During a recent run, due to low blood sugar, I developed double vision. Otherwise, I felt great. Out of prudence, I quit and decided to walk home. I had eaten nothing for 14 hours before the run, so I am sure the problem was low blood sugar.

The double vision started half way through a 6-mile run. Had I continued on with the annoying double vision, what might have realistically happened? I am in seriously good shape, so what realistically might have happened? What would have been the next symptoms?

  • I think runners call it "hitting the wall" when you get close to 0 carbs. That phrase should return some good Google results. I'm no runner myself, or I'd try to answer.
    – Tyler
    Commented Jan 22, 2015 at 5:20

2 Answers 2


Basically, your body will start shutting down external systems (muscle control, digestion, etc) to preserve glucose to keep the brain ticking over. For a good example of this, look at the video of Gabriela Andersen-Scheiss (Then Gabriela Andersen) at the Olympic marathon in 1984.

You can see that over the last lap, she has little control and keeps wandering around, collapsing, and barely making progress. You can also see it in the women's Ironman finish from 1997, with Sian Welch and Wendy Ingraham. This was fatigue, muscle cramping and hypoglycemia.

Eventually, if you keep exercising in that state, your body will shut down, and in extreme cases it can lead to death.

  • In layman terms: During the aerobic process your muscles burn muscle-sugar (glycogen). Your blood sugar (glucose) is used to replenish the muscle stores. Therefore; if you continue to run, your blood sugar will continue to drop. It is important to note that in ultra-marathons the dissociations, like the one in @JohnP's video, can be caused by hyponatremia. Or, too low blood sodium. Because your body wants to stay in homeostasis, if your sodium drops too low, it will pull sodium from your nervous system. This can cause motor-function loss and even death.
    – BryceH
    Commented Jan 22, 2015 at 17:55

As noted by JohnP, there's not a lot of direct side effects at first, the effect being the result of your body routing energy to essential systems at the cost of others, but the effect is much like being drunk in that you lose coordination, reaction speed, and common sense. In much the same way that drinking alcohol sharply increases your risk of trauma, continuing your exercise after "hitting the wall" can result in serious injuries as you're more likely to trip, and less likely to catch yourself in the process. At that, you don't even get the mild improvement in survival of head trauma that moderate alcohol consumption provides.

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