Humans require antioxidant intake (especially from the diet in the form of fruits and vegetables) in order to combat free radicals.  Excessive free radical production has been correlated with all chronic diseases, and can occur due to stress and toxin exposure.  While you can measure antioxidant status in serum, you can also measure it non-invasively through the skin.  This can tell you whether you are consuming enough fruits and vegetables to keep the oxidative stress in balance.  Complete Health offers non-invasive antioxidant status measurement.

Here is more education about oxidative stress:
https://www.medicalnewstoday.com/articles/324863#prevention

The relationships between skin carotenoid levels and metabolic syndrome:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8772725/

Skin carotenoids reflect fruit and vegetable intake:
https://nutritionj.biomedcentral.com/articles/10.1186/s12937-019-0500-0

Here is an excellent study looking at skin carotenoids, serum carotenoids, vegetable intake, and how these correlate with biomarkers of circulatory diseases and metabolic syndrome:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7353351/