Vitamin C comes from fruits and vegetables, with some of the richest sources being red bell peppers, citrus fruits, kiwifruit, and broccoli. Unlike most mammals, humans lost the ability to produce vitamin C internally roughly 40 million years ago, which means every milligram you need has to come from food or supplements. The vitamin C in supplements, meanwhile, starts as plain glucose and is converted through industrial fermentation.
Top Food Sources
When most people think of vitamin C, they think of oranges. Oranges are a solid source at about 70 mg per medium fruit, but they’re far from the top of the list. A half cup of raw red bell pepper delivers 95 mg, making it one of the most concentrated everyday sources available. Three-quarters of a cup of orange juice comes close at 93 mg.
Here are the standout foods ranked by vitamin C per typical serving:
- Red bell pepper (½ cup, raw): 95 mg
- Orange juice (¾ cup): 93 mg
- Orange (1 medium): 70 mg
- Grapefruit juice (¾ cup): 70 mg
- Kiwifruit (1 medium): 64 mg
- Green bell pepper (½ cup, raw): 60 mg
- Broccoli (½ cup, cooked): 51 mg
- Strawberries (½ cup, sliced): 49 mg
- Brussels sprouts (½ cup, cooked): 48 mg
- Grapefruit (½ medium): 39 mg
A couple of things stand out from this list. Vegetables hold their own against fruits, and cooking doesn’t destroy as much vitamin C as people assume. Half a cup of cooked broccoli still provides over half a day’s requirement. That said, vitamin C is water-soluble and sensitive to heat, so steaming or quick cooking preserves more than boiling for a long time.
How Plants Make Vitamin C
Plants build vitamin C from simple sugars through a series of chemical reactions. The main production route, called the galactose pathway, starts with a sugar called GDP-mannose and converts it through four intermediate steps into L-ascorbic acid, which is the chemical name for vitamin C. This pathway has been well mapped in species like kiwifruit and the model plant Arabidopsis.
Plants ramp up vitamin C production in response to light and stress, which is one reason sun-ripened produce tends to have higher levels than fruit picked early. The vitamin serves as an antioxidant for the plant itself, protecting its cells from damage caused by sunlight and environmental threats.
Why Humans Can’t Make Their Own
Most mammals synthesize vitamin C in their liver. Dogs, cats, goats, and cows all produce it internally. Humans cannot, and the reason is a broken gene. The enzyme needed for the final step of vitamin C production has been nonfunctional in primates for over 40 million years due to accumulated mutations in the gene that codes for it. Guinea pigs and some bat species share this same genetic quirk.
This means vitamin C is, by definition, a vitamin for humans: a nutrient essential for survival that the body cannot produce. Without dietary intake, you develop scurvy within weeks to months, a disease characterized by bleeding gums, joint pain, and poor wound healing. It was famously common among sailors on long voyages before the connection to citrus fruit was understood in the 18th century.
Where Supplement Vitamin C Comes From
The vitamin C in supplements and fortified foods is almost entirely manufactured, not extracted from oranges or other produce. The raw material is glucose, typically derived from corn. From there, manufacturers use one of several processes to convert it into ascorbic acid.
The oldest method, called the Reichstein process and still in use today, combines six chemical synthesis steps with one microbial fermentation step. It has high conversion efficiency and uses cheap raw materials. A newer two-step fermentation process improves on this by reducing toxic byproducts and lowering costs. It starts by converting glucose into sorbitol, then uses bacteria in two successive fermentation stages to reach the final product. More recently, a one-step fermentation method has been developed that uses engineered microorganisms to go directly from glucose or sorbitol to vitamin C in a single stage.
Regardless of the method, the end product is the same molecule found in a bell pepper or an orange. Natural and synthetic L-ascorbic acid are chemically identical, with no known differences in biological activity. Research from the Linus Pauling Institute found that ascorbic acid consumed in cooked broccoli, orange juice, orange slices, and synthetic tablets all produced the same blood levels in a study of 68 men. A separate study even found synthetic vitamin C powder dissolved in water to be slightly better absorbed than orange juice when measured by blood levels.
Food Sources vs. Supplements
If synthetic vitamin C is absorbed just as well as the natural form, you might wonder why food sources matter at all. The answer has less to do with the vitamin C itself and more to do with everything else in the food. Fruits and vegetables come packaged with fiber, potassium, flavonoids, and other plant compounds that work together in ways a single-ingredient supplement does not replicate. People who get their vitamin C primarily from food tend to have better overall nutrient intake simply because they’re eating more produce.
That said, supplements are a practical backup. If your diet is limited, whether due to food access, allergies, or personal preference, a basic ascorbic acid tablet will deliver the same molecule your body would get from a kiwi. There’s no need to pay a premium for “natural” vitamin C supplements, since the absorption is equivalent. Adults generally need about 75 to 90 mg per day, a target easily met by a single serving of most items on the list above. Smokers need roughly 35 mg more per day because smoking increases oxidative stress and depletes vitamin C faster.