Rather than reinvent the wheel here, and not being a scientist myself or a doctor with any medical training, I submit for you here two different theories and let you decide for yourself. The link contains 22 pages of some pretty scientific reading.
MORE ON VITAMIN C and Possible Interaction:
Pharmaceutical drugs often react with foods and supplements, including vitamin C. And as we have seen, excessive vitamin C intake can interact with some nutritional compounds. Sometimes these interactions are beneficial, such as increasing the absorption of minerals. And sometimes it leads to problems such as iron overload or copper and B12 deficiencies.
Another area that is often overlooked is the interference of laboratory tests by excessive intake of vitamin C. For example, excessive vitamin C intake may lead to false high or low bilirubin levels, depending on the assay test being used. Lactate dehydrogenase, cholesterol, and triglyceride levels will read erroneously low. Aspartate aminotransferase levels may read erroneously high. The National Institute of Health (NIH) reported on a case in which a woman with unexplained anemia was taking 2,000mg of vitamin C daily. When tested for occult blood in the stool, repeated tests showed negative results. The woman discontinued taking the vitamin C for 4 days, and when retested stool samples tested positive for blood. It was also found that taking 750mg of vitamin C daily can interfere with detecting blood in the urine.
Vitamin C interferes with several glucose tests, including tests diabetics use at home. Urinary glucose test strips will test false positive with as little as 2,000mg of vitamin C daily. Home test strips can show normal blood glucose readings, even when glucose levels are elevated, also at 2g of vitamin c daily. Laboratory glucose tests may show erroneously low glucose levels with excessive vitamin C intake.
To decrease the risk of false laboratory readings it is recommended that all supplements be stopped at least 48 hours before having any lab work done.
I still routinely take 1000 to 2000 mg of Vitamin C on a daily basis. it has NOT seemed to adversely affect my own BG readings either UP or DOWN.
Psalm 139:14 (New International Version)
New International Version (NIV)
14 I praise you because I am fearfully and wonderfully made;
your works are wonderful,
I know that full well.
Science is a wonderful thing and I hate to think where medical science would be without the research that has gone on for decades. At the same time, I believe we are all UNIQUE (one of a kind) and it is sometimes difficult to know why one substance will adversely affect one person and not the other.
None of us have the same fingerprints or the same DNA and even siblings from the same parents do NOT inherit EXACTLY the same genetic code.