I think the issue is function, not just donation status. We all lose function as we age, more or less, and while our remaining kidney rebounds some after we donate, there has been a loss of nephron mass and number. Supplementation of D, the B complex, calcium, and more is increasingly recommended for those over fifty, presumably to do with decreasing function in absorption and synthesis from dietary sources. Perhaps we just fit an older digestive profile? Some studies I found:
Canada:
Bone and mineral metabolism and fibroblast growth factor 23 levels after kidney donation.
Young A, Hodsman AB, Boudville N, Geddes C, Gill J, Goltzman D, Jassal SV, Klarenbach S, Knoll G, Muirhead N, Prasad GV, Treleaven D, Garg AX; Donor Nephrectomy Outcomes Research (DONOR) Network.
Am J Kidney Dis. 2012 Jun;59(6):761-9. doi: 10.1053/j.ajkd.2011.09.019. Epub 2011 Nov 16.
https://www.ncbi.nlm.nih.gov/pubmed/22093959"The FGF-23 pathway may be activated in living kidney donors who show early biochemical changes compatible with chronic kidney disease-mineral and bone disorder. Whether these changes influence bone mineral density and fracture rates warrants consideration."
Germany:
Vitamin A metabolism is changed in donors after living-kidney transplantation: an observational study.
Henze A, Raila J, Kempf C, Reinke P, Sefrin A, Querfeld U, Schweigert FJ.
Lipids Health Dis. 2011 Dec 7;10:231. doi: 10.1186/1476-511X-10-231.
https://www.ncbi.nlm.nih.gov/pubmed/22151790"LDKT is generally regarded as beneficial for allograft recipients and not particularly detrimental for the donors. However, it could be demonstrated in this study that a moderate reduction of kidney function by unilateral nephrectomy, resulted in an imbalance of components of vitamin A metabolism with a significant increase of retinol and RBP4 and apoRBP4 concentration in serum of donors."
Poland:
Serum concentration of vitamin D and parathyroid hormone after living kidney donation.
Bieniasz M, Kwiatkowski A, Domagała P, Gozdowska J, Kieszek R, Ostrowski K, Deptuła A, Durlik M, Paczek L, Chmura A.
Transplant Proc. 2009 Oct;41(

:3067-8. doi: 10.1016/j.transproceed.2009.09.039.
https://www.ncbi.nlm.nih.gov/pubmed/19857678"Prospective studies may be essential to determine metabolic changes after nephrectomy among living kidney donors."