Calcium and Vitamin D in a plant-based diet

All you need to know about the role of calcium and Vitamin D in a plant-based diet, from science to practice in helping people achieve their protein recommendation.

Summary

• Calcium is the most abundant mineral in the human body. Predominantly stored in bone tissue, calcium is essential for bone strength as well as function of the nervous system, blood clotting and muscle control.

• Vitamin D is a fat-soluble vitamin that promotes calcium absorption in the gut and helps to maintain adequate serum calcium and phosphate concentrations.

• Calcium is obtained through dietary intake. The predominant source of calcium across Europe is dairy and dairy products however fortified products and calcium containing plant foods also provide a notable contribution.

• Most of our vitamin D is obtained through synthesis in our skin on exposure to sunlight, with small quantities coming from dietary sources and fortified foods.

• Those following vegetarian and vegan diets may be at slightly higher risk of vitamin D and calcium deficiency in the absence of a well-planned diet; however low intakes and stores have been found across the population in all dietary patterns.

• Attention should be paid to calcium intake and vitamin D status in all populations to support bone health, and this may be particularly important for those on plant-based diets.

• Use of calcium and vitamin D fortified foods as well as supplementation is recommended to safeguard skeletal health.

Introduction

What is calcium?

Calcium is a mineral that is required for all life. Found naturally in our environment, plants absorb calcium through their roots before integrating it into their plant cell structure.

Calcium also supports the structure of animals as an integral component of shells, bone tissue and teeth.

Calcium is the most abundant mineral in the human body with 99% stored within bone in the form of hydroxyapatite, with the remaining 1% supporting the function of the nervous system, blood clotting and muscle control(1).

Without sufficient dietary sources of calcium, we are at increased risk of weakened bones, osteoporosis and bone fracture.

What is vitamin D?

Vitamin D is a fat-soluble vitamin, that is also vital for bone health due to its role in regulating calcium and phosphate levels in the body.

Vitamin D circulates in our blood in the form of ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3). Vitamin D deficiency can also lead to osteoporosis as well as muscle weakness(2).

How do calcium and vitamin D interact?

Calcium and vitamin D work together. Calcium is actively absorbed through the lining of the small intestine when vitamin D is present(3).

Optimal levels are necessary to increase absorption from 10-15% of dietary calcium to 30-40%3. Unabsorbed calcium is lost through urine, faeces, skin and sweat(4).

Where do we get calcium and vitamin D from?

Our diet and lifestyle play a major role in achieving sufficient levels of both calcium and vitamin D.

Calcium is obtained through dietary intake with dairy and dairy products being the most recognised sources.

The contribution of dairy products such as milk, yogurt and cheese to total calcium intakes across Europe varies from 43% in the UK(5) to 59% in Italy(4).

Despite the variation, they are generally the main dietary sources of calcium in all European countries studied(4).

National dietary guidelines, such as UK’s Eat Well Guide(6) highlight alternative sources of calcium in addition to dairy.

Calcium-rich foods that also provide notable contributions to intake include calcium-fortified foods (such as cereals, plant-based milks and tofu), grains, dark green vegetables, legumes, nuts, fish with bones, and even water in hard water areas(7).

Not everyone can utilise dairy as a calcium source. Up to 65% of the world population is lactose intolerant(8) and are therefore required to choose lactose-free dairy or plant-based milks.

Although incidence of lactose intolerance is lower in populations of European descent at 28%, it can be highly prevalent, up to 70%, in other ethnic populations(9). Other reasons for avoidance of dairy and dairy-containing products include milk allergy, ethical and environmental motivations based on the production techniques, and those who are seeking general improved health outcomes(10).

Vitamin D is unique in that 80-90% is synthesised in our skin from 7-dehydrocholesterol on exposure to UVB rays(11, 12).

Due to insufficient exposure to sunlight because of our modern lifestyles, seasonally low concentrations have led to the recommendation of vitamin D supplementation during the European winter (October to March).

Vitamin D is available in 2 distinct forms, namely, ergocalciferol or vitamin D2 and cholecalciferol or vitamin D3. The naturally occurring plant-derived form, vitamin D2, was produced in the early 1920s through UV exposure of foods, such as yeast and mushrooms(1).

Vitamin D3 is synthesized in the skin of humans from 7-dehydrocholesterol and is also present in animal-based foods, such as egg yolks and oily fish.

Both vitamin D3 and vitamin D2 are synthesized commercially and found in dietary supplements or fortified foods(2).

Do we get enough calcium and Vitamin D?

Current intakes

Average calcium intakes and vitamin D status vary across countries in Europe as shown in table 1.

Recent research has found that most countries in Europe have an intake higher than 700mg per day(13), however for some of the countries with the highest calcium intakes, such as Iceland, data collected includes dietary supplementation.

Intake is also known to vary between subgroups, for example teenage girls in both the UK and the Netherlands have been found to have calcium intakes below recommendations(14, 15).

Vitamin D deficiency [serum 25(OH)D <25nmol/l] has been reported in 2-30% of adults in Europe(16), and some studies suggest that this could rise to 80% in the institutionalised elderly(17).

It has been noted that deficiency is entirely avoidable in western countries as access to vitamin D supplementation is both affordable and widely available.

Table 1. Calcium intake and vitamin D status in 21 European countries from currently available data (mg/day)(13)

Recommendations

National and organisational recommendations for dietary calcium vary minimally around the world.

The European Food Safety Authority(4) recommend that all adults over 25 years obtain 950mg of calcium per day, and more for teenagers and young adults between 11 and 24 years due to rapid growth of the skeleton as shown in table 2.

This is approximately consistent with the Institute of Medicine (IOM) of the US National Academy of Sciences, but who also recommend increased levels at 1200mg/day for post-menopausal women and 1000mg/day in pregnancy and lactation(18).

