Bone Health: Calcium, Vitamin D and Vitamin K2

Bones: Living Tissue in Constant Renewal

At first glance, bones appear rigid and unchanging. In reality, they are highly active tissue that continuously renews itself. This process is called bone remodeling. Specialised cells known as osteoclasts break down old bone tissue. At the same time, osteoblasts form new bone tissue. Through this process, the entire skeleton is completely renewed every 7 to 10 years.

In younger years, formation outpaces resorption. Bone density reaches its peak (peak bone mass) at approximately 25 to 30 years of age. After that, a slow, natural decline of about 0.5 to 1% bone mass per year begins. In women, this process accelerates significantly after menopause, as declining oestrogen levels promote bone resorption. Weaver et al. (2016) emphasise in a comprehensive review that nutrition plays a central role in bone health across all stages of life.

Calcium: More Than Just Dairy

Calcium is the most abundant mineral in the human body. Approximately 99% of all calcium is stored in bones and teeth. The remaining 1% circulates in the blood and is involved in critical functions such as muscle contraction, blood clotting, and nerve signal transmission.

The recommended daily intake is 1,000 mg for adults and increases to 1,200 mg for individuals over 50. Many people automatically associate calcium with dairy products. However, there are numerous plant-based sources with good bioavailability.

Plant-Based Calcium Sources

Food	Calcium per 100 g	Note
Sesame seeds (unhulled)	approx. 780 mg	Very high content, tahini as alternative
Kale	approx. 210 mg	High bioavailability (~50%)
Broccoli	approx. 60 mg	High bioavailability (~60%)
Dried figs	approx. 160 mg	Also a good potassium source
Mineral water (calcium-rich)	300-600 mg/L	Excellent bioavailability
Almonds	approx. 260 mg	Phytic acid may reduce absorption

Weaver et al. (2016) demonstrated that the bioavailability of calcium from cruciferous vegetables (broccoli, kale, bok choy) at 50 to 60% is actually higher than from milk (approx. 32%). The reason: cruciferous vegetables contain little oxalic acid, which can bind calcium in the gut and inhibit absorption. Spinach contains a substantial amount of calcium, but due to its high oxalic acid content, only about 5% is actually absorbed.

Vitamin D: The Sunshine Vitamin

Vitamin D occupies a special position among vitamins. It is technically classified as a prohormone because the body can produce it on its own. When sunlight reaches the skin, 7-dehydrocholesterol is converted into vitamin D3 (cholecalciferol). This is then converted in the liver to 25-hydroxyvitamin D and subsequently in the kidneys to the active form 1,25-dihydroxyvitamin D (calcitriol).

Calcitriol regulates calcium absorption in the intestine. Without sufficient vitamin D, the body can absorb only 10 to 15% of ingested calcium. With adequate vitamin D status, absorption increases to 30 to 40%. Holick (2007) describes this relationship in a widely cited review and characterises vitamin D deficiency as a global health crisis.

Vitamin D Deficiency in Northern Europe

At northern latitudes, sunlight from October to March is insufficient to enable adequate vitamin D synthesis in the skin. UV-B radiation reaches too shallow an angle during these months. Studies show that in Switzerland and Germany, 60 to 80% of the population has insufficient vitamin D levels during winter (below 50 nmol/L 25(OH)D in serum).

Risk factors for vitamin D deficiency include:

• Geographic location: North of the 42nd parallel (north of Rome), synthesis is severely limited in winter.
• Skin pigmentation: Darker skin requires longer sun exposure for the same vitamin D production.
• Age: The skin’s synthesis capacity declines with age.
• Sunscreen: SPF 30 reduces vitamin D synthesis by over 95%.
• Office work: Limited time outdoors during midday hours.

Dietary sources of vitamin D are limited. Fatty fish, egg yolks, and mushrooms (UV-exposed) contain relevant amounts but rarely reach the recommended daily intake of 800 to 1,000 IU. For this reason, many professional societies recommend supplementation during the winter months.

Vitamin K2: The Underestimated Component

Vitamin K exists in two main forms. Vitamin K1 (phylloquinone) is found in green leafy vegetables and is primarily involved in blood clotting. Vitamin K2 (menaquinone) has a different, less well-known but equally important function. It activates proteins that regulate calcium metabolism.

