Vitamin C is a vital nutrient for health. It is central in the functioning of the immune system and building strong bones, skin, and blood vessels. Yet despite vitamin C being abundant in fruits and vegetables, it is also one of the most popular supplements taken, despite widespread deficiency in the population not being an issue. In this podcast, I’ll look closer at the roles of vitamin C and uncover the evidence for some of the many health claims made about it. 

Vitamin C is probably one of the most well-known vitamins and also one of the most popular supplements. Its history traces back to the 17th century where the Scottish doctor James Lind of the British Royal Navy undertook one of first-ever nutrition clinical trials as a way to prevent scurvy. He found that supplementing the diets of sailors undergoing long sea voyages with a range of different foods revealed that it was those sailors who received citrus fruits were the ones who quickly recovered from scurvy. The anti-scurvy factor in limes and other foods was dubbed the antiscorbutic factor. Nearly 200 years later, the factor was isolated and given its name of ascorbic acid – that’s vitamin C to you and me.

Roles

Vitamin C has a range of functions in the body. A key one is as an antioxidant. Vitamin C losses its electrons easily which allows it to neutralise potentially damaging free radicals. A free radical is a molecule with one or more unpaired electrons, which makes it unstable and highly reactive and damaging to other molecules.

Vitamin C also helps to form the fibrous structural protein of connective tissues known as collagen. 

Collagen is the main component of connective tissue and is the most abundant protein in humans. It is a vital component in tendons, ligaments, skin, bones and blood vessels. Cells are held together largely by collagen. This is especially important in the artery walls, which must expand and contract with each beat of the heart, and in the thin capillary walls, which must withstand a pulse of blood every second or so without giving way. The bleeding characteristic of scurvy is because of the breakdown in collagen strength and fragility of tissues and capillaries.

Vitamin C also serves as a cofactor in the synthesis of several other compounds such as the production of carnitine which is a compound that transports long-chain fatty acids into the mitochondria of a cell for energy metabolism. It also participates in the conversions of the amino acids tryptophan and tyrosine to the neurotransmitters serotonin and norepinephrine, respectively. Vitamin C also assists in the making of hormones, including thyroid hormone, which regulates metabolism, digestive function, bone health and brain development.

Colds

So, let’s look at one of the big health claims made about vitamin C which is its use as a popular option for the prevention and treatment of the common cold. Vitamin C levels can decrease in immune cells during the common cold, so perhaps supplementation may help improve the vitamin C status of these immune cells. But research supporting such claims has been conflicting and controversial. Most studies find no relationship between vitamin C and the occurrence of the common cold. Some studies do though show a benefit, but only in a small, specific population undergoing severe physical stress like elite athletes so here, the effect is not really generalisable to most people as vitamin C has not been shown to prevent colds in the general community.

A major Cochrane review of 42 studies on vitamin C in the treatment and prevention of the common cold found no consistent effect of vitamin C in reducing the risk of getting a cold. It did though find a very small benefit for taking vitamin C in reducing how low you have a cold for which was an 8% shorter duration for adults and 14% shorter duration in children. So, there could be something to be said for taking some vitamin C once the symptoms of a cold appear, but the benefit is very small. The overall conclusion from this review is that routine vitamin C supplementation is not justified for the general population. However, supplementation may be of benefit in some groups such as athletes undergoing periods of intense training, as severe physical exercise can suppress the immune system. I’ll link to the study in the show notes. https://www.cochranelibrary.com/cdsr/doi/10.1002/14651858.CD000980.pub4/full

Considerable experimental evidence suggests that vitamin C plays an important role in immune function, so getting too little vitamin C may negatively affect immune function. Some studies suggest that people with low vitamin C status may have an increased risk of infections and perhaps this in fact due to inadequate vitamin C. But if you are healthy, maintain a good diet, do not smoke or have disease, it is highly questionable if taking more vitamin C will boost your immune function any further.

On to the big topic in the world at the moment, that being the coronavirus, there is a clinical trial underway of using 12 grams of i.v. vitamin C for severe COVID-19-induced pneumonia. The work is currently ongoing and from the preliminary results I’ve seen which are unpublished I would add it is by no means a cure, but it may be pointing to a slightly shorter stay in ICU which actually meshes quite well with the small benefit that vitamin C has in reducing the duration of the common cold. Let’s wait to the full results our out through, there is far too much noise in this space for all manner of treatments competing for attention.

Observational studies point to people with diets higher in vitamin C having a lower risk of cancer, but this is most likely because of the foods it is found in which are fruits and vegetables. When you look at studies that supplement with vitamin C for long periods of time, there is little benefit in reducing cancer risk. And a similar situation applies when you look at research into vitamin C and heart disease where diets high in vitamin C containing foods are linked to a lower risk, but randomised controlled trials giving vitamin C to people at risk of heart disease show no benefit.

