The Science:

Beetroot contains 250mg per 100g of Nitric Oxide (Hord, 2008) which is a vasodilator. Vasodilation causes the lumen (the area were blood flows) of the blood vessels to expand. This is of benefit as it increases the blood flow of oxygenated blood to the working muscles. In addition the increased blood flow increases stroke volume (amount of blood pumped in one beat of the heart) consequently this decreases heart rate therefore making the body more efficient.

Who?

Research has demonstrated that beetroot does not always help performance. Studies show that the intake of beetroot before anaerobic exercise (sprinting) has no impact on performance. However there is evidence to suggest that during aerobic or sub-maximal exercise (endurance) beetroot can lower oxygen consumption. This is good as it means breathing and heart rate does not need to work as hard for the same result. Partially due to the increased blood flow beetroot causes as mentioned earlier. 

When? 

Beetroot juice is considered a ‘medium’ on the Glycemic Index. Meaning the body will not metabolise beetroot quickly. It has therefore been suggested by Jones (2014) and Webb (2008) that consumption is best 2-3 hours prior to exercise for optimum benefit.

References

Hord, N., Tang, Y. and Bryan, N. (2009). Food sources of nitrates and nitrites: the physiologic context for potential health benefits. The American Journal of Clinical Nutrition, 90(1), pp.1-10.

Webb, A., Patel, N., Loukogeorgakis, S., Okorie, M., Aboud, Z., Misra, S., Rashid, R., Miall, P., Deanfield, J., Benjamin, N., MacAllister, R., Hobbs, A. and Ahluwalia, A. (2008). Acute Blood Pressure Lowering, Vasoprotective, and Antiplatelet Properties of Dietary Nitrate via Bioconversion to Nitrite. Hypertension, 51(3), pp.784-790.

Jones, A. (2014). Dietary Nitrate Supplementation and Exercise Performance. Sports Medicine, 44(S1), pp.35-45.