This occurs when your bone marrow produces unusually large, immature red blood cells that cant carry oxygen properly
Birn H, Zhai X, Holm J, Hansen SI, Jacobsen C, Christensen EI, Moestrup SK
(Second on my list would be vitamin D, but its much harder to develop low vitamin D levels if you take a daily supplement, as I explain in this post
A peptides certificate of analysis should be independently verifiable meaning it comes from a third-party lab rather than the suppliers in-house testing alone since that independence is what gives the purity claim scientific credibility
Physical activity guidelines: UK Chief Medical Officers' report
Similarly, B12 is classified not for its chemical nature (a vitamin) but for the high-risk administration route (injection) and the severity of the condition (Pernicious Anaemia) it treats