When I get into longer workouts as I increase training towards an Ironman, I often lose my appetite for the remainder of the day. In other words, on the weekend when I have my long ride and long run, I often experience a very low appetite, where I would expect to be ravenous.

I have no idea if this is common or an anomaly. I hear athletes talk about insatiable hunger during high-volume periods. I usually eat more, but I’m not sure I experience what other athletes do.

I don’t know why I lose my appetite, and until now my only theory was that it was related to over training. However, I just learned something that could pose a new theory.

I am in the midst of listening to this podcast with Dom D’Agostino. He covers ketosis, extending a healthy lifespan, and other related topics.

What I learned relates to ketosis and endurance athletes, but first a really brief background based on energy and ketosis as I understand it. Remember, I’m not a scientist! If I have gotten something wrong, please let me know.

Our typical energy source is glycogen. We have limited stores of glycogen. Another fuel source is ketones, which we can survive off of for FAR longer than the glycogen stores we have. (This is surely different for everyone, but you can go days without eating, and when you do this, your fuel source is ketones). You may have heard of the ketogenic diet which is a current fad, and all about burning ketones instead of glycogen.

One can get into ketosis by fasting, eating a very high fat and low carb diet, and taking exogenous ketones can also help. Dom D’Agostino says that some people who do endurance sports also get into ketosis on a regular basis. When people get into ketosis, their bodies become adapted to this state and thus able to switch to burning ketones more quickly. He also said that ketosis has an appetite-suppressing effect.

Thus…I am now wondering if my lack of appetite is because sometimes I get into ketosis during the workout, and the appetite-suppressing effect somehow sticks around for the day?

It is probably really unlikely, but I thought it was an interesting hypothesis and wanted to throw it out there.

Do you ever lose your appetite after long workouts? Any educated thoughts on this possibility?