I don't have many of the studies on vitamin C listed here.  In general, the research shows definite benefits from eating fruits and vegetables high in vitamin C, but less benefit from taking vitamin C supplements.  However, I have seen no evidence of harm from the supplements and think that some supplementation might be worth it in view of the very low cost.  My current recommendation is 500 mg per day.  It may be wise for diabetics not to take vitamin C.

2 grams per day OK: No harmful effects from this higher dosage although 200 mg/d gives maximum plasma concentration with any additional simply excreted. This level is achieved in menus following the recommended 5 or more fruits or vegetables/day guidelines. 33 of 46 studies looking at vitamin C and cancer found vitamin C to be protective. 21 or 29 studies looking at fruit consumption report higher consumption protective. Supplements were associated with lower overall mortality and lower heart disease in Boston elderly. Supplements and higher dietary intake lower heart disease in several studies. Sahyoun, Nutr 13:835-6, 97

High Blood Pressure: Vitamin C No Help: 5 year follow-up of 539 patients at high risk for gastric cancer treated with vitamin C 500mg/d vs. 50mg/d found no difference in blood pressure in either group or in dropouts. Lack of long-term effect of vitamin C supplementation on blood pressure. Kim MK, Sasaki S, Sasazuki S, Okubo S, Hayashi M, Tsugane S., Hypertension 2002 Dec;40(6):797-803

Cataracts: Vitamin C Reduces: Only vitamin C intake remained significantly associated (P =.003 for trend) with the prevalence of nuclear opacities. Study of 480 from 18yr nurses study. Tufts U. Arch Ophthalmol 2001 Jul;119(7):1009-19

Cataracts: Vitamin C Reduce, Lycopene Increase: Blood levels of vitamin C above 49 micromol/L were associated with a 64% reduced odds for cataract (P < 0.0001). Dietary intake of vitamins C, E and selenium were marginally associated with decreased odds (P = 0.09, P = 0.09, P = 0.07, respectively), whereas moderately high levels of blood lycopene (>0.30 micromol/L) were associated with a 46% increased odds of cataract (P = 0.04). Study of 343 Italians with cataracts vs. controls. J Nutr 2002 Jun;132(6):1299-306 Vitamin C is associated with reduced risk of cataract in a Mediterranean population. Valero MP, Fletcher AE, De Stavola BL, Vioque J, Alepuz VC

Common Cold: Vitamin C 500 mg Helped Prevent But Not Treat Common Cold: In a 5-year DB PC study of 305 Japanese adults with atrophic gastritis, the number of common colds (per 1000 person-months) was 21.3 for vitamin C 50 mg/d vs. 17.1 for vitamin C 500 mg/d. After adjustment for several factors, the relative risks was 0.34 for the high-dose group. No apparent reduction was seen for the severity and duration of the common cold. Effect of vitamin C on common cold: randomized controlled trial. Sasazuki S, et al. National Cancer Center, Tokyo, Japan. European Journal of Clinical Nutrition 24 August 2005. For more, see the Common Cold.

Death, Cardio, and Cancer: Vitamin C Fruits Reduced: In a 4 year prospective study of 19,000 adults, there was a powerful effect of vitamin C rich fruits and vegetables after controlling for other factors including supplements. This is contrary to the NHANES 12 year study. Researchers found 50% lower rate of death in highest vs. lowest quintile. This study used actual blood level measurements. Lancet 2001;357:657-63

Death Strongly Associated with Low Vitamin C Blood Level, But Not Intake: Low blood level of vitamin C is strongly predictive of mortality in a study of 1,214 Brits aged 75 to 84 years, randomly selected, with a median follow-up was 4.4 years. The lowest quintile for ascorbate concentration (<17 µmol/L) had the highest mortality, whereas those in the highest quintile (>66 µmol/L) had the lowest mortality (hazard ratio = 0.51). Tocopherol, beta-carotene, and retinol levels did not predict total mortality, nor were dietary antioxidants measured by the food-frequency questionnaire associated with all-cause or cardiovascular disease mortality. Enthusiasm for vitamin supplementation has been tempered by the negative results from randomized trials, which were conducted predominantly in middle-aged populations. The best recommendation for older persons, as for middle-aged and younger persons, is to maintain a diet rich in a variety of antioxidant micronutrients. Astrid E. Fletcher, London School of Hygiene and Tropical Medicine, Am J Clin Nutr. Nov/2003;78:999-1010

Death Increased in Diabetics on Vitamin C Supplement: In a 15-year follow-up study of 2,000 postmenopausal women with diabetes, those who took heavy doses of vitamin C supplements -- 300 milligrams (mg) a day or more -- were roughly twice as likely to die of heart disease or stroke compared with women who took no supplemental C. David R. Jacobs Jr., et al. University of Minnesota. American Journal of Clinical Nutrition, November 2004.

