Deficiencies in folate (Vitamin B9) and Vitamin B12 are systematically linked to the modern epidemic of chronic fatigue.

Study: Effects of B vitamins and magnesium on fatigue, disease activity and quality of life in inflammatory bowel disease

Giving people B vitamins (including B12) made their fatigue better in a study of people with a gut disease, which suggests that low B vitamins might be one reason people feel super tired all the time.

Study: Effects of Vitamin B12 and Folic Acid Supplementation on Negative Symptoms as the Primary Outcome and Positive and Cognitive Symptoms as Secondary Outcomes in Schizophrenia: A Randomized Controlled Trial

This study found that giving people with schizophrenia extra B12 and folate made them feel less tired and less emotionally flat—symptoms very similar to chronic fatigue. This suggests low levels of these vitamins might cause fatigue-like feelings, which supports the idea that modern vitamin deficiencies could be contributing to the rise in chronic fatigue.

Study: Association of Vitamin B12 Deficiency with Fatigue and Depression after Lacunar Stroke

This study found that stroke patients with low B12 levels felt much more tired than those with normal levels, suggesting that low B12 might make people feel fatigued. While it doesn't prove this causes the modern rise in fatigue, it shows a clear connection.