Can supplements affect DNA? (Could 10,000% Methylcobalamin affect MTHFR genotype)
What the DNAfit test does is analyse some of your genes for common variants (called SNPs - single nucleotide polymorphisms). These are hard-written in your genetic code, they won't ever change (you could in theory have mutations in some individual cells, but we can ignore that).

Taking supplements or any other environmental influences will therefore not affect these tests.

The environment can affect certain markers on the DNA, that control how active a given gene is (how strong its expressed) - this is called epigenetics. Since these markers are written on the DNA they are also potentially heritable and the two effects (the genetic code itself and the markers on it) are often thrown into one pot when some news report about 'changes to your DNA'.