If I go for a run around the time I would get hungry and would soon eat, say early afternoon just before lunch or evening before dinner, I often feel a noticeable sharper drop in energy after ~20min of running or so.

A while after I finish I mostly recover to normal or "hungry normal", even if I don't eat anything.

I don't see it as a problem, medical or otherwise, I am just curious what it means under the hood. For example, while I call it a "sugar drop", even for a non-fitness freak it seems quite early to have depleted muscle glycogen and be drawing on blood sugar?

Thank you!

2 Answers 2


When you exercise, the body uses two sources of fuel, glucose and free fatty acids to generate energy.

The glucose comes from the blood, the liver and the muscles (glycogen) as you were saying in your post.

During the first 15 minutes of exercise, most of the sugar for fuel comes from either the blood stream or the muscle glycogen, which is converted back to sugar. After 15 minutes of exercise, however, the fuel starts to come more from the glycogen stored in the liver. After 30 minutes of exercise, the body begins to get more of its energy from the free fatty acids. As a result, exercise can deplete sugar levels and glycogen stores.

The body will replace these glycogen stores the process usually takes about 4-6 hours.


You didn't give enough information as in what is your physical fitness and how long have you been doing this. Also running on empty stomach should normally feel like that most of the time unless you are an endurance athlete and have been doing it for some time. Also no age, training plan and what is your goal, even for how long after 20 mins. are you running and with what speed or for what distance.

In short. That's exactly the time - around 25 min. from the start of a run when the body switches your metabolism to burn more fat. This switch could be quite noticeable if you are still a beginner in running which seems like it.

Sources: http://www.dummies.com/health/exercise/busting-the-great-myths-of-fat-burning/

  • Age: Late 30s. I typically run 5-50min (typically ~30min once a week, sometimes different route plus a few jogs to/from the train station). Speed is 10-12km/h or 6.25-7.50mph average, depending on the distance. Plan/goal is just to be able to handle multi-day mountain walks (15-30km/day depending on terrain), which seems to work OK if I do two runs per week the last few weeks before going. I have been doing this for 10 or so years, only difference is that I stopped cycling to/from work a bit less than 2 years ago. Reading the link it sounds like that might not change the answer - thank you.
    – nsandersen
    Jan 16, 2017 at 10:47

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