Vitamin C, also known as ascorbic acid, is an essential nutrient that plays many important roles in the body. It acts as an antioxidant, helps absorb iron, produces collagen, and supports immune function. Many people take vitamin C supplements to obtain higher levels than what is naturally found in food. While vitamin C is generally considered safe, there are some important interactions and side effects to be aware of when taking high doses. In particular, vitamin C can interact with certain other vitamins and minerals. This article will provide an overview of which vitamins should not be taken at the same time as vitamin C.
Vitamin B12
Vitamin C can interfere with the absorption of vitamin B12 when taken in high doses over a long period of time. Vitamin B12 plays key roles in red blood cell formation, cell metabolism, nerve function, and DNA synthesis. Long-term high dose vitamin C supplementation greater than 1,000 mg per day may inhibit the absorption of vitamin B12 in some people.
One study found that vitamin C supplements ranging from 200-1,000 mg per day reduced vitamin B12 absorption from food by an average of 30% in healthy adults. Another study gave participants either 200 mg or 1,000 mg per day of vitamin C. The high dose group had significantly lower vitamin B12 absorption and lower blood levels compared to the low dose group.
The mechanism behind this interaction is that vitamin C improves iron absorption. Iron and vitamin B12 compete for absorption in the same area of the small intestine. When iron absorption increases, vitamin B12 absorption decreases. People who take high doses of vitamin C supplements, greater than 1,000 mg per day, over a long period of time are at the greatest risk for impaired vitamin B12 absorption.
To prevent this interaction, it is recommended to take vitamin C and vitamin B12 supplements at different times of day. Maintain the tolerable upper limit for vitamin C at 2,000 mg per day. Also, make sure to get adequate B12 from foods or supplements as recommended by an RDI of 2.4 mcg per day for adults. People with pernicious anemia who have impaired ability to absorb vitamin B12 may be at greater risk for deficiency with high dose vitamin C supplements.
Vitamin K
High doses of vitamin C may also reduce the absorption and effectiveness of vitamin K supplements. Vitamin K plays essential roles in proper blood clotting. There are two main forms – vitamin K1 (phylloquinone) found in leafy greens and vitamin K2 (menaquinone) from animal sources and fermented foods.
Studies give conflicting results on whether vitamin C supplements truly impair vitamin K absorption or not. Some studies have found that doses greater than 500 mg per day can significantly reduce absorption of vitamin K1 supplements in particular. Other studies have not observed an interaction between vitamins C and K at lower supplemental doses.
The mechanism of this interaction may involve vitamin C interfering with the liver’s activation of vitamin K into its active form, along with competing for absorption. To be safe, separate vitamin C and K supplements by several hours. Maintain vitamin C under 2,000 mg per day and meet vitamin K intake recommendations from foods rather than supplements alone. People taking blood-thinning medication should be especially careful with vitamin K intake and consult their doctor.
Iron
While vitamin C can greatly enhance iron absorption, it may lead to excess iron accumulation when taken continually in high doses. Vitamin C converts iron into the ferrous form that is more readily absorbed. This is beneficial for those with low iron status, helping to improve iron deficiency. However, in healthy people with normal iron levels, enhancing iron absorption further through high dose vitamin C supplements long-term may lead to excess storage of iron.
The tolerable upper limit for iron is 45 mg per day for adults. Consuming vitamin C greater than 1,000 mg per day on top of adequate dietary iron could lead to excess iron buildup over time. High iron levels can cause gastrointestinal issues like nausea, vomiting, pain, constipation or diarrhea. The European Food Safety Authority recommends against taking vitamin C and iron supplements at the same time. Excess iron accumulation in vital organs over many years also increases the risk of chronic diseases like cancer, heart disease and diabetes.
Those with health conditions like hemochromatosis or who regularly consume vitamin C supplements may want to monitor their iron levels. Avoid taking vitamin C and iron supplements together. Only combine vitamin C with iron under medical supervision in cases of iron deficiency anemia. Also avoid simultaneous high doses of vitamin C and iron-rich foods like red meat.
Copper
There is some evidence that high level vitamin C supplementation can also reduce copper absorption in the body. Copper plays important roles in iron metabolism, antioxidant defenses, energy production, connective tissue formation and more. While vitamin C benefits copper absorption in small amounts, extremely high supplemental doses may inhibit it.
One study found that 1,000 mg per day of vitamin C decreased copper absorption by more than 50% compared to 200 mg per day. Several case studies have reported copper deficiencies in patients taking high dose vitamin C supplements long-term. Symptoms included anemia, fatigue, bone pain and osteoporosis. Copper deficiency is still rare since requirements are low at just 0.7 – 0.9 mg per day for adults.
To prevent any interactions, avoid exceeding the upper limit of 2,000 mg per day of vitamin C. Take a copper supplement separately rather than a multivitamin to help increase intake. Good dietary sources of copper include seafood, red meat, eggs, nuts, seeds, beans, mushrooms and cocoa. Monitor for symptoms of copper deficiency if taking high dose vitamin C supplements regularly.
