I’ve read some good things about beetroot juice specifically for endurance.
An even newer supplement to the market, beetroot juice has received a ton of hype over the last couple of years after several studies demonstrated its remarkable ability to increase endurance in tasks of varying intensity and duration.
It’s assumed that beetroot juice exerts this effect because of its high concentration of nitrates, which are related to nitric oxide (NO) and its effects on increasing blood vessel dilation and blood flow to working muscles. Some research has shown that the juice may improve time to exhaustion in high intensity activities by up to as much as 15%.
The best time to use beetroot juice is 30-60 minutes before training, with an effective dose of 300-500mL of the juice itself. It’s also thought that it may take several days of drinking the juice to really see the benefits, so it’s important to drink it even on non-training days for at least the first week or two.
8weeksout dot com/2015/10/01/conditioning-supplements-that-really-work/