What would cause folate (folic acid) deficiency in an 18 y/o male?

Inadequate diet: This is caused by not eating enough foods rich in folate: dark leafy greens (spinach), beans & lentils, cruciferous vegetables like broccoli.You can also obtain folate from supplements, though most supps have folic acid, a synthetic form you need to convert to folate. 1/3 of people can't make that conversion & thus should take l-5-mthf. See http://www.Whfoods.Com/genpage.Php?Tname=nutrient&dbid=63.