Updated: Nov 21, 2022
Unlike most animals that are able to synthesise vitamin C from glucose in their kidney or liver, humans cannot produce vitamin C in their body. The theory says we lost this ability a few millions years ago due to the inactivation of a certain gene involved in the last step of vitamin C synthesis. From then on, humans and some other primates became dependent on dietary intake of vitamin C. Why did it happen and what was the evolutionary advantage to this lost ability is an interesting debate, but we will keep it for another blog post. I’ve been rather curious to explore if there is an optimal daily amount of vitamin C for us.
As you know, many countries establish dietary reference values to tell the population how much daily amount is needed to ensure an adequate intake. And vitamin C is an interesting case. While it takes very little vitamin C to prevent a deficiency, the challenge is actually to define how much we need daily to maintain adequate health given the many metabolic processes that vitamin C is involved in.
In 2013, the European Food Safety Authority evaluated that the average requirement to keep bodily vitamin C at healthy levels is an intake of 90 mg/day for men and 80 mg/day for women. The population reference intake or an ideal level for the majority of people was set at 110 mg/day for men and 95 mg/day for women.
The good news is that these levels were finally increased compared to previous public advice. And it’s great, because recommending a tiny amount of vitamins just to prevent deficiency and disease such as scurvy is not really an ideal approach for optimal health. But are these new levels really optimal or again, just sufficiently safe for us to keep going?
Unfortunately, these dietary references are not updated very often to take into account new research findings. And research is not dormant in this field. Of course, it’s still a challenge to determine a suitable functional parameter to derive vitamin C reference value, but we might have an interesting candidate.In recent years, there is new emerging evidence that neutrophil motility (ability of our defence immune cells to move spontaneously) may be considered as a possible marker of vitamin C requirements, putting public authority recommendations in perspective. Neutrophils are the most abundant type of leukocytes (40% to 75%) and hugely important for our innate immune system. These cells are highly mobile and are able to migrate from the blood into the affected tissues. They also contain high concentrations of vitamin C compared to blood plasma levels, and it is thought that they function best if adequate amounts of vitamin C are available or when their concentration in vitamin C is saturated. Potentially, the neutrophil function could be a suitable functional marker to define vitamin C intake level in the future, but we need well designed human studies to make real progress. It’s just an example to show you that it’s difficult to take into account all body processes to establish dietary recommendation.
So how do we know if our body doesn't need more vitamin C than currently recommended 80/90 mg a day?
Well, let's look at two extreme examples. When we get 15 mg of vitamin C, the amount found in about a quarter of an orange, our body absorbs nearly 90 percent of it. However, if we instead take a supplement containing 1000 mg of vitamin C, our body seems to realise that’s too much and clamps down on absorption at the intestinal lining level, and we end up absorbing less than half. Some people will start experiencing tummy pain, diarrhea and stomach cramps even at 500 mg, but not everyone. Even if you don't have digestive discomfort after taking a large dose of vitamin C, it doesn't mean you absorb it. And don't forget that very high doses of vitamin C are quite dangerous for those with compromised stomachs or suffering from gastritis or ulcer.
So, experimenting with various levels of vitamin C researchers start getting a better idea whether the body is still absorbing vit C or starts to say, “That’s enough.”
The magic level of intake looks to be between 200 and 250 mg a day. When we take up to 200 mg daily, our body absorbs it all. Above that level, however, the body tries to reduce further absorption, suggesting that perhaps our “intestinal vitamin C transport mechanisms have evolved to fully absorb up to about 200 mg of vitamin C” a day.
In a study that I mentioned earlier, neutrophil motility was optimal at intakes ≥250 mg/day. Additionally, it was shown that at a plasma level of 75 µmol/L, which is reached with vitamin C intakes ≥200 mg/day, incidences of cardiovascular disease were lowest.
Interestingly, vitamin C is reabsorbed in our kidneys back into our bloodstream to maintain our vitamin C blood levels around 70 or 80 µmol/L, which is what we reach at a vitamin C intake of about 200 mg a day. Even if we take ten times as much in vitamin C supplements, 2,000 mg a day, our body will just excrete the excess to keep our blood levels in that narrow range of 70 to 80 µmol/L. The urinary excretion of vitamin C has been also proposed as a sign of adequate intake. Unfortunately, this is not a functional marker and reflects only one aspect of vitamin C plasma homeostasis. But it still shows that the bioavailability is complete for 200 mg vitamin C as a single dose. Maybe, 200 mg is actually closer to the optimal daily intake than current recommendations.
What else do we know about vitamin C dose from recently published data. Daily dietary intake of vitamin C of 200 mg is associated with the lowest stroke risk. But curiously it doesn’t have the same effect from vitamin C supplements, which is consistent with the overall body of evidence showing that antioxidant supplements in general don’t seem to protect against heart attacks or strokes.
Moreover, combined with established knowledge from pharmacokinetic studies and studies on cardiovascular diseases and common cold, intake recommendations are also to be increased to ≥200 mg/day which would be beneficial for the functioning of the immune system. It is assumed that the high vitamin C concentration protects neutrophils from the reactive oxygen species (ROS) they generate to kill pathogens such as bacteria and viruses.
These findings are supported by the observation that the vitamin C level is affected in people with infections, chronic diseases, and higher oxidative stress, as they lead to higher metabolic losses. Required amount seems to be higher, but there is no consensus on how much vitamin C is needed in chronic disease.
So now you probably think whether it is possible to get up to an intake of 200 mg of vitamin C a day without taking supplements? Sure. It’s really not that difficult, especially if you love vegetables and fruits. A kiwi or an orange will already provide between 65 and 70 mg of vit C. A single serving of vegetables may have about 50 mg each, but some veggies such as spinach will offer you up to 200mg in one cup. A cup of broccoli or kale is already 100 mg. So just mix various fruits and veggies a day to get you to ideal blood levels. In addition, other components in fruits and veggies will contribute to beneficial effects.
However, if you live in a highly polluted area or you smoke, you need at least 35 mg/day in addition to our magic level of 200-250, because you are under increased oxidative stress from the toxins and generally have lower blood levels of vitamin C.