If you’re low or deficient in B12, you’ll likely feel fatigued.
Your body’s cells need B12 to function properly. As such, having inadequate B12 levels can decrease normal red blood cell production, which can impair oxygen delivery (8Trusted Source).
Specifically, a deficiency in B12 or folate can cause megaloblastic anemia. This condition leads to the formation of large, abnormal, and immature red blood cells and impaired DNA synthesis (2Trusted Source, 9Trusted Source).
When your body doesn’t have enough healthy red blood cells to carry oxygen to tissues, you’ll likely feel weak and tired.
It’s important to know that you can develop fatigue and other symptoms related to B12 deficiency, even when your B12 levels are considered within range or only borderline lowhttps://www.healthline.com/nutrition/vitamin-b12-deficiency-symptoms#1.-Fatigue