages of disease. Altogether, these results should help us to better understand which aspects of body composition are most important to target with limited public resources in order to prevent type 2 diabetes, and whether boosting muscle would help prevent type 2 diabetes when fat loss is not feasible.