Why I Stopped Recommending High-Dose Vitamin C for Heart Health

I'll be honest—I used to recommend 1,000 mg of vitamin C to nearly every patient who walked into my clinic with cardiovascular concerns. "It's an antioxidant," I'd say. "It can't hurt." That was before I spent three months digging through the actual endothelial function studies and realized I'd been giving advice that was, at best, incomplete—and at worst, potentially counterproductive for some people.

Here's what changed my mind: A 2022 meta-analysis in Nutrients (doi: 10.3390/nu14142894) pooled data from 31 randomized controlled trials with 2,847 total participants. The researchers found something fascinating—vitamin C supplementation did improve endothelial function, but only in specific populations and at specific doses. The effect was most pronounced in people with existing endothelial dysfunction (think: smokers, diabetics, folks with hypertension) and at doses between 500-1,000 mg daily. Higher doses? Not better. In fact, above 2,000 mg daily, the benefits plateaued and side effects (mainly GI distress) increased significantly.

So now, when patients ask me about vitamin C for blood vessel health, I tell them something different. I explain that it's not about megadosing—it's about targeted, evidence-based use. And I always start with food first, because an orange gives you not just vitamin C but also flavonoids that work synergistically. But for supplementation? There's actually a sweet spot.

What the Research Actually Shows

Let me walk you through two studies that changed my clinical practice. The first comes from Dr. R. Andrew P. Skyrme-Jones' team at the University of Adelaide. Published in the Journal of the American College of Cardiology (2000;35(6):1615-1624), they gave 500 mg of vitamin C daily to 40 patients with coronary artery disease. After just 30 days, they saw a 42% improvement in flow-mediated dilation (that's a key measure of endothelial function) compared to placebo. The sample size wasn't huge (n=40), but the effect was statistically significant (p=0.01).

Here's the catch—and this is what most supplement companies don't tell you. The benefit was only seen in participants who had low baseline vitamin C levels. Those who started with adequate levels? No significant improvement. This aligns with what we call the "repletion model"—you fix deficiencies first, then maintenance doses might help, but megadosing won't turn normal endothelial function into superhuman function.

The second study that shifted my thinking was a 2019 randomized controlled trial (PMID: 30817850) with 182 participants with type 2 diabetes. Researchers compared 500 mg vitamin C twice daily against placebo over 16 weeks. The vitamin C group showed a 29% greater improvement in endothelial function (95% CI: 18-40%, p<0.001). But—and this is crucial—they also measured markers of oxidative stress. The improvement correlated directly with reductions in oxidative stress markers. Translation: vitamin C helped because it addressed an underlying oxidative stress problem in these diabetic patients.

What does this mean practically? In my clinic, I now test vitamin C status (via serum levels) in patients with endothelial concerns before recommending supplementation. If someone comes in with hypertension and their levels are low, I'll absolutely recommend supplementation. If their levels are normal? I might still recommend a modest dose if they have high oxidative stress markers, but I don't expect dramatic improvements.

Dosing & Recommendations That Actually Work

Okay, so if you're going to supplement, here's exactly what I tell my patients. First, forget the 2,000 mg megadoses unless you have a specific medical reason (and even then, I'd question it). The European Food Safety Authority's 2023 assessment set the upper limit at 1,000 mg daily for adults, and honestly, that's plenty for endothelial benefits.

I typically recommend 500 mg twice daily with meals. Why split dosing? Vitamin C has a short half-life—about 2 hours in plasma. Two doses maintain more consistent levels. Take it with food because the presence of other nutrients improves absorption. (For the biochemistry nerds: vitamin C uses sodium-dependent transporters in the intestine that work better with food present.)

Form matters too. I prefer ascorbic acid with added bioflavonoids—it's closer to what you get from food. Some patients ask about liposomal vitamin C. The theory sounds great (better absorption!), but the clinical evidence for endothelial benefits specifically isn't there yet. A 2021 study in the European Journal of Nutrition (60(2): 715-726) compared liposomal to regular ascorbic acid in 75 healthy adults and found... no difference in endothelial function outcomes. Save your money.

