Why Humans Can't Make Vitamin C (and Why You Need It Every Day)
A dog, a cat or a cow makes its own vitamin C in the liver, every single day. You don't. Humans belong to a small group of animals that lost this ability millions of years ago, which is why we depend completely on what we eat.
In this article we explain why it happens, why it pays to get vitamin C regularly, and how to cover your needs in a simple way.
A curious fact: we're one of the few that can't make it
Most mammals produce their own vitamin C. Humans, monkeys and apes, guinea pigs and some bat species are the exceptions.
Why we can't make it: a gene that stopped working
Animals that produce vitamin C make it from glucose, in the liver, thanks to an enzyme called L-gulonolactone oxidase (GULO).
In humans, the gene for that enzyme picked up mutations tens of millions of years ago and stopped working. We still carry a broken copy of it in our DNA, known as a "pseudogene," that no longer produces anything useful.
We're not alone. This limitation is shared by:
- Haplorhine primates: monkeys, apes, tarsiers and humans.
- Guinea pigs.
- Some bat species (others still keep the enzyme).
Rats, mice, rabbits, dogs, cats, pigs and cows, on the other hand, make it with no problem. Biochemists Grollman and Lehninger confirmed this in 1957, when they looked for the enzyme in different animals and couldn't find it in humans, guinea pigs or monkeys.
Why did evolution "allow" this loss? The most widely accepted explanation is that our ancestors ate so much fresh fruit and leafy food that making vitamin C was no longer necessary to survive.
Why you should get it every day
Vitamin C is water-soluble: it dissolves in water and doesn't build up in body fat the way vitamins A, D, E and K do. That changes completely how your body handles it.
We do keep a reserve, but it's small. An adult body holds between about 300 mg (when close to deficiency) and about 2 g of vitamin C. It is concentrated mainly in white blood cells, the eyes, the adrenal glands and the brain.
Extra is flushed out in urine. At 30 to 180 mg a day, the gut absorbs 70% to 90%. Above 1 g a day, absorption drops below 50%, and whatever the body doesn't need is excreted.
The reserve runs out in weeks, not months. If someone gets less than 10 mg a day, the first signs of scurvy can appear in as little as one month.
What your body needs it for
Vitamin C takes part in processes that happen in your body every day:
- Collagen: it's essential to make it. Collagen supports your skin, blood vessels, tendons and gums, and it's key for wound healing.
- Antioxidant: it helps limit damage from free radicals and regenerates other antioxidants, such as vitamin E.
- Immune system: it contributes to the normal function of your defenses.
- Iron absorption: it improves absorption of plant-based iron (beans, spinach, grains).
- Energy and nervous system: it's involved in producing L-carnitine and some neurotransmitters.
As for the common cold, the evidence is more modest than many people think. Taking it regularly may slightly shorten a cold, but it doesn't prevent you from catching one, and starting it once symptoms appear doesn't seem to help.
How much you need and where to find it
An adult needs 75 to 90 mg a day, an amount that's easy to cover with fresh fruit and vegetables. These are the recommendations from the U.S. National Institutes of Health (NIH):
| Group (19 years and older) | Daily recommendation |
|---|---|
| Men | 90 mg |
| Women | 75 mg |
| Pregnancy | 85 mg |
| Breastfeeding | 120 mg |
| Smokers | 35 mg more than the base recommendation |
The tolerable upper limit for adults is 2,000 mg a day. Above that amount, digestive discomfort such as diarrhea, nausea or cramps becomes more likely.
One orange and one kiwi a day already cover an adult's recommendation. Eating five varied servings of fruits and vegetables provides more than 200 mg.
What happens when you don't get enough
Severe deficiency is called scurvy, the disease that devastated sailors on long voyages until the 18th century. Today it's rare, but it still happens in people with very limited diets.
Early signs often include:
- Tiredness and general malaise.
- Swollen or bleeding gums.
- Bruising easily.
- Wounds that are slow to heal.
- Joint pain.
People more likely to fall short include smokers and those exposed to secondhand smoke, people with a very limited variety of foods, and people with intestinal absorption problems or kidney disease on dialysis.
The bottom line
Your body can't make vitamin C and only keeps a small reserve that runs out in weeks. That's why the key isn't taking a lot once in a while, but a little every day: one or two pieces of fresh fruit and some raw vegetables with your meals are enough for most adults.
If your diet or lifestyle makes it hard to reach the recommended amount, a supplement can help. Check with your doctor first if you're pregnant, take medication or have a health condition.
Sources: NIH Office of Dietary Supplements: Vitamin C Fact Sheet · Journal of Molecular Evolution (2024): GULO gene evolution · PLoS ONE: Vitamin C synthesis loss in bats