I'd echo the above post. Most of my longer sessions on a weekend morning with no breakfast, save for a small fruit juice etc. No problems with this, but my higher intensity sessions usually on a weekday evening after a meal.

Although interestingly when I was in Thailand I would do a morning session including 10km run, skipping, and then all manner of boxing and conditioning, mostly at high intensity, on no breakfast, 6 days a week, followed by a similar afternoon session, and no problems either.

Even during the week I never have breakfast, mainly as weight management, and going against all advice my largest meal is before bed (can't face going to bed hungry). I am around 82kg which is massive for a fell runner, however I'm 6ft 2" and my "fighting weight" was 78kg so definitely not carrying too much extra.