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Focus on vitamin D

Vitamin D is an essential vitamin for elements such as bones, teeth and the immune system. It is present in fortified dairy but many people do not consume enough. Let’s focus on it.

What is vitamin D?

Vitamin D is a fat-soluble vitamin. It exists on 2 different forms, D2 (ergocalciferol), mainly produced by some plants and D3 (cholecalciferol). The human body synthesizes vitamin D3 in the skin, under the effect of ultraviolet rays.

Vitamin D is known for its significant role in calcium homeostasis and metabolism but it plays a multitude of effects on the body besides bones and calcium metabolism by contributing to:

  • the normal function of the immune system
  • inflammatory response
  • muscle functions

The roles of vitamin D

Vitamin D promotes healthy bones by aiding calcium absorption through modulation of active transport absorption and maintaining a good ratio of calcium and phosphate for bone mineralisation.

It reduces inflammation, modulates cell growth, glucose metabolism, neuromuscular and immune function through the modulation of genes encoding proteins that regulate these processes.

functions of vitamin D - YINI

A chronic deficiency in vitamin D leads to improper bone mineralisation, as less calcium and phosphorous are absorbed in the small intestine and can induce hypocalcaemia (low levels of calcium in the blood) and phosphaturia (phosphate in urine) causing accelerated bone demineralization. This can result in bone mineralisation diseases such as osteomalacia, osteoporosis, and rickets in children. Adequate levels of vitamin D combined with calcium can help prevent these diseases.

Vitamin D consumed in the diet is absorbed through passive diffusion in the gut and this absorption is enhanced by fat.

Diet recommendations

Vitamin D is synthesised by the skin when exposed to UV light. However, the amount synthesised depends on a multitude of factors such as where you live, weather, skin melanin levels, and wearing sunscreen.

Diet recommendations usually assume skin endogenous production of vitamin D is equal to 0 to ensure guidelines provide enough vitamin D through diet alone. The recommended daily intake is 15µg per day for adults. The ability of the body to produce and absorb vitamin D decreases with age which is why this recommendation increases to 20µg per day for people over the age of 70.

It is important not to consume too much vitamin D because excess vitamin D in the body is toxic and can lead to symptoms such as headache, nausea and vomiting, weight loss or fatigue. It is recommended to not consume more than 100µg per day. However, this happens almost only in cases of supplement overuse and it is not something the majority of the population has to worry about. In fact, most people fall short of meeting recommended intakes. In the USA, over 90% of adults do not consume enough vitamin D and this is similar in most northern hemisphere countries.

Dietary sources of vitamin D

Dietary sources of vitamin D are quite rare. Some vitamin D can be found in fatty fish, fish liver oils, beef liver, egg yolk, cheese and fortified dairy. For example, in the USA, almost all milk is fortified with vitamin D and it is compulsory in Canada.

Some mushrooms can have vitamin D if they are treated with UV light which makes the mushroom produce vitamin D.

Dietary sources of vitamin D - YINI

Vitamin D in dairy

As dairy products are often fortified in vitamin D, it is a major source of this nutrient for populations. For example, in France, 25% of vitamin D intake comes from dairy products.

Dairy products were chosen to be fortified as it doesn’t change the taste and it also helps in absorbing the high content of calcium and ensure healthy bones.

Fortified dairy products are an interesting source of vitamin D as they provide a large portion of the dietary recommendation.

Data from the USA National Health Nutrition and Examination Survey (NHANES), the Canadian Community Health Survey, and the UK National Diet and Nutrition Survey show that yogurt consumers have higher daily intakes of several key nutrients including vitamin D.

Research shows that people that consume yogurt have stronger bones and lower bone resorption markers. In older adults, yogurt consumption is linked to increased bone mineral density and physical function. Therefore, encouraging older people to eat yogurt more often, particularly vitamin D- and calcium-fortified yogurt, may be a valuable public health strategy to stave off osteoporosis.

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