Jeff Rothschild, who has the website eat.sleep.fit (nutrition advice for active people), co-authored an interesting paper  on nutrition strategies to get improved fitness and health benefits from exercise. It turns out it is very helpful to try to increase signaling from the enzyme AMPK (short for the mouthful “5ʹ adenosine monophosphate (AMP)-activated protein kinase”). This improves metabolic health as we age, including insulin sensitivity, as discussed here, here, and here. The paper discusses how nutrition affects the amount of AMPK signaling during exercise. The paper is pretty technical, but Jeff gave a 16-minute youtube summary of it. It turns out one of the most important factors is the amount of glycogen left in your muscles at the end of your exercise session, or how empty your fuel tank is.
The emptier, the better, for producing the AMPK benefits. So doing a long relatively easy workout in the fasted state is one good approach. I was already doing this for the purpose of achieving fat-adaptation, and now it seems to have an additional benefit. In the video, Jeff also discusses why fueling instead of fasting before a higher-intensity workout might be a good idea.
While running your glycogen fuel tank low during a workout can be beneficial, it can be overdone. You don’t want to be miles from home on empty, what cyclists call “bonking” or runners call “hitting the wall”. It’s a pretty bad feeling. Low but not empty is good. So I’m going to continue with my combination of skipping breakfast on days I go at a longer and easier pace, but eating breakfast on days I do strength training or higher-intensity workouts.
Rothschild, J, et al, “Factors Influencing AMPK Activation During Cycling Exercise: A Pooled Analysis and Meta-Regression”, Sports Medicine, 2021. Online here.