Nitric Oxide (NO) boosters have been a popular supplement category since their arrival in the late ’90s. NO relaxes blood vessels, dilating them to allow more blood flow, and thus more oxygen and nutrients, to the muscles. The end result is more endurance and strength, as research confirms, and in short, NO can help you train harder for longer. The following ingredients are must-haves in any NO supplement.

ARGININE
This amino acid is the most widely used NO booster around. A recent study found that subjects who took arginine before a biceps workout experienced a 100% boost in biceps blood volume during training. If a product doesn’t contain arginine but does have citrulline, that’s just as good. Citrulline converts to arginine in the body and may even be absorbed more readily.

PYCNOGENOL
A flavonoid from the French maritime pine tree, pycnogenol is a potent antioxidant. It increases NO levels in two ways, by catalyzing the conversion of more arginine into NO, and by quelling free radicals that can break down NO, resulting in higher NO levels for longer.

NORVALINE
This derivative of the branched-chain amino acid valine has been shown to boost NO levels by more than 50 percent on its own. It works by inhibiting the enzyme arginase, which normally breaks down arginine in the body. Inhibition of this enzyme results in higher arginine levels, which can lead to higher NO levels.