Sources

How much vitamin C does a person need? The minimum requirement which is the amount needed to prevent the overt symptoms of scurvy is a tiny 10 milligrams daily – that’s less than a quarter of an orange. However, 10 milligrams a day does not saturate all the body tissues so higher amounts are required. The RDI in Australia has been set at 45 milligrams per day for adult men and women. At about 200 milligrams, absorption reaches a maximum, and there is little, if any, increase in blood concentrations at higher doses.

Fruits and vegetables can easily provide a generous amount of vitamin C. Citrus fruits, cantaloupe, strawberries and mangoes are notable sources among fruits. Cabbage-type vegetables, dark green vegetables, lettuce, tomatoes and even potatoes are some of the vegetables that provide vitamin C 

A glass of orange juice at breakfast, a salad for lunch, and a stalk of broccoli and a potato for dinner alone provide more than 300 milligrams -that’s many times above the RDI which shows how unlikely true vitamin C would be in the general population.

There are some people who might be at risk of vitamin C inadequacy and these include those with a very poor diet, alcoholism, anorexia nervosa and smokers. Deficiency symptoms include anaemia, atherosclerotic plaques, pinpoint haemorrhages, bone fragility, poor wound healing, bleeding gums and loose teeth, muscle degeneration, rough skin and blotchy bruises.

Toxicity

On the other end of the spectrum to do with toxicity, the body has specialised vitamin C transporters in the small intestine that can take up only a certain amount of vitamin C at any one time, so the rest is excreted. And because vitamin C is water-soluble, it is not stored in the body. Therefore, the body takes what it needs from the blood and the rest is excreted in the urine.

But you can take too much. Although vitamin C has an overall low toxicity risk in healthy individuals, consuming too much of it can cause some adverse gastrointestinal side effects, including cramps, nausea, and diarrhoea. Additionally, since a high vitamin C intake increases the body’s absorption of non-haem iron, consuming too much vitamin C could cause problems for people with hemochromatosis, a condition in which the body retains too much iron and too much free iron causes the kind of cellular damage typical of free radicals.

Large amounts of vitamin C excreted in the urine can obscure the results of tests used to detect diabetes. While people taking anticlotting medications may unwittingly counteract the effect if they also take large doses of vitamin C.

Those with kidney disease, a tendency towards gout or a genetic abnormality that alters vitamin C’s breakdown to its excretion products are prone to forming kidney stones if they take large doses of vitamin C.

It is not possible to set with certainty a safe upper level of intake for vitamin C is, but 1000 milligrams per day is considered a prudent limit. Few instances warrant consuming more than 200 milligrams a day.

Vitamin C is widely found in the food supply and deficiency of it is rare and let's face it, if you are hardly eating any fruits and vegetables, you’d be faced with many health problems outside of those caused by vitamin C deficiency. A broad healthy diet gives ample vitamin C with cases for using supplements being a rare exception rather than a rule.

Research Wrap Up

Now on to my research wrap up segment where I profile a study that has grabbed my attention during the week. And for this week it is all about speed eating with new research showing that fast eaters are more at risk of weight gain and metabolic disease.

The advice seems credible: eat slowly so that you have a chance to feel full before you can overeat. But how true is this? Just how much credence to give to this advice was the topic of research out of Japan looking at how eating speed could influence future weight gain and risk of metabolic disease. And I’ll link to the research in the show notes. https://www.ahajournals.org/doi/abs/10.1161/circ.136.suppl_1.20249

Just over 1000 adult men and women took part in the study which tracked their health and eating habits over 5 years. Based on their own self-assessment, the participants were divided into three groups depending on if they considered themselves slow, normal or fast eaters.

So, after 5 years had passed, how did the weight and health of the different groups of eaters fare? Just over 11 percent of the fast eaters had developed metabolic syndrome, compared with 6.5 percent for the normal speed eaters, and just 2.3 percent for the slower eaters.

Faster eating speed was also linked with a greater risk of weight gain, greater waist circumference and a more adverse blood lipid profile.

The findings from this work support previous short-term laboratory eating studies that quick eaters feel less full and are more likely to overeat later. It takes time for mechanical and hormonal signals to reach the brain from the stomach to tell us we’re full so eating slowly allows this feedback system to work properly. Eating quickly also causes greater fluctuations in blood glucose, which can lead to insulin resistance.

So how fast is too fast? Previous research has found that a meal should take about 20 minutes to consume so this should be a time target to aim for.

Eating slowly is easier said than done. But one way to achieve this is to put your cutlery down between each bite. Better yet, try eating food with chopsticks if you’re not so adept with them. Keep your mind on your teeth and jaw and get them working together to really chew each mouthful of food fully and mindfully.

So that’s it for today’s show. You can find the show notes either in the app you’re listening to this podcast on if it supports it, or else head over to my webpage www.thinkingnutrition.com.au and click on the podcast section to find this episode to read the show notes.

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I’m Tim Crowe and you’ve been listening to Thinking Nutrition