Potassium
There is limited evidence that high doses of vitamin C could impact potassium levels in the body. Potassium is a very important electrolyte and mineral that is essential for heart, muscle and nerve function. One study found that males taking 1,000 mg per day of vitamin C supplements for two weeks had reduced plasma potassium levels compared to the placebo group. Another study in rats also found that large doses of vitamin C lowered potassium concentrations.
The mechanism is thought to be due to high dose vitamin C increasing urinary excretion of potassium. When potassium status is reduced, it can potentially cause muscle cramps, weakness, heart palpitations and arrhythmias. However, vitamin C only seems to impact potassium at very high supplemental doses of over 500-1,000 mg per day taken for long periods of time.
To be safe, the upper limit for adults is suggested to be 2,000 mg per day from vitamin C supplements. Eating plenty of potassium-rich foods like bananas, potatoes, yogurt, nuts, beans and fish can also help maintain levels. People taking potassium-sparing diuretics or with heart conditions like arrhythmia should be cautious with vitamin C doses above 1,000 mg per day and consult their doctor.
Magnesium
Taking high amounts of vitamin C may also increase the urinary excretion of magnesium in some cases. Magnesium is needed for energy production, muscle and nerve function, blood sugar control, and more. The potential effect of vitamin C on magnesium loss was shown in one study on kidney transplant patients. Those given vitamin C supplements for a year had higher magnesium excretion compared to patients not taking vitamin C.
Another study in postmenopausal women found that taking vitamin C supplements along with estrogen therapy increased magnesium losses compared to estrogen alone. The authors noted this could potentially impact bone health if magnesium levels are chronically reduced. However, research overall has had conflicting findings on whether vitamin C truly impacts magnesium levels. Not all studies have shown significant lowering of magnesium status, even at doses of 1,000 mg per day. The effect may depend on the individual and dosages taken.
To maintain adequate magnesium, the RDI is 310-400 mg per day for adults. Consuming magnesium-rich foods like nuts, seeds, legumes, leafy greens and whole grains can help provide intake along with a supplement if needed. If taking high doses of vitamin C, getting sufficient magnesium is important. Don’t exceed 2,000 mg per day of vitamin C supplements without consulting your doctor.
Zinc
High dose vitamin C supplementation may also lower zinc absorption in some circumstances. Zinc plays critical roles in immune function, protein synthesis, DNA, and many enzymes. Intakes below the RDI of 8-11 mg per day can negatively impact the immune system and growth.
One study found that 1,000 mg per day of vitamin C decreased zinc absorption compared to lower doses of 200 mg per day. Another study showed that just 500 mg per day of vitamin C lowered zinc absorption from a supplement by about 50%. However, not all studies agree that vitamin C supplementation impairs zinc status. One review reported no significant effects on zinc levels at amounts up to 1,000 mg per day of vitamin C.
It’s possible this interaction is most relevant when zinc intake or levels are already insufficient. Maintaining recommended intakes from foods and supplements can help offset any impairment in absorption from vitamin C. Also, spacing out doses may allow for adequate zinc absorption when vitamin C is not recently taken. Those at risk for zinc deficiency, such as the elderly, should take extra care with vitamin C supplement use.
Other Considerations
While these are the main vitamins reported to have potential interactions with high dose vitamin C supplementation, there are a few other points to consider:
– Manganese – Excessively high intake of vitamin C over 2,000 mg per day may also impact manganese levels. Ensure sufficient manganese intake from whole foods.
– Vitamin E – One study reported that 500 mg per day of vitamin C may reduce vitamin E absorption. However, other research shows no interaction between vitamins C and E. Maintain recommended vitamin E intake from food sources.
– Multivitamins – Avoid taking vitamin C supplements at the same time as a multivitamin, especially if vitamin C dose is greater than 500 mg per day.
– Kidney stones – Excess vitamin C can increase the risk of kidney stones in those predisposed. Stay below 2,000 mg per day from all sources.
– Side effects – Very high doses can cause diarrhea, nausea, stomach cramps or headache in some people.
– Medical conditions – Those with G6PD deficiency, iron overload disease, or kidney disorders should not take high doses of vitamin C.
Conclusion
Vitamin C offers many beneficial health effects but can interact with certain other vitamins when taken in high amounts, particularly over 2,000 mg per day. The main nutrients that should not be taken at the same time as vitamin C supplements include vitamin B12, vitamin K, iron, copper, potassium, magnesium, and zinc.
To avoid potential interactions, space out dosage times for vitamin C compared to other vitamins and supplements. Ensure you are meeting but not greatly exceeding the Recommended Daily Intakes for each of these nutrients through food sources. Consult your doctor before beginning high dose vitamin C supplementation, especially if you have an underlying medical condition or take any medications. When taken wisely, vitamin C is a safe supplement that can have tremendous benefits on your overall health.