Can vitamin C help type 2 diabetes?

The researchers carried out a ‘double-blind, placebo-controlled, cross-over study’.¹ They investigated whether vitamin C supplementation could decrease daily post-meal blood sugar levels in people with type 2 diabetes which was already managed with diet or oral antihyperglycaemic medicines.

They also investigated whether vitamin C supplements affected blood pressure.

A total of 31 people (26 men and 5 women) with type 2 diabetes were randomly split into two groups.

One group took a capsule containing 500 mg of vitamin C (test capsule) twice daily, and the other group took a placebo capsule. The placebo capsules were the same as the test capsules except that they did not contain any vitamin C. This helped make sure that no-one knew what type of capsule they were taking, to improve the quality of the results. After 4 months the participants stopped taking their capsules for at least 1 month to let any supplement leave their bodies (this is called a washout period). Then each group was switched to the other type of capsule for another 4 months.

Participants also continued to take their usual prescription medicines over the course of the study.

Both groups had their blood sugar levels measured over two 48-hour periods using glucose monitors, once while taking the vitamin C supplement and once while taking the placebo.

For the 27 participants who completed the study, post-meal blood sugar levels were significantly lower following 4 months of vitamin C supplementation compared with 4 months of placebo treatment.

After taking vitamin C supplements, participants also had fewer hours per day with hyperglycaemia (high blood sugar levels) or post-meal hyperglycaemia, and had lower blood pressure.

Although results of the above study are interesting, a closer look shows several weaknesses in the findings.

• The study investigated only a very small number of people with type 2 diabetes, and most were male. This makes it difficult to apply the study findings to everyone with type 2 diabetes.
• The effect of vitamin C supplementation alone cannot be determined, because people were already being treated with different diabetes medicines, heart medicines, or lifestyle treatments.

Interestingly, the researchers found that glycated haemoglobin (HbA_1c) levels did not improve with vitamin C supplementation.

The HbA_1c level is important because a result of 48 mmol/mol (6.5%) or more can suggest a diagnosis of diabetes, and it can help track long-term blood sugar control.²

The authors suggested that this result may have been because the 1-month washout period was too short, meaning that vitamin C may still have had an effect in participants switched to placebo (and vice versa).

It is thought that damage to the body’s cells and tissues caused by substances known as free radicals may lead to the development of type 2 diabetes.¹ Vitamin C is an antioxidant which is thought to have a protective role in diabetes by reducing the damage caused by free radicals.^1,3 There are other ways you can reduce damage from free radicals, for example , not smoking and limiting alcohol intake.

More research studying a larger number of people is needed to determine whether people with type 2 diabetes can benefit from vitamin C supplements.

Initially, type 2 diabetes can often be managed with lifestyle changes such as healthy eating and regular exercise. Most people with type 2 diabetes will also need oral prescription medicines and many will eventually require insulin.⁴ Type 2 diabetes needs to be managed effectively to prevent complications.

If you are thinking about starting a vitamin C supplement or any other new medicine, including complementary medicines, we recommend that you talk to your GP or diabetes educator.