John Saxton, senior research fellow in Exercise Physiology, replies: Eating little but often is a good way to maintain blood sugar levels, but make sure your diet contains a good mix of carbohydrates. The light-headedness could be due to a transient decrease in blood pressure, known as post-exercise hypotension. This phenomenon is likely to occur soon after your run or between sets of resistance training.
During exercise, blood pressure increases and this drives more blood to the active muscles, while maintaining flow to vital organs, including the brain. The contraction and relaxation cycles of the active muscles massage the blood back to the heart. But when exercise stops the heart continues to pump high volumes of blood into muscles that are no longer working. This can lead to a pooling of blood in the muscles and a diminished return of blood back to the heart. The knock-on effect is a reduction in blood pressure that can compromise blood flow to the brain, leading to light-headedness.
Some individuals are more susceptible to this and you should get your resting blood pressure checked by your GP. However, to reduce the likelihood of this happening, keep active muscles moving during rest cycles within your training session (particularly the large leg muscles), ensure you always drink plenty of fluids to maintain blood volume and implement a proper cool-down at the end of training that gradually brings your heart rate back to resting levels.