Eating too little doesn't make you a tougher runner. It just leaves you less equipped to handle the training.
You bonk more easily (i.e. you run out of energy), recover less well, and you increase the risk of injury. Over time, a negative energy balance can also affect your hormones, your periods and your bone health.
- See also
- On carb loading
So is sugar healthy, then? Everything has to be seen in context! When you train for a long time, sugar, that is fast carbohydrates, is first and foremost readily available fuel.
And by the way, you get pretty grumpy walking around hungry.