06/25/2026
The word deficiency gets used as if it means one thing, but the body runs every vitamin on its own clock, and the clocks are not close. Some vitamins sit in a reserve deep enough to last years.
Others hold barely a few weeks of buffer, and once intake stops the tank drains fast. The difference is not about how important the vitamin is. It is about where and how much the body can physically store, and that single fact decides how quickly a shortfall turns into a problem.
The big tanks belong to the fat-soluble vitamins and to B12. Fat-soluble vitamins dissolve into tissue rather than washing out in urine, so the body can stockpile them. Vitamin A is held in the liver in quantities measured in months to years of supply, confirmed in direct liver-store measurements going back decades. Vitamin D and vitamin E bank into fat tissue and turn over slowly, with circulating vitamin D carrying a half-life measured in weeks and a reserve that buffers for months. B12 is the standout among the water-soluble vitamins: the liver holds a store large enough that deficiency from low intake alone takes years to surface, which is why true dietary B12 deficiency is slow while the faster cases usually come from an absorption problem rather than an empty tank.
The middle tier holds vitamins with a real but limited reserve. Vitamin C is the classic example. The body maintains a pool of roughly 1500 milligrams, and experimental studies in which volunteers ate no vitamin C at all found that scurvy did not appear until the pool fell below about 300 milligrams, which took around three months. Folate stores in the liver buffer for a few months as well, with megaloblastic changes appearing after roughly four months on a deficient diet. Vitamin K sits at the lower edge of this group, with a smaller store and faster turnover that buys weeks rather than months.
Most of the B-complex vitamins are water-soluble, barely stored, and continuously flushed, so the reserve is days to weeks rather than months. Thiamine is the sharpest case with real data behind it: in a controlled restriction study, measurable depletion appeared in just four to five weeks. Riboflavin, B6, niacin, B5, and biotin share the same basic problem of minimal storage, which is why deficiencies in this group can develop quickly when intake drops, when needs rise, or when something interferes with absorption.
A gap in a deep-tank vitamin like A or B12 can go unnoticed for a very long time, while a gap in a small-tank vitamin like thiamine shows up on a timescale of weeks. When intake suddenly drops, whether from a restrictive diet, heavy alcohol use, or an illness that wrecks appetite, the small tanks empty first and are the ones worth thinking about soonest. The deep reserves are a luxury the body only affords for a handful of vitamins.
Stabler, NEJM 2013
Hoppner et al., CMAJ 1969
Jones, Am J Clin Nutr 2008
Hodges et al., Am J Clin Nutr 1969
Golding, BMC Hematol 2018
Wood et al., Am J Clin Nutr 1980