In addition, specific medical conditions may require increased calcium intake for example up to 1500mg in coeliac disease(19) and up to 1000mg in inflammatory bowel disease when on corticosteroids(20).

The IOM recommend that vitamin D levels of >50nmol/l is ‘sufficient’ for almost the whole population(16).

Table 2. EFSA summary of Dietary Reference Values for calcium and vitamin D for infants, children and adults

Calcium and Vitamin D intakes across dietary patterns

Comparing intake of omnivorous and plant-based dietary patterns

 A systematic review in 2022 found that dietary calcium intakes were slightly lower in vegans when compared to vegetarians and meat-eaters (22).

However, more than one third of studies reported intakes below the estimated average requirements for all dietary patterns, indicating inadequate intakes in the general population.

Similarly, vegetarians and vegans may be at slightly higher risk of vitamin D deficiency due to reduced intake of foods such as fish and eggs, however dietary sources cannot be relied upon for any dietary patterns and all European populations are at risk of deficiency regardless of diet (23, 24).

Plant-based diets and bone health

 Several large investigations have been carried out to explore the impact of plant-based dietary patterns on bone health (25,26,27,28).

These studies have raised concerning headlines due to the higher risk of total bone fracture found for those on vegan diets when compared to meat-eaters.

There were however many contributing factors in these results that were not directly related to dietary pattern. For example, in one study, a 30% increased risk of fracture found in vegans disappeared when adjusting for calcium intake(25).

Another study found that vegetarians had an increased risk of fracture, but also consumed less protein, iron, iodine, niacin, selenium, vitamin B12 and vitamin D, plus they only achieved a daily average of 22.6g of fibre and similar levels of saturated fat to meat-eaters(28).

Although these vegetarians had lower rates of heart disease, cancer and type 2 diabetes, the results suggest that they were not achieving a high-quality diet supportive of bone health(28).

In fact, studies investigating postmenopausal women have found a protective effect of healthy plant-based diets compared to unhealthy plant-based diets on bone loss and risk of osteoporosis(29,30). 

As well as the potential for low levels of certain nutrients, those on a fully plant-based diets had a lower body mass index (BMI) which has been linked to increased bone fracture regardless of dietary pattern(31).

In addition, most results related to females, rather than males, who also had lower use of hormone replacement therapy, low levels of vitamin D and B12 supplementation and higher levels of physical activity, which has the potential to increase impact fracture rates, although this was not explored.

These investigations highlight the importance of a high-quality diet, appropriate supplementation, use of fortified foods and a bone-healthy lifestyle on any dietary pattern. These factors may be of even more importance when following a plant-based dietary pattern.

Ways to meet calcium and vitamin D requirements on a plant-based diet

Sources

It is important for everyone to meet calcium and vitamin D requirements regardless of dietary preferences.

It is possible to meet calcium requirements on a planned plant-based diet with the use of fortified products alongside naturally calcium-rich plants. Plant-based alternatives to dairy products, such as soya, almond and oat drinks, are regularly fortified with calcium and have a similar bioavailability to cow’s milk(32).

Naturally calcium-rich plant foods can also support dietary intake but are not recommended to be relied upon as main sources due to their lower calcium content.

Some plant foods with a higher calcium content also contain higher levels of phytates and oxalates which adversely impact absorption, such as spinach and rhubarb, and these are not recommended to be used as calcium sources(32).

Table 3 displays the calcium content of various animal and plant-based foods(20).

Table 3. Calcium content of various animal and plant-based foods(20,33)

Table 4 provides some food sources of vitamin D, however dietary intake should not be relied upon for sufficient levels.

Table 4. Main food sources of vitamin D(34)

Supplementation advice

Sufficient calcium can most often be achieved through diet alone, which provides the benefits of a food matrix with additional and complementary nutrients.

Calcium supplementation is usually only required in specific circumstances when dietary calcium is deficient. There have been some concerns over the safety of taking high dose calcium supplementation(35), however the UK’s national health service (NHS) recommends that calcium supplementation up to 1500mg per day is unlikely to cause harm(36).

As most Europeans do not reach dietary recommendations for vitamin D, fortification programmes and supplements play a key role in supporting intakes.

In the UK, it is recommended that in addition to at-risk groups such as older adults, pregnant and breastfeeding women and those with darker skin pigmentation should take a daily 400 IU (10µg) supplement all year round, all adults should consider doing the same, particularly in the autumn and winter(11).

Ways to increase calcium and vitamin D on a plant-based or dairy-free diet:

• Choose a calcium and vitamin D fortified plant-based milk alternative throughout the day in cereal or porridge, hot drinks and cooking.

• Add a calcium and vitamin D fortified plant-based alternative to yogurt to your breakfast, snacks and desserts.

• Choose tofu that is calcium-set, indicated by the word ‘calcium’ in the ingredients list in the form of calcium sulphate or similar.

• Regularly add low-oxalate leafy green vegetables such as spring greens, kale and broccoli to meals.

• Top up with additional calcium-rich plants such as oranges, almonds, dried figs and tahini.

• Consider taking a 400 IU (10µg) supplement during the winter season, and all year round if falling into an at-risk population.

Calcium-rich plant-based meal ideas (approximate values)

Breakfast – Porridge made with fortified almond drink and flaxseed topped with chopped dried figs and almonds – 320mg

Lunch – Wholewheat wrap with rocket salad, hummus and tahini dressing – 160mg

Snack – Berries and orange with fortified soya yogurt and chia seeds – 200mg

Dinner – Calcium-set tofu stir-fry with broccoli, spring greens and brown rice – 270mg

Factsheet

Download our Calcium and Vitamin D factsheet - a series of helpful infographics covering key facts and health benefits.

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