Calcium and vitamin D supplementation plays a vital role in the management of osteoporosis, a condition affecting approximately 3 million people in the UK. These nutrients work together to maintain bone strength: calcium provides the structural framework of bone tissue, whilst vitamin D enhances calcium absorption and regulates bone metabolism. For individuals with osteoporosis or those receiving bone-protective treatments, ensuring adequate intake is essential to reduce fracture risk and support treatment effectiveness. This article examines the evidence-based use of calcium and vitamin D supplementation, recommended dosages aligned with NHS and NICE guidance, dietary sources, potential side effects, and when to seek medical advice.

Understanding Osteoporosis and Bone Health

Osteoporosis is a progressive skeletal condition characterised by reduced bone mineral density and deterioration of bone microarchitecture, leading to increased bone fragility and susceptibility to fractures. In the UK, approximately 3 million people are affected by osteoporosis, with fragility fractures representing a significant public health burden. The condition develops when bone resorption (breakdown) exceeds bone formation, resulting in porous, weakened bones that are vulnerable to fracture from minimal trauma.

Bone is a dynamic tissue that undergoes continuous remodelling throughout life, with osteoclasts breaking down old bone and osteoblasts forming new bone tissue. During childhood and adolescence, bone formation exceeds resorption, allowing skeletal growth and the accumulation of peak bone mass, typically achieved by the mid-twenties. After approximately age 35, bone loss gradually begins to exceed formation. In women, this process accelerates significantly following menopause due to declining oestrogen levels, which normally inhibit bone resorption.

Risk factors for osteoporosis include:

• Advanced age (particularly over 65 years)

• Female sex, especially post-menopausal women

• Family history of osteoporosis or fragility fractures

• Low body mass index (BMI under 19 kg/m²)

• Prolonged corticosteroid use (≥3 months at ≥5mg prednisolone daily)

• Certain medical conditions (rheumatoid arthritis, hyperthyroidism, malabsorption disorders)

• Medications including aromatase inhibitors and androgen-deprivation therapy

• Lifestyle factors including smoking, excessive alcohol consumption, and physical inactivity

The most common sites for osteoporotic fractures are the hip, spine (vertebrae), and wrist. Hip fractures are particularly serious, with significant mortality and morbidity, and many patients experiencing loss of independence. Early identification of individuals at risk and appropriate preventive measures, including adequate calcium and vitamin D intake, are essential components of osteoporosis management.

The Role of Calcium and Vitamin D in Osteoporosis Management

Calcium and vitamin D work synergistically to maintain skeletal health and are important to both the prevention and management of osteoporosis. Calcium is the primary mineral component of bone, comprising approximately 99% of the body's total calcium stores. It provides the structural framework that gives bones their strength and rigidity. Throughout life, calcium is continuously deposited into and withdrawn from bone tissue as part of normal bone remodelling. When dietary calcium intake is insufficient, the body mobilises calcium from the skeleton to maintain essential physiological functions, including nerve transmission, muscle contraction, and blood clotting, thereby compromising bone density over time.

Vitamin D plays a crucial regulatory role in calcium homeostasis and bone metabolism. Its primary function is to enhance intestinal absorption of dietary calcium, with absorption significantly reduced in vitamin D deficiency. Vitamin D is converted in the liver to 25-hydroxyvitamin D [25(OH)D], the main circulating form used to assess vitamin D status, and subsequently in the kidneys to the active hormone 1,25-dihydroxyvitamin D [1,25(OH)₂D]. This active form stimulates calcium absorption in the small intestine and, when necessary, promotes calcium release from bone to maintain serum calcium levels.

Vitamin D deficiency leads to secondary hyperparathyroidism, where elevated parathyroid hormone (PTH) levels increase bone resorption to maintain serum calcium, accelerating bone loss. In the UK, vitamin D insufficiency is common due to limited sunlight exposure, especially during winter months (October to March), when ultraviolet B (UVB) radiation is insufficient for cutaneous vitamin D synthesis.

For individuals with osteoporosis, calcium and vitamin D are essential to correct deficiency and support the effectiveness of anti-resorptive therapies such as bisphosphonates or denosumab. For patients receiving denosumab or intravenous bisphosphonates, it is particularly important to correct vitamin D deficiency and ensure adequate calcium intake before starting treatment, with monitoring of serum calcium as recommended in the product information. While calcium and vitamin D supplementation alone is generally insufficient to prevent fractures in community-dwelling adults, ensuring adequacy is an important component of comprehensive osteoporosis management.

Recommended Dosages and NHS Guidelines

The recommended daily intake of calcium and vitamin D varies according to age, sex, and individual risk factors. For adults, the Reference Nutrient Intake (RNI) for calcium in the UK is 700mg per day, though individuals with osteoporosis or at high risk of fracture may require higher intakes of 1000-1200mg daily. The recommended vitamin D intake is 10 micrograms (400 IU) daily for the general adult population, as advised by the UK Health Security Agency (UKHSA, formerly Public Health England) and the Scientific Advisory Committee on Nutrition (SACN).

