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Vitamin B12 is stored in your liver and can be reused by your body for up to 10 years.
Because of this, when you don’t get enough B12 from your food, your body may not show signs of deficiency right away. This can make it tricky to know if you need more B12 by checking your blood levels alone.
However, there’s another way to find out if you have a deficiency at a cellular level early on. It involves looking at a substance called MMA, which can be found in both your blood and urine.
By measuring the levels of MMA in your urine (while correcting for creatinine levels), you can get a better indication if you need more B12. This test is more sensitive and can detect even small changes in MMA levels.
One advantage of testing MMA levels in urine is that it helps reduce the chances of kidney issues giving false results, which is not the case with blood MMA levels. Additionally, the urine sample can stay good for several weeks without degrading, so it can be accurately tested in the lab.