[Ozark Tom]

Lithium Depletion in Fully Convective Pre–Main-Sequence Stars

https://iopscience.iop.org/article/10.1086/304151/fulltext/35775.text.html

[Fred Hayek]

Lithium 7 when cooked becomes Lithium 6, which when further cooked becomes Tritium, a really good fusion fuel. Not taking Lithium 7 into account resulted in a disaster known as the Castle Bravo test.
Thus much of the Lithium 7 was consumed during pre main sequence, before the proton-proton reaction would start . Lithium is also a fuel material in brown dwarfs.