For individuals receiving bone-protective treatments for osteoporosis, guidance advises ensuring adequate calcium intake (at least 700-1200mg daily) and vitamin D supplementation (800 IU or 20 micrograms daily). These higher vitamin D doses have been shown in clinical trials to reduce fracture risk when combined with adequate calcium intake.

For older adults in care homes or those who are housebound, the risk of vitamin D deficiency is substantially elevated due to limited sun exposure. The Department of Health and Social Care recommends that these individuals should take a daily supplement containing 10 micrograms (400 IU) of vitamin D throughout the year, with higher doses often prescribed following assessment of vitamin D status.

When vitamin D deficiency is confirmed through blood testing (serum 25(OH)D levels below 25 nmol/L), treatment regimens typically involve loading doses to replenish stores, followed by maintenance therapy. A common approach is a total loading dose of approximately 300,000 IU over 6-10 weeks (for example, 20,000 IU twice weekly for 7 weeks or 50,000 IU weekly for 6 weeks), followed by maintenance doses of 800-2000 IU daily. The specific regimen should be individualised based on the severity of deficiency, body weight, and presence of malabsorption conditions.

Supplemental calcium should be used to make up dietary shortfalls to reach the recommended intake, rather than exceeding upper intake levels. The total calcium intake from diet and supplements combined should generally not exceed 1500mg daily from supplements or 2000-2500mg daily in total.

NHS prescribing guidance supports the provision of calcium and vitamin D supplements for individuals with confirmed osteoporosis, those receiving osteoporosis treatments, and older adults at high risk of deficiency. Combined calcium and vitamin D preparations are available on NHS prescription, offering convenience and improved adherence compared to taking separate supplements.

Food Sources vs Supplements for Bone Health

Obtaining calcium and vitamin D through dietary sources is generally preferable to supplementation when adequate intake can be achieved, as foods provide additional nutrients that support bone health. Calcium-rich foods include:

• Dairy products: Milk, yoghurt, and cheese are excellent sources, with a 200ml glass of semi-skimmed milk providing approximately 240mg of calcium

• Fortified plant-based alternatives: Soya, almond, and oat drinks fortified with calcium (check labels, as calcium content varies)

• Leafy green vegetables: Kale, broccoli, and pak choi (though calcium from spinach is less bioavailable due to oxalate content)

• Fish with edible bones: Tinned sardines and salmon provide both calcium and vitamin D

• Nuts and seeds: Almonds, sesame seeds, and tahini

• Fortified foods: Some breakfast cereals and bread are fortified with calcium

Three servings of dairy or fortified alternatives daily typically provide approximately 700-900mg of calcium, meeting or approaching recommended intakes for most adults.

Vitamin D food sources are more limited. The primary dietary sources include:

• Oily fish: Salmon, mackerel, sardines, and herring (a 100g portion of salmon provides approximately 10 micrograms)

• Egg yolks: Contain small amounts (approximately 1-2 micrograms per egg)

• Some fat spreads and fortified foods: Check labels for vitamin D content

• Red meat and liver: Provide modest amounts

However, it is challenging to obtain sufficient vitamin D from diet alone, particularly during UK winter months when sunlight exposure is inadequate for cutaneous synthesis. The NHS advises short, regular periods of sun exposure to the skin between March and September, without burning. Many people need a daily 10 microgram supplement during autumn and winter months.

Supplementation becomes necessary when dietary intake is insufficient, sun exposure is limited, or malabsorption conditions are present. For individuals with osteoporosis, lactose intolerance, vegan diets, or those unable to achieve adequate intake through diet, supplements provide a reliable means of meeting requirements. Combined calcium and vitamin D supplements offer convenience, though some individuals may prefer separate supplements to allow flexible dosing. Calcium supplements are available as calcium carbonate (requiring acid for absorption, best taken with food) or calcium citrate (better absorbed on an empty stomach and suitable for those with reduced gastric acid). Supplemental calcium doses should be split (no more than 500-600mg per dose) to optimise absorption.

Potential Side Effects and Safety Considerations

Whilst calcium and vitamin D supplementation is generally safe when taken at recommended doses, potential adverse effects and drug interactions warrant consideration. Common side effects of calcium supplementation include:

• Gastrointestinal disturbances: Constipation (most common), bloating, flatulence, and abdominal discomfort occur in approximately 10-15% of users

• Nausea: Particularly with higher doses or when taken on an empty stomach

To minimise gastrointestinal effects, calcium supplements should be taken with meals, and the total daily dose divided into smaller amounts (no more than 500-600mg per dose, as absorption efficiency decreases with larger single doses). Adequate fluid intake and dietary fibre can help prevent constipation.

