Do vitamin C & beta-carotene reduce the risk of type 2 diabetes?
A diet high in vitamin C and beta-carotene may provide some protection against type 2 diabetes. This is suggested by an epidemiological study from Japan recently published in the Journal of Nutritional Science and Vitaminology.


Researchers at Tokai University School of Medicine studied data from 506 male and 493 female clients of an anti-aging clinic.The researchers determined the concentration of vitamin C and beta-carotene in the blood of the study participants, plus the concentration of the sugared iron-containing protein HbA1c in the red blood cells. HbA1c is a marker of glucose exposure. The more HbA1c red blood cells contain, the more glucose has circulated in the blood for a longer period of time. HbA1c is a marker for type-2 diabetes.
Diabetes, vitamin C & beta-carotene
A trial published in 2009 in which researchers wanted to alleviate type-2 diabetes with supplements containing vitamin C and beta-carotene had disappointing results. [Am J Clin Nutr. 2009 Aug;90(2):429-37.]

The results of the epidemiological studies into the protective effect of vitamin C and beta-carotene intake are less clear-cut. To any study that finds no association [Eur J Clin Nutr. 2011 May;65(5):590-7.] there is one in which vitamin C or beta-carotene does protect against diabetes. [Arch Intern Med. 2008 Jul 28;168(14):1493-9.]
The theory that dietary antioxidants can reduce the risk of type 2 diabetes is not at all unlikely. The high glucose level that occurs in untreated forms of diabetes damages cells and tissues, among other things via oxidant reactions, and antioxidants such as vitamin C and beta-carotene may slow those reactions. You would think.
The more vitamin C and beta-carotene the researchers found in the blood of the study participants, the lower the concentration of HbA1c in their red blood cells.

The researchers knocked out a few study participants who supplemented with extreme amounts of vitamins from their sample. They didn't do that with regular supplement users.
The researchers did not use statistics to brush away the influence of BMI and other lifestyle factors. That's a minor minus pooint of this research, said the ergoblogger euphemistically.
Food sources
The two tables below give you an idea of the main dietary sources of vitamin C and beta-carotene respectively. We got them from Wikipedia. They are based on data from the US Department of Agriculture.

Click on them for a larger version.

Nutr Sci Vitaminol (Tokyo). 2020;66(4):289-95.