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.

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.

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.

See also

• Boost your vitamin D with fortified yogurt
• Eat your way to stronger bones: fermented dairy may help

Sources

• Office of Dietary Supplements – Vitamin D.

• Sizar O, Khare S, Goyal A, Givler A. Vitamin D Deficiency. StatPearls. Treasure Island (FL): StatPearls Publishing; 2024.

• Vitamin D: why do I need an adequate intake and how can I make sure I get it ? In: Anses – Agence nationale de sécurité sanitaire de l’alimentation, de l’environnement et du travail [Internet]. 2 Mar 2022 [cited 13 Feb 2024].

• EUFIC – Vitamin D: foods, functions, how much do you need & more.

• CIQUAL – La table de composition nutritionnelle des aliments. ANSES, France.

• Zhu Y, Jain N, Holschuh N, et al. Associations between frequency of yogurt consumption and nutrient intake and diet quality in the United Kingdom. J Nutr Sci. 2021;10:e85.

• US Department of Agriculture and US Department of Health and Human Services. Dietary Guidelines for Americans, 2020–2025. 9th Edition. December 2020.

• Cifelli CJ, Agarwal S, Fulgoni VL. Association of yogurt consumption with nutrient intakes, nutrient adequacy, and diet quality in American children and adults. Nutrients. 2020;12:3435.

• Vatanparast H, Islam N, Prakash Patil R, et al. Consumption of yogurt in Canada and its contribution to nutrient intake and diet quality among Canadians. Nutrients. 2019;11:1203.

• He M, Yang YX, Han H, et al. Effects of yogurt supplementation on the growth of preschool children in Beijing suburbs. Biomed Environ Sci. 2005;18:192–7

• Laird E, Molloy AM, McNulty H, et al. Greater yogurt consumption is associated with increased bone mineral density and physical function in older adults. Osteoporos Int. 2017;28:2409–19.

• Bonjour JP, Benoit V, Payen F, et al. Consumption of yogurts fortified in vitamin D and calcium reduces serum parathyroid hormone and markers of bone resorption: a double-blind randomized controlled trial in institutionalized elderly women. J Clin Endocrinol Metab. 2013;98:2915–21.

• Bonjour JP, Benoit V, Atkin S, et al. Fortification of yogurts with vitamin D and calcium enhances the inhibition of serum parathyroid hormone and bone resorption markers: a double blind randomized controlled trial in women over 60 living in a community dwelling home. J Nutr Health Aging. 2015;19:563–9.

• Sahni S, Mangano KM, Kiel DP, et al. Dairy intake is protective against bone loss in older vitamin D supplement users: the Framingham study. J Nutr. 2017;147:645–52.

YINI attended the 2024 ECO congress to gather scientific updates and expert’s advice on the role of diet and dairy in the management and prevention of obesity. The congress offered a great opportunity to meet with experts from various fields of expertise. Here Niam Arthurs, pediatric dietitian and researcher in the area of child and adolescent obesity based in Dublin, Ireland, speaks about her research on child obesity.

Can you introduce yourself and tell us what you work on?

I’m a senior pediatric dietitian and researcher in the area of child and adolescent obesity based in Dublin, Ireland. My team works on food habits among children with obesity. Our main objective is to gather intelligence about our young patients’ habits before implementing any obesity treatment in order to maximize the effectiveness of personalized dietary approaches.

Firstly, we examine the nutritional quality of children’s reported food and fluid intakes. For this specific aspect, we use a 28-item food frequency questionnaire, conducted with the parents.

Secondly we evaluate the level of parental knowledge about nutrition, their self-reported cooking confidence and their relationship to food consumption. Ultimately, these elements guide us in determining the relationship between parental nutritional knowledge and children’s dietary habits. That is the basis for tailoring personalized treatment.

What have you learned about personalized dietary treatment for children with obesity using this method?

Getting a baseline idea about what children and teenagers are actually eating, combined with parents’ nutrition knowledge and their ease with cooking preparations allowed us to avoid making assumptions about children’s environments. This truly helped us make the connection between gathered information and its relationship with food consumption.

Our study found that parents who reported lower levels of cooking confidence tended to have households with higher consumption of convenience meals and takeaway foods. For dietitians aiming at framing their patients’ specific needs, such elements represent key targets for designing their interventions.

Assessing levels of knowledge in nutrition also enables understanding of general food and nutrient recommendations knowledge. We observed that the consumption of dairy products was a lot lower than the recommended amounts, in all age groups. From 5- to 88-year-olds, only about over 50% of the studied population met the recommended daily intakes for dairy products.

In Ireland, dairy products are the main source of calcium in children’s and teenagers’ diets. This nutrient is of pivotal importance during this age of growth. However, none of the 9- to 16-year-olds included in the survey met the recommended daily intakes for calcium, from either dairy products or other calcium-rich foods.

Again, the study aims at identifying such key target points to let dietitians design relevant interventions.

How do these results translate in your day-to-day consultation?

Expanding on the earlier example of calcium deficiency, the study highlighted that our interventions need to focus more on dairy products. This implies addressing the myths around dairy products and exploring with the families the reasons why they do not consume enough dairy products, particularly children and teenagers.

In this instance, it appears that a lot of the parents simply do not know what the recommendations for dairy foods are – i.e. 3 portions of dairy foods per day from up to 8 years, and 5/day after 8 years old, in Ireland – nor what a serving size corresponds to.

That information is crucial for health and nutrition literacy, and we need to incorporate it more into our one-on-one consultations and group education programs to ensure that individuals are receiving adequate levels of necessary knowledge.

Do you provide them with simple ways to incorporate more dairy into their meals?

This is where tailoring comes in. It’s important to understand what children and teenagers are currently eating to make small, specific suggestions for incorporating more dairy. For example, many children begin skipping breakfast when they transition from primary to secondary school, often due to time constraints or lack of appetite. However, breakfast is an important opportunity to consume calcium-rich foods.

We can suggest alternatives such as eating something once they get to school or choosing quicker options for the morning to overcome time barriers. Additionally, they can incorporate more calcium-rich foods into their lunch. For instance, they could add cheese to their sandwiches or bring pouch yogurts, which are convenient and less likely to burst in their bags. Drinkable yogurts are also a good option. These types of recommendations are often more acceptable and practical for their routines.

In March 2024, the FDA published a qualified health claim about the role of yogurt in the prevention of type 2 diabetes. Can this have an impact on the recommendations? Do you think it can be an opportunity for patients?

It’s incredibly valuable to have strong, evidence-based claims because there’s so much misinformation surrounding dairy foods. For instance, myths about dairy contributing to acne, containing hormones, or causing inflammation are not supported by scientific evidence. Unfortunately, the science community isn’t reaching these target groups effectively. Families are increasingly getting their information from social media, so we need to be much louder in our communication. Having well-supported claims can help us do that.

Moreover, these claims are particularly significant in our world today, where we face rising rates and high prevalence of conditions like type 2 diabetes. These conditions can severely impact an individual’s quality of life and health if not properly managed. Strong, scientifically-backed claims not only help us communicate more effectively but also attract media attention. This can encourage individuals, families, and teenagers who might be avoiding calcium-rich foods, such as dairy products, to reconsider their choices.

Eating ultra-processed foods is linked to the risk of developing long-term health problems, according to the latest evidence from an umbrella study conducted by an international team of researchers (1).

But questions remain on just how much do these ready-to-eat products affect our well-being and should they all be assessed under the same umbrella.

An umbrella approach to assessing ultra-processed food effects