Great question Tommy08. Like most things in our body, there are multiple issues that cause our BH4 levels drop. We must examine the precursors that make BH4 and what might recycle BH4. Genes and diet effect both the precursors and our recycling ability. Eating more leafy greens increases folate levels which increases BH4 production.
www.sciencedirect.com/science/article/pii/0165032789900621When we have infections, (especially fungal/yeasts) our phenylalanine excretion level goes way up. There are tests for this. It's been shown that high phenylalanine levels affect blood levels of biopterin. If it's in the blood, it's not in the cells where it needs to be to make BH4. Reduce the infection and reduce phenylalanine intake and this should help make BH4. Phenylalanine is in milk, eggs, and meat.
www.sciencedirect.com/science/article/pii/000989819390139USulfa drugs like Septra DS or Bactrim DS reduce BH4 levels.
Here is a quote from a reference: Tetrahydrobiopterin biosynthesis within cultured dopamine neurons of the hypothalamus and mesencephalon thus appears to be regulated by a cAMP-dependent mechanism involving enhanced gene expression of enzyme(s) involved in cofactor biosynthesis.
I find this interesting since the polyamines bind cAMP and directly affect this pathway. That's a much longer and deeper discussion that would take more research. Element wise, a decrease in body calcium, magnesium, and/or molybdenum can affect BH4 levels. They do it in differing manors which are too lengthy to discuss.
IMO, the best way to increase BH4 levels is by diet. I know it's not popular or fun, but it has been effective for me and many others who choose to adhere to the strict guidelines of the Wahls Diet. I ate it with over 90% compliance for 1 month and saw a drastic change in health. Research into why it improves health did not turn up any direct references, but my opinion based on multiple inferences from references is as follows. The Wahls Diet decreases phenylalanine intake, it increases antioxidant intake, it increases folate intake, it increases mineral levels (polyvalent cations) and it provides a source of flavins from the bioflavinoid intake. BH4 is a flavin. All of these things increase BH4 levels.
I think when we become sick, our body becomes overburdened because it doesn't have enough "raw materials" to function like it did in the past. Aging is the same. So much processing is going on that we become deficient in the chemical moieties that we need. I believe this is why eating polyphenols helps me. It is very complicated and much more to it than I know, but looking at it simply, 2 + 2 = 4. It always has and it always will. Our body works on the same principle but after we become sick, we're trying to make 2 + 1 = 4 and it just doesn't work. We just need to add a little bit to it and it will work like it's supposed to.