Take a lesson from Athletics?
You know in good time who the chosen 'nauts are.
Six months to launch, take some NK cells out of their body, grow them on in the lab, whilst the body naturally replenishes them.
A day or two before lift off inject the NK cells back into the body. Packing them in.
Double the benefit, they're healthier for the journey AND they get to the finish line, Mars, that much quicker.
Oh, and pack plenty of ginseng tea for the journey.