Diabetes: Vitamin C May Help: An inverse association was found between plasma vitamin C and HbA1c. Dietary measures to increase plasma vitamin C may be an important public health strategy for reducing the prevalence of diabetes. Diabetes Care 2000 Jun;23(6):726-32

Gallstones: Low Vitamin C Assoc: In a study of over 7000 women, low blood vitamin C was associated with an increased risk of both symptomatic and asymptomatic gallstones. Joel Simon, UC San Fran., Arch Int Med 2/01

Gastric Cancer: Vitamin C May Help; E and Multi Don’t: Cancer Prevention Study-II 16 f/u of 1,000,000 US adults with 1725 cases of stomach cancer. Those taking Vitamin C had a 17% reduced risk, i.e. an RR 0.83, but only for those using less than 10 years. No assoc for Vitamin E or a multivitamin. Cancer Epidemiol Biomarkers Prev 2002 Jan;11(1):35-41

Osteoporosis: Vitamin C May Help: Not Vitamin A, Carotene, Vitamin E, or Selenium: In a study of 11,068 women ages 50-79 in the Women's Health Initiative Observational Study, vitamin A, retinol, beta-carotene, vitamin C, vitamin E, and selenium were estimated by using a self-reported food-frequency questionnaire and a random subset (n = 379) had serum levels of retinol, carotenoids, and tocopherols measured. After adjustment for important BMD-related covariates, only vitamin C (lower three-fourths compared with highest one-fourth) and use of hormone therapy (HT) (P < 0.01) were linked to higher bone density in the femoral neck, total-body (P < 0.045), spine (P = 0.03), and total-hip BMDs (P = 0.029). Lack of a relation between vitamin and mineral antioxidants and bone mineral density: results from the Women's Health Initiative. Wolf RL, et al. Columbia University. . Am J Clin Nutr. 2005 Sep;82(3):581-8.

Schizophrenia: Vitamin C Helped Schizophrenics: In an 8-week DB PC crossover study of 40 schizophrenic patients, increased serum MDA and decreased plasma ascorbic acid levels were found in schizophrenic patients. These levels were reversed significantly after treatment with vitamin C along with atypical antipsychotics compared to placebo with atypical antipsychotics. BPRS change scores at 8 weeks improved statistically significant with vitamin C as compared to placebo. Supplementation of vitamin C with atypical antipsychotics reduces oxidative stress and improves the outcome of schizophrenia. Dakhale GN, et al. Government Medical College, Nagpur, India. Psychopharmacology (Berl). 2005 Aug 13:1-5

Stroke Reduced in Rotterdam Study Especially for Smokers: A 6.4 year follow-up of over 5,000 Dutch adults found a 34% lower rate of ischemic stroke in the highest tertile of vitamin C rich foods vs. the lowest tertile.  Supplements made no difference and it was especially smokers benefiting to the tune of 72%. Vitamin E rich food benefited smokers 23% but no benefit for non-smokers. Neurology. 2003;61:1273-1275, 11/03.

Uric Acid Levels: Vitamin C Lowers: In a 2-month DB PC study of 184 nonsmokers, vitamin C 500 mg/d reduced uric acid levels by -0.5 mg/dl vs. +0.09 for the placebo group (P < 0.0001). The same pattern of results was evident in subgroups defined by age, sex, race, body mass index, chronic illness, diuretic use, and quartiles of baseline serum ascorbic acid levels. In the subgroups, from the lowest to the highest quartile of baseline serum uric acid, changes in serum uric acid with vitamin C supplementation were -0.4, -0.5, -0.5, and -1.0 mg/dl (P = 0.06, 0.005, 0.003, and 0.002, respectively). Compared with placebo, vitamin C increased the estimated glomerular filtration rate. Vitamin C might be beneficial in the prevention and management of gout and other urate-related diseases. The effects of vitamin C supplementation on serum concentrations of uric acid: Results of a randomized controlled trial. Huang HY, Appel LJ, et al. Johns Hopkins. Arthritis Rheum. 2005 Jun 2;52(6):1843-1847

Vaginitis: Vitamin C Vaginally Helped: In a 6-day DB PC study of 100 women with non-specific vaginitis (discharge, fishy odor, and presence of clue cells), vitamin C vaginal tablets (250 mg) was given once a day. Two weeks after the end of treatment significantly more patients were still affected by non-specific vaginitis after placebo (36%) vs. 14% with vitamin C. The clue cells disappeared in 79% after vitamin C vs. 53% on placebo. Bacteria disappeared in 77% on vitamin C vs. 54% placebo, while lactobacilli reappeared in 79.1 and 53.3%, respectively. Efficacy and safety of Vitamin C vaginal tablets in the treatment of non-specific vaginitis; A randomised, double blind, placebo-controlled study. Petersen EE, Magnani P., Freiburg D-79106, Germany. Eur J Obstet Gynecol Reprod Biol. 2004 Nov 10;117(1):70-75.