A Review of Recent Vitamin C Research

References

It seems that every year more and more research is published about vitamins, especially vitamin C. How can you keep up with it all? It’s not easy. However, my colleagues Tim N. Ziegenfuss, Ph.D., and Jamie Landis, M,D., Ph.D., have written a review which will appear in the journal Current Sports Medicine Reports (Vol. 5, No. 4, 2006), and examines recent key research conducted on vitamin C.

It’s important to understand vitamin C’s role in human health. The chief role of vitamin C, also known as ascorbate and ascorbic acid, is as a water-soluble antioxidant. Vitamin C is also required for the maintenance of normal connective tissue, for wound healing and for the remodeling of bone. Vitamin C is also needed as a cofactor in the synthesis of epinephrine (adrenalin) and bile acids, as well as thyroxin (thyroid hormone) synthesis, amino acid metabolism, strengthening resistance to infection, and aiding in the absorption of iron.¹ This array of mechanisms contributes to vitamin C’s impact on many major health problems Americans face.

When excess glucose (blood sugar) binds with proteins in the blood, diabetics experience the characteristic and sometimes life-threatening complications of diabetes. Also, oxidative stress plays a role in the disease progression of diabetes. The good news is that antioxidants like vitamin C can reduce the binding of glucose to proteins and help counter oxidative stress.

One diabetic condition linked to vitamin C is diabetic retinopathy, a progressive eye disease that can cause decreased vision and blindness. In a large study of over 1,300 subjects with Type 2 diabetes, long-term users of vitamin C, vitamin E or multis had a two-fold lower risk of prevalent retinopathy, compared with nonusers.² Diabetics are also at an increased risk of cardiovascular disease (CVD). Although some researchers concluded a high vitamin C intake from supplements is associated with an increased risk of cardiovascular disease mortality in postmenopausal women with diabetes,³ other researchers presented data indicating that vitamin C supplementation (as well as vitamin E and alpha-lipoic acid) decreases oxidative stress and aids the management of diabetic CVD, and actually promotes vascular benefits of insulin in Type 2 diabetics.^4,5,6

Heart Disease

Since vitamin C is an antioxidant and reduces C-reactive protein (CRP)—a substance that can support the progression of CVD—it may help slow the progression of CVD in some individuals. In fact, in a study of active and passive smokers, vitamin C supplementation (515 mg daily) resulted in a 24-percent reduction in plasma CRP.⁷ In examining the relation between the intake of antioxidant vitamins C and E, and coronary heart disease (CHD) from nine studies, a researcher found a reduced incidence of major CHD events with supplemental vitamin C intakes greater than 700 mg daily.⁸

Since large doses of vitamin C have been shown to convey an antihistamine effect, this nutrient may have value in asthma and allergies. In a study involving over 7,500 youths, researchers found that an increase in beta-carotene was associated with a 10-percent reduction in asthma in those not exposed to smoke and a 40-percent reduction in young persons who had passive smoke exposure.⁹ The pattern for vitamin C was similar to betacarotene results. An increase in selenium was associated with a 10- to 20-percent decrease in asthma prevalence, and a 50-percent decrease in asthma prevalence in youth with passive smoke exposure.

Researchers found that a maternal diet rich in natural sources of vitamin C during breastfeeding could reduce the risk of atopic dermatitis (a type of allergy) in high-risk infants.¹⁰

Studies conducted by Linus Pauling and associates in the 1970s suggested very large doses of vitamin C were helpful in increasing the survival time and improving quality of life of terminal cancer patients.¹¹ More recently, researchers found greater vitamin C intakes were associated with fewer therapy delays, less toxicity and fewer days spent in the hospital in children being treated for leukemia with chemotherapy.¹²

In another study, researchers found fruit and vitamin C intake reduced risk of noncardiac cancer (the major form of stomach cancer in most parts of the world) by approximately 45 percent.¹³

Vitamin C is required for the synthesis of carnitine, the amino acid responsible for transporting fatty acids into the cells to be oxidized (burned as fuel). Researchers found that subjects with poor vitamin C status oxidized less fat than subjects with adequate vitamin C status during a walking test.¹⁴ Similarly, subjects depleted of vitamin C who then underwent two weeks of vitamin C supplementation (500 mg daily), were able to perform 10-percent more work during a 90-minute walk.¹⁵

"A Review of Recent Vitamin C Research" References

1. Whitney EN, Cataldo CB, Rolfes SR. Understanding Normal and Clinical Nutrition. Wadsworth/Thompson Learning, Belmont, CA, 2002.

2. Millen AE et al. "Relation between intake of vitamins C and E and risk of diabetic retinopathy in the Atherosclerosis Risk in Communities Study." Am J Clin Nutr. 79:865-73, 2004.

3. Lee D-K et al. "Does supplemental vitamin C increase cardiovascular disease risk in women with diabetes?" Am J Clin Nutr. 80:1194-200, 2004.

4. Lee KW, Mossine V, Ortwerth BJ. "The relative ability of glucose and ascorbate to glycate and crosslink lens proteins in vitro." Exp Eye Res. 67:95-104, 1998.

5. Vega-Lopez S, Devaraj S, JialalI. "Oxidative stress and antioxidant-supplementation in the management of diabetic cardiovascular disease." J Invest Med. 52:24-3, 2004.

6. Evans M et al. "Effects of insulin lispro and chronic vitamin C therapy on postprandial lipaemia, oxidative stress and endothelial function in patients with type 2 diabetes mellitus." Eur J Clin Invest. 33:231-238, 2003.

7. Block G et al. "Plasma C-reactive protein concentrations in active and passive smokers: influence of antioxidant supplementation." J Am Coll Nutr. 23:141-147, 2004.

8. Knekt P et al. "Antioxidant vitamins and coronary heart disease risk: a pooled analysis of 9 cohorts." Am J Clin Nutr. 80:1508-1520, 2004.

9. Rubin RN, Navon L, Cassano PA. "Relationship of serum antioxidants to asthma prevalence in youth." Am J Respir Crit Care Med. 169:393-98, 2004.

10. Hoppu U et al. "Vitamin C in breast milk may reduce the risk of atopy in the infant." Eur J Clin Nutr. 59:123-8, 2005.

11. Cameron E, Pauling L. "Supplemental ascorbate in supportive treatment of cancer: prolongation of survival times in terminal human cancer." Proc Natl Acad Sci USA. 73:3685-89, 1976.

12. Kennedy DD et al. "Low antioxidant vitamin intakes are associated with increases in adverse effects of chemotherapy in children with acute lymphoblastic leukemia." Am J Clin Nutr. 79:1029-1036, 2004.

13. Nouraie M et al. "Fruits, vegetables, and antioxidants, and risk of gastric cancer." Frontiers in Cancer Prevention Research. Poster Session #C69:166, 2004.

14. Johnston CS. "Strategies for healthy weight loss: from vitamin C to the glycemic response." J Am Coll Nutr. 24:158-165, 2005.