Hypercalcaemia (elevated blood calcium) is rare with standard supplementation but may occur with excessive intake (typically >2500mg daily) or in individuals with certain medical conditions, including primary hyperparathyroidism, sarcoidosis, or some malignancies. Symptoms include fatigue, confusion, excessive thirst, frequent urination, and nausea. Individuals with a history of kidney stones, particularly calcium oxalate stones, should consult their GP before starting calcium supplements, as supplementation may increase recurrence risk in susceptible individuals, though evidence suggests that dietary calcium may actually reduce stone formation.

Vitamin D toxicity is extremely rare and typically occurs only with very high doses taken over prolonged periods. Symptoms of vitamin D toxicity result from hypercalcaemia and include nausea, vomiting, weakness, and in severe cases, kidney damage. The tolerable upper intake level for vitamin D is 4000 IU (100 micrograms) daily for adults, as established by the Scientific Advisory Committee on Nutrition (SACN).

Drug interactions require attention:

• Bisphosphonates: Calcium significantly impairs absorption; alendronate and risedronate should be taken at least 30 minutes before calcium supplements or food; ibandronate requires 60 minutes separation. Always follow the specific product information

• Levothyroxine: Calcium can reduce absorption; separate administration by at least 4 hours

• Some antibiotics (tetracyclines, quinolones): Calcium forms complexes that reduce antibiotic absorption; separate by 2-4 hours

• Thiazide diuretics: May increase calcium retention, potentially increasing hypercalcaemia risk

• Corticosteroids: Reduce calcium absorption and increase requirements

• Medications affecting vitamin D: Orlistat, bile-acid sequestrants (cholestyramine), rifampicin, and anticonvulsants (phenytoin, carbamazepine) may reduce vitamin D levels

Individuals with chronic kidney disease require specialist supervision, as impaired renal function affects vitamin D metabolism and calcium handling, potentially leading to vascular calcification with excessive supplementation. Those with sarcoidosis or other granulomatous diseases may be more sensitive to vitamin D due to extra-renal conversion to active forms.

Suspected adverse effects from supplements or medicines should be reported via the MHRA Yellow Card Scheme (yellowcard.mhra.gov.uk or the Yellow Card app).

When to Seek Medical Advice About Supplementation

Whilst calcium and vitamin D supplements are available over the counter, certain circumstances warrant medical consultation before commencing or continuing supplementation. Individuals should contact their GP or healthcare provider if they:

• Have been diagnosed with osteoporosis or osteopenia and are uncertain about appropriate supplementation

• Experience persistent side effects such as severe constipation, abdominal pain, or nausea that does not resolve with dose adjustment

• Have a history of kidney stones, particularly calcium-containing stones, before starting calcium supplements

• Have chronic kidney disease, as specialist guidance is needed regarding appropriate calcium and vitamin D formulations and doses

• Are taking multiple medications, particularly those with known interactions (bisphosphonates, levothyroxine, digoxin, certain antibiotics)

• Have symptoms suggesting hypercalcaemia: excessive thirst, frequent urination, confusion, severe fatigue, bone pain, or nausea

• Have conditions affecting calcium metabolism, including hyperparathyroidism, sarcoidosis, or certain malignancies

• Experience sudden severe back pain after minimal trauma, new height loss, kyphosis (curvature of the spine), or neurological symptoms – seek urgent assessment for possible vertebral fracture

Routine monitoring may be appropriate for certain individuals. Those receiving high-dose vitamin D therapy for deficiency should have serum 25(OH)D levels checked after 3-4 months of treatment to ensure adequacy and avoid toxicity. For patients starting denosumab or intravenous bisphosphonates, checking adjusted serum calcium after vitamin D loading and before initiating treatment is advisable. Individuals with chronic kidney disease require regular monitoring of serum calcium, phosphate, and parathyroid hormone levels.

Fracture risk assessment should be considered for:

• Women aged 65 years and over

• Men aged 75 years and over

• Younger individuals with risk factors (early menopause, prolonged corticosteroid use, family history of hip fracture, low BMI)

GPs can arrange DEXA (dual-energy X-ray absorptiometry) scans to measure bone mineral density when clinically indicated, and may use fracture risk assessment tools such as FRAX® or QFracture to guide management decisions. If osteoporosis is confirmed, calcium and vitamin D supplementation forms part of a comprehensive management strategy that may include prescription bone-protective medications, falls prevention strategies, and lifestyle modifications including weight-bearing exercise, smoking cessation, and moderation of alcohol intake.

Patients should inform their healthcare provider about all supplements they are taking, including over-the-counter preparations, to ensure safe and effective management of their bone health. Regular medication reviews, particularly for older adults taking multiple medications, help identify potential interactions and optimise therapeutic regimens.

Frequently Asked Questions