Diet and exercise are not separate ways to improve the health

Muscle tissue can remodel itself, which is one reason why exercise becomes easier when we do it regularly. Over time, aerobic exercise such as running or swimming can alter muscle fibers to become more efficient at using oxygen during exercise and increase our aerobic fitness levels. Joslin Diabetes Center studies have demonstrated that people with diabetes or chronically high levels of blood sugar struggle to improve their aerobic exercise capacity compared to people with normal blood sugar levels.
High levels of blood sugar may prevent muscle remodeling and the muscle was not adapting to the aerobic challenge as muscle normally would.
Study shows diet and exercise are not separate ways to improve the health and suggests that we may want to consider them together to maximize the health benefits of aerobic exercise.
In order to increase the benefits of aerobic exercise, the study suggests for people with higher-than-normal blood sugar levels, adopting a diet designed to keep blood sugar levels low or to take existing diabetes drugs designed to keep blood sugar levels in normal ranges.

Source: https://www.joslin.org/about/news-media/increased-blood-sugar-levels-may-decrease-benefits-aerobic-exercise