Hypertension and Vitamin C

Vitamin C may have help reduce high blood pressure. A number of studies have been done to test the vitamin for this purpose.

The exact role of vitamin C in lowering blood pressure is not yet clear. Vitamin C is a powerful antioxidant that neutralizes damaging natural substances called free radicals. It works in water, both inside and outside of cells. Antioxidant actions may help treat hypertension by reducing and repairing damage to the lining (endothelium) of blood vessels, enhancing their function and lowering blood pressure.

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In a 30-day, double-blind, placebo-controlled study of 39 people taking medications for hypertension, treatment with 500 mg of vitamin C daily reduced blood pressure by about 10%.³³ Smaller benefits were seen in studies of people with normal blood pressure or borderline hypertension.^34,35 One double-blind study compared 500 mg, 1,000 mg, and 2,000 mg of vitamin C, and found an equivalent level of benefit in all three groups.⁶⁰

Other studies have failed to find evidence of benefit with vitamin C.^36,37,66

Unexpectedly, one study found that a combination of vitamin C (500 mg daily) and grape seed OPCs (1000 mg daily) slightly increased blood pressure.⁷³ Whether this was a fluke of statistics or a real combined effect remains unclear.

Vitamin C is an essential nutrient that must be obtained from food or supplements; the body cannot manufacture it. It is present in many foods, including citrus fruits, red chili peppers, sweet peppers, kale, parsley, collards, turnip greens, broccoli, Brussel sprouts, watercress, cauliflower, cabbage, and strawberries. To get therapeutic doses, you will probably need to take a supplement. Because vitamin C is the most popular supplement in the United States, supplements are affordable and widely available.

Many nutritional experts recommend a total of 500 mg of vitamin C daily. This dose is almost undoubtedly safe. The daily doses of vitamin C used in these studies were 500 mg, 1,000 mg, and 2,000 mg. One study demonstrated that blood pressure-lowering benefits increased with the larger doses of the vitamin.⁶⁰ These higher doses are considered safe for adults age 19 and older. (For more information, see Side Effects & Warnings.)

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Safety Issues

The US government has issued recommendations regarding tolerable upper intake levels (ULs) for vitamin C. The UL can be thought of as the highest daily intake over a prolonged time known to pose no risks to most members of a healthy population. The ULs for vitamin C are as follows:

• Children
  • 1-3 years: 400 mg
  • 4-8 years: 650 mg
  • 9-13 years: 1,200 mg

• Males and Females
  • 14-18 years: 1,800 mg
  • 19 years and older: 2,000 mg

• Pregnant Women
  • 18 years old or younger: 1,800 mg
  • 19 years and older: 2,000 mg

• Nursing Women
  • 18 years old or younger: 1,800 mg
  • 19 years and older: 2,000 mg

However, even within the safe intake range for vitamin C, some individuals may develop diarrhea. This side effect will likely go away with continued use of vitamin C, but you might have to cut down your dosage for a while and then gradually build up again.

Concerns have been raised that long-term vitamin C treatment can cause kidney stones . ¹ However, in large-scale observational studies, individuals who consume large amounts of vitamin C have shown either no change or a decreased risk of kidney stone formation. ^{2 3} Still, there may be certain individuals who are particularly at risk for vitamin C-induced kidney stones. ⁴ People with a history of kidney stones and those with kidney failure who have a defect in vitamin C or oxalate metabolism should probably restrict vitamin C intake to approximately 100 mg daily. You should also avoid high-dose vitamin C if you have glucose-6-phosphate dehydrogenase deficiency, iron overload, or a history of intestinal surgery.

Vitamin C supplements increase absorption of iron . ⁵ Since it isn’t good to get more iron than you need, individuals using iron supplements shouldn’t take vitamin C at the same time except under a physician’s supervision.

One study from the 1970s suggests that very high doses of vitamin C (3 g daily) might increase the levels of acetaminophen (such as Tylenol) in the body. ⁶ This could potentially put you at higher risk for acetaminophen toxicity. This interaction is probably relatively unimportant when acetaminophen is taken in single doses for pain and fever, or for a few days during a cold. However, if you use acetaminophen daily or have kidney or liver problems, simultaneous use of high-dose vitamin C is probably not advisable.

Weak evidence suggests that vitamin C, when taken in high doses, might reduce the blood-thinning effects of warfarin (Coumadin) and heparin . ^{7 8 9} As noted above, one study found that vitamin C at a dose of 1 g daily substantially reduced blood levels of the drug indinavir, a protease inhibitor used for the treatment of HIV infection. ¹⁰ Heated disagreement exists regarding whether it is safe or appropriate to combine antioxidants such as vitamin C with standard chemotherapy drugs. The reasoning behind the concern is that some chemotherapy drugs may work in part by creating free radicals that destroy cancer cells, and antioxidants might interfere with this beneficial effect. ¹¹ However, there is no good evidence that antioxidants actually interfere with chemotherapy drugs, but there is growing evidence that they do not. ¹² The maximum safe dosages of vitamin C for people with severe liver or kidney disease have not been determined.

Interactions You Should Know About

If you are taking:

• Aspirin , other anti-inflammatory drugs, or oral contraceptives : You may need more vitamin C.

• Acetaminophen (eg, Tylenol) : The risk of liver damage from high doses of acetaminophen may be increased if you also take large doses of vitamin C.

• Warfarin (Coumadin) or heparin : High-dose vitamin C might reduce their effectiveness.

• Iron supplements: High-dose vitamin C can cause you to absorb too much iron. This is especially a problem for people with diseases that cause them to store too much iron.

• Medications in the nitrate family: Vitamin C may help maintain their effectiveness. Note: Angina is too serious a disease for self-treatment. If you have angina, do not take vitamin C (or any other supplement) except on a physician’s advice.

• Protease inhibitors for HIV: High-dose vitamin C may reduce their effectiveness.

• Cancer chemotherapy : Do not use vitamin C except on physician’s advice.

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