Two Mortality Studies on Danger of Low Vitamin C Blood Levels. Both studies were based on blood measurements and the results should have made world-wide news.

Relation between plasma ascorbic acid and mortality in men and women in EPIC-Norfolk prospective study: a prospective population study

https://www.thelancet.com/journals/lancet/article/PIIS0140-6736%2800%2904128-3/fulltext

Cambridge Medical School Researchers Findings

"Plasma ascorbic acid concentration was inversely related to mortality from all-causes, and from cardiovascular disease, and ischaemic heart disease in men and women.

Risk of mortality in the top ascorbic acid quintile was about half the risk in the lowest quintile (p<0·0001).

The relation with mortality was continuous through the whole distribution of ascorbic acid concentrations. 20 μmol/L rise in plasma ascorbic acid concentration, equivalent to about 50 g per day increase in fruit and vegetable intake, was associated with about a 20% reduction in risk of all-cause mortality (p<0·0001), independent of age, systolic blood pressure, blood cholesterol, cigarette smoking habit, diabetes, and supplement use. Ascorbic acid was inversely related to cancer mortality in men but not women.

Another mortality study:

Relation of Serum Ascorbic Acid to Mortality Among US Adults

https://www.tandfonline.com/doi/abs/10.1080/07315724.2001.10719040

"Conclusions: Low SAA (Serum Ascorbic Acid/Vitamin C) levels were significantly associated with an increased risk for all-cause mortality."