What the C677T variant changes
MTHFR is the enzyme that produces the active methyl form of folate used to recycle homocysteine. A common genetic variant, C677T, lowers enzyme activity. People who carry two copies (homozygotes) have reduced activity (often described as roughly 30 to 70 percent lower depending on the source and conditions), which can modestly raise homocysteine, especially when folate intake is low.
The variant is fairly common in the general population, and many carriers have no health problem from it, particularly when folate status is adequate.
Putting the variant in proportion
Because the affected step makes methylfolate, supplemental 5-MTHF (methylfolate) can bypass the slower conversion. That logic is sound, but the broader clinical claims attached to MTHFR online often run far ahead of the evidence; routine testing is not recommended for most people, and folate from diet or standard supplements is usually sufficient. Adequate folate generally normalizes homocysteine in carriers.
This is general educational information, not medical advice. MTHFR results should be interpreted by a qualified clinician, not used to justify high-dose self-supplementation.