As a runner who’s also a dietitian, I’m often asked for nutrition related running advice, and I also dive deep into research related to running nutrition. I am frequently asked, “Is there benefit to starvation training (training your body to sustain continued activity with low caloric intake)?” I have attempted to define parameters to this many times over the years, to establish the sweet spot for starvation training, which turns out to be a fine line between benefit and detriment.

I have had several friends make comments to me, such as,

“Why don’t I feel good anymore when I run?”

“I can’t seem to run at the same capacity that I used to.”

“It seems like every run is a struggle these days.”

The one I hear the most: “I just don’t feel strong anymore.” Can you guess what I have found to be the common denominator among those who ask me these questions? They all frequently do starvation runs, even if unintentionally. Some say they simply never eat when they run, and they never have. I get it; it’s inconvenient to carry nutrition and the extra planning involved. Another common denominator among these runners tends to be completely normal blood work. Which has raised a question in my brain, and lead me to research this further; this is a summary of that research, in my own words.

Glycogen, the storage form of carbohydrates, is the primary fuel source for runners, once any food in our stomach is used up. Glycogen is found in our muscles, with a small amount also stored in the liver. Men store more glycogen than women, at approximately 400 grams vs 300 grams, respectively. You can also increase your glycogen storage capacity through exercise, provided you replace carbs in a timely fashion after activity has stopped. The other factor that plays a role here is effort; essentially meaning you could burn through the same amount of glycogen running a 5k as you could a half marathon, if you sprint the 5k and jog the half marathon.

So what happens when glycogen is gone? Your body begins to use fat for energy. Perhaps you’re thinking, “Well, that sounds pretty awesome. Let’s do that more!” Unfortunately, this also has a time limit. Let me explain why your muscles will eventually feed you. While protein is not a preferred fuel substrate for endurance exercise, you leave your body no other option when you enter the “starvation” phase of a run. This is essentially cannibalization of your muscle tissue.

Perhaps you are thinking, “Well, I have plenty of stored energy, so this wouldn’t happen to me.” Let me tell you this: It will happen to anyone who runs for hours at a time without taking in fuel. When I say “hours”, I mean almost any run of more than 2 hours. The resulting damage happens slowly and over time. Recall that I mentioned friends who have been running for several years coming to me with the concerns of no longer feeling strong. That’s because it takes time for the damage to truly manifest itself.

Another issue that runners often deal with at several years into endurance training is feeling like they eat less than they used to, and now deal with weight gain. Why is this occurring? Because muscle burns more calories than fat and when your muscles have to fuel your run, they become smaller over time. As they become smaller, your caloric requirement is less. This cycle perpetuates itself by the runner feeling like he or she can’t afford the calories, and muscles suffer the consequences at an accelerated rate. You don’t feel strong anymore because you’re literally not strong anymore. Your metabolism slows because you have successfully trained your body to run on empty. You have successfully adapted your body to starvation training.

Congratulations! Now it’s time for the long road to recovery.

So what does this all mean? Is all hope lost? While prevention in the first place would be ideal, all hope is not lost. It is possible to rebuild and restore. The road to recovery means fueling your run with good nutrition. It means consuming calories that contain a good ratio of carbs/fats/proteins within 30 minutes after a run, for optimal glycogen uptake into the muscles.

So what does prevention look like? Keep in mind that the below guidelines are meant to be generalized. As individuals, we are not a one size fits all species. Figure out what works for you.

  • If you run on empty, limit your run to 60-90 minutes.
    • Tip: ideal fat burning does occur when running on empty for 60-90 minutes.
  • If you consume calories before you run, it should sustain you for 2 hours.
  • If you are planning to run for more than 2 hours, start consuming calories 1 hour into the run, regardless of if you ate prior to the run or not.
    • Tip: you should always eat something first if you are planning to run for more than 2 hours.
  • Ideally consume 100-150 calories per hour during sustained activity.
  • Consumption of large amounts of calories during activity will likely lead to GI upset, greatly dependent on your exertion level.
  • If you plan to consume calories during a race, you need to consume calories during training. If you adapt to starvation, you can also adapt to digestion.

Keep in mind that this blog is to address those who read this and say “this is totally what I’m dealing with!” This is not intended to be a diagnosis in anyway, but hopefully to help keep others from going down a road so many of us have traveled. This is also not meant to be an excuse to ignore something that may be more serious. You should always start with your doctor if you are not feeling well. There may be other factors playing a role that should not be ignored.

For my next blog, I’ll be discussing, “Why am I craving sugar, pickle juice, or peanut butter?”