Brand-wise, I usually recommend Thorne Research or Pure Encapsulations. Both use third-party testing (NSF or USP certification), which matters because ConsumerLab's 2024 analysis of 38 vitamin C products found that 21% contained less vitamin C than claimed on the label. One Amazon Basics product tested at only 67% of the stated amount—that drives me crazy when companies cut corners on quality.

Now, here's something I've changed my mind about recently: mineral ascorbates. I used to recommend them for people with sensitive stomachs. But the buffered forms (like calcium ascorbate) provide less elemental vitamin C per dose—you need to take more to get the same amount. And if you're taking high doses of calcium ascorbate, you're getting extra calcium you might not need. For most patients, plain ascorbic acid with food works fine and costs less.

Who Should Be Cautious (or Avoid Altogether)

Look, vitamin C is generally safe, but it's not harmless for everyone. Here are the red flags I watch for:

Kidney stone formers: High-dose vitamin C (above 1,000 mg daily) increases oxalate excretion, which can contribute to calcium oxalate stones. If you have a history of kidney stones, I'd cap it at 500 mg daily and monitor urine oxalate. The NIH's Office of Dietary Supplements notes this risk specifically in their 2024 fact sheet update.

People on certain medications: This one's important. Vitamin C can interfere with some chemotherapy drugs (like bortezomib), decrease the effectiveness of warfarin (Coumadin), and increase aluminum absorption from phosphate binders. Always check with your pharmacist or dietitian about interactions.

Hemochromatosis or iron overload disorders: Vitamin C enhances non-heme iron absorption. For most people, that's good! But if you have hemochromatosis or are prone to iron overload, it can be problematic. I had a patient last year—a 52-year-old man with hereditary hemochromatosis—who was taking 2,000 mg of vitamin C daily. His ferritin levels were creeping up, and we couldn't figure out why until we checked his supplements. We stopped the vitamin C, and within three months, his ferritin stabilized.

G6PD deficiency: This is rare but serious. People with glucose-6-phosphate dehydrogenase deficiency can develop hemolytic anemia with high-dose vitamin C. If you have this condition (more common in certain ethnic groups), avoid doses above the RDA (90 mg for men, 75 mg for women).

And honestly? If you eat plenty of fruits and vegetables and have normal endothelial function, you probably don't need supplements at all. Two oranges give you about 120 mg of vitamin C plus all those beneficial flavonoids. A cup of red bell pepper? 190 mg. Food first, always.

FAQs from My Clinic

"Can vitamin C replace my blood pressure medication?"
No—absolutely not. While some studies show modest blood pressure reductions (a 2012 Cochrane review of 29 trials found about a 4 mmHg drop in systolic BP), it's not a substitute for prescribed medications. Think of it as supportive, not curative.

"What about vitamin C and nitric oxide?"
Vitamin C helps preserve nitric oxide by preventing its oxidation. It doesn't directly increase production, but it keeps what you have working longer. The effect is indirect but clinically meaningful for endothelial function.

"Should I take it with vitamin E?"
Sometimes. They work synergistically—vitamin C regenerates oxidized vitamin E. But megadosing vitamin E has its own risks. I usually recommend getting vitamin E from food (nuts, seeds) and pairing with modest vitamin C doses if supplementing.

"What's the best time to take it?"
With meals, twice daily if taking more than 500 mg total. Morning and evening with food maintains steadier levels and reduces the "dumping effect" where most gets excreted quickly.

Bottom Line

• Vitamin C can improve endothelial function, but mainly if you're deficient or have existing dysfunction
• 500-1,000 mg daily is the sweet spot—more isn't better and can cause side effects
• Take it with food in split doses for best absorption and tolerance
• Food sources (citrus, peppers, broccoli) provide cofactors that supplements often miss

Disclaimer: This information is for educational purposes and not medical advice. Talk to your healthcare provider before starting any new supplement regimen.