Diabetes drags down fitness, but staying active closes part of the gap

Cardiorespiratory fitness is one of the strongest predictors of how long people live, and low fitness raises the risk of cardiovascular disease by up to eightfold. That makes it a critical question whether diabetes itself is associated with impaired fitness, because people with diabetes already carry elevated cardiovascular risk. The existing literature has been contradictory: some studies found clear fitness deficits in people with type 1 or type 2 diabetes, while others found none. This review set out to settle whether the deficit is real, how big it is, and which clinical factors travel with it.

The researchers pooled 68 studies comparing cardiorespiratory fitness in people with type 1 or type 2 diabetes against people without diabetes. They calculated standardised differences between groups and ran meta-regressions to identify which clinical characteristics predicted larger fitness deficits. That allowed them to test whether physical activity levels, sedentary behaviour, diabetes complications, blood sugar control and body mass index were associated with the size of the gap, and to compare the two types of diabetes against each other.

Diabetes was associated with a large negative effect on cardiorespiratory fitness overall. High physical activity partially softened the deficit, but even active people with diabetes remained measurably less fit than people without it. A sedentary lifestyle predicted a bigger gap, and the presence of diabetes-related clinical complications predicted the biggest gap of all. Both types of diabetes were independently associated with impaired fitness, but the effect was significantly larger in type 2. In meta-regression, the fitness deficit tracked with HbA1c, a marker of long-term blood sugar, in type 1 diabetes, and with body mass index in type 2.

This analysis largely resolves a contradictory literature in favour of a real, sizeable fitness deficit in diabetes, and it adds texture by showing the deficit is not uniform. The finding that activity mitigates but does not erase the gap complicates the simple story that people with diabetes are merely less active. Because the pooled studies compared existing groups, the direction of causation stays open: does diabetes impair fitness, does low fitness precede diabetes, or both? The associations with blood sugar control and body mass point to where future intervention trials might look.

For a reader with diabetes, or watching their risk of it, this is context worth having: fitness is a headline risk factor, and diabetes is associated with running lower on it. The comparison between active and sedentary groups is the most useful part, since the fitness gap was smallest in highly active people and largest alongside sedentary living and complications. The associations with blood sugar control in type 1 and body weight in type 2 suggest the levers may differ by diabetes type. None of this dictates a programme, but it is a reason to take fitness measurement seriously if diabetes is part of your picture.