Most animals can synthesise vitamin C from glucose. However, humans are missing the L-gluconolactone enzyme required for this reaction. As a result humans are dependent on their diet in order to supply adequate vitamin C. Because vitamin C is synthesised from glucose its structure bears some resemblance to glucose, and it is known for example that both glucose and vitamin C can interact with the same proteins, including transporters on cell membranes. In addition, vitamin C also required insulin in order to enter cells, and so vitamin C is able to compete with glucose for entry to cells. High levels of fasting blood glucose, as are seen with insulin resistance, metabolic syndrome and type 2 diabetes, may therefore prevent vitamin C for being able to correctly enter somatic cells, and may thus deprive them of vitamin C. Controlling blood glucose levels is therefore pivotal to maintaining health by allowing adequate vitamin C to reach the interior of the cells.
However, vitamin C also appears to have blood glucose lowering properties. It is not fully understood how vitamin C is able to cause decreases in fasting blood glucose but as has been suggested previously, it may relate to a general antioxidant effect. Vitamin C is a water soluble antioxidant, and in this role may inhibit the free radical chain reactions that occur in cells due to the nutrient overload syndrome. Nutrient overload refers to the oversupply of energy to cells, resulting in free radicals that damage the insulin receptor. In one study, researchers fed 84 patients with type 2 diabetes either 500 or 1000 mg of vitamin C per day for 6 weeks. At the end of this study time, the researchers observed significant reductions in fasting blood sugar, triglycerides, low density lipoprotein (LDL) cholesterol, glycated haemoglobin, and serum insulin at the 1000 mg, but not 500 mg, dose of vitamin C. Vitamin C therefore appears to have similar effects to other antioxidants in that it is able to reduce the symptoms of insulin resistance.
Dr Robert Barrington’s Nutritional Comment: Vitamin C rich diets are important to health. The best plant food diets with lots of fruits and vegetables can supply around 600 mg of vitamin C. However, to take advantage of the blood glucose lowering effects of the vitamin, supplements will likely be needed in order to provide the 1000 mg that was found to be effective in this study. Around 1 to 3 grams of vitamin C is recommended for physically active people, and it is not possible to supply this amount from food alone. Vitamin C is cheap and safe and so vitamin C tablets should be part of a healthy diet. Another possible explanation for the beneficial effects of vitamin C on fasting blood glucose levels seen in this study relates to the competition between vitamin C and glucose for intestinal absorption. Because of their similar structure the two compounds may compete for absorption, with high amounts of vitamin C in the diet decreasing the absorption of glucose to the blood. Dietary polyphenols may also have this effect.
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