Two proteins are at the centre of K2 research:

• Osteocalcin: This protein, produced by osteoblasts, binds calcium and directs it into the bone matrix. Without sufficient vitamin K2, osteocalcin remains in its inactive (undercarboxylated) form and cannot effectively incorporate calcium into bones.
• Matrix Gla Protein (MGP): This protein prevents calcium deposition in soft tissue, particularly in the arteries. Activated MGP is one of the most potent natural inhibitors of vascular calcification.

Knapen et al. (2013) investigated the effects of vitamin K2 (MK-7, 180 mcg/day) on bone density and vascular elasticity in postmenopausal women over a three-year randomised controlled trial. The results showed that the K2 group exhibited improved bone strength and less age-related stiffness of the carotid artery.

Natural K2 Sources

The richest source of vitamin K2 is natto, a fermented soy product from Japan. Other sources include hard cheese (particularly Gouda and Emmental), egg yolks, and fermented foods such as sauerkraut. The Western diet typically provides considerably less vitamin K2 than the traditional Japanese diet.

The Synergy: D3, K2, and Calcium

The three nutrients function as a unit. Vitamin D3 increases calcium absorption from the intestine. Vitamin K2 ensures that this calcium is directed into the bones rather than deposited in the arteries. Calcium provides the building material itself.

Without vitamin K2, high calcium intake combined with vitamin D3 could theoretically increase the risk of vascular calcification. Masterjohn (2007) describes this interplay as the “calcium paradox”: in some populations, elevated rates of vascular calcification were observed despite high calcium intake, which may be linked to a concurrent K2 deficiency.

The practical implication: anyone supplementing with calcium and vitamin D should also ensure adequate vitamin K2 intake.

Strength Training and Bone Density: Wolff’s Law

Julius Wolff formulated the principle in 1892 that bones adapt to the mechanical loads placed upon them. Greater loading leads to denser, stronger bones. Reduced loading leads to bone loss. Astronauts lose up to 1 to 2% of bone mass per month in zero gravity.

For bone health, exercises that generate high mechanical loading are particularly effective:

• Resistance training with progressive overload: Squats, deadlifts, and lunges load the hip and spine bones, which are most commonly affected by osteoporosis.
• Jumping and plyometrics: The impact forces from jumping stimulate bone formation particularly effectively.
• Standing exercises: Exercises performed standing place greater load on the skeleton than seated exercises.

Swimming and cycling are excellent cardiovascular activities but generate little bone loading and contribute minimally to bone density. For bone health, they should be supplemented with resistance training.

Osteoporosis Prevention: Start Early

Osteoporosis develops over decades. Prevention ideally begins long before the first symptoms appear. Since bone density reaches its peak around age 30, the building phase during adolescence and young adulthood is particularly critical.

Preventive measures include:

• Calcium-rich diet: 1,000 to 1,200 mg calcium daily from food and supplements if needed.
• Check vitamin D status: Especially during winter months. A blood level of 50 to 75 nmol/L 25(OH)D is considered optimal.
• Incorporate vitamin K2: Fermented foods or targeted supplementation (100 to 200 mcg MK-7 per day).
• Strength training: At least 2 to 3 sessions per week with progressive overload.
• Minimise risk factors: Smoking, excessive alcohol consumption, and physical inactivity accelerate bone loss.

Conclusion

Bone health does not depend on a single nutrient but on the interplay of several factors. Calcium provides the building material, vitamin D3 optimises absorption, and vitamin K2 directs calcium to the right place. Strength training provides the mechanical stimulus that signals bones to become stronger. Combining these factors and paying attention to bone health from a young age lays the foundation for a stable skeleton later in life.

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Sources:

• Weaver CM et al. (2016). Calcium plus vitamin D supplementation and risk of fractures: an updated meta-analysis from the National Osteoporosis Foundation. Osteoporosis International, 27(1), 367-376.
• Holick MF (2007). Vitamin D deficiency. New England Journal of Medicine, 357(3), 266-281.
• Knapen MHJ et al. (2013). Three-year low-dose menaquinone-7 supplementation helps decrease bone loss in healthy postmenopausal women. Osteoporosis International, 24(9), 2499-2507.
• Masterjohn C (2007). Vitamin D toxicity redefined: vitamin K and the molecular mechanism. Medical Hypotheses, 68(5), 1026-1034.