Calcium is an important mineral that helps build strong bones and teeth. It also supports nerve signaling, muscle contraction, and hormone secretion. About 99% of the calcium in your body is stored in your bones and teeth. The other 1% circulates in your blood and other tissues.
Your body carefully regulates calcium levels in your blood through the actions of three hormones: parathyroid hormone (PTH), calcitonin, and vitamin D. When blood calcium levels drop too low, PTH signals your bones to release calcium into the bloodstream. When levels are too high, calcitonin and vitamin D work to lower blood calcium by promoting calcium deposition in bones and reducing calcium absorption from your digestive tract.
A proper balance of calcium in your blood and bones is important for many aspects of health. However, certain factors can disrupt this balance and leach calcium from your bones, leading to weak, brittle bones that are prone to fracture.
What is Bone Resorption?
Bone undergoes a continuous process of remodeling throughout life. Old, worn out bone is broken down and replaced with new bone. The scientific terms for bone breakdown and buildup are resorption and formation.
Osteoclasts are specialized cells that remove old bone in a process called bone resorption. Afterwards, osteoblasts move in and lay down new bone in a process called bone formation. Under normal conditions, the rates of bone resorption and formation are balanced so your bone mass remains stable.
However, when the rate of resorption exceeds the rate of formation, you develop a net loss of bone – which over time leads to weaker, more fragile bones. Some degree of bone loss is normal with aging. But excessive resorption can cause osteopenia (low bone mass) and osteoporosis (severe loss of bone density).
Factors that accelerate bone resorption leach calcium from bones by stimulating osteoclast activity. This tips the balance towards excessive bone breakdown.
How Does Bone Resorption Cause Calcium Loss?
Bones are made up of a matrix of collagen fibers reinforced with mineral salts – mostly calcium and phosphorus in the form of calcium-phosphate crystals called hydroxyapatite. These mineral salts give bones their hardness and strength.
During bone resorption, osteoclasts secrete acids and enzymes that dissolve the mineral and protein matrix of bone, releasing calcium and other minerals into the bloodstream. The rate at which osteoclasts resorb bone determines the amount of calcium leached from bones.
Excessive osteoclast activity and bone resorption can lead to:
- Decreased bone mineral density
- Weak, brittle bones prone to fracture
- Gradual bone loss over time (osteoporosis)
- Low blood calcium levels (hypocalcemia)
Maintaining healthy bones requires finding the right balance between bone resorption and formation over your lifetime.
Risk Factors for Excessive Bone Resorption
Many factors can disrupt the balance between bone resorption and formation, tipping the scales towards excessive osteoclast activity and calcium loss. The main risk factors include:
As you age, bone remodeling becomes imbalanced. Resorption outpaces formation, causing a gradual loss of bone mass. This leads to weaker, more porous bones and higher fracture risk. Women especially have accelerated bone loss in the first 5-10 years after menopause due to declining estrogen. Up to 20% of bone mass may be lost during this time.
Inadequate Calcium Intake
Getting insufficient calcium from your diet signals your body to draw more calcium from your bones, increasing resorption. The recommended dietary allowance (RDA) for calcium is:
- 1,000 mg per day for adults under 50
- 1,200 mg per day for women over 50
- 1,000-1,200 mg per day for men over 70
Low calcium intake is common – estimated at just 748 mg per day for women ages 20-39. Consuming less than 600 mg per day markedly increases bone resorption.
Vitamin D Deficiency
Vitamin D promotes calcium absorption in your gut and helps deposit calcium in bones. Low vitamin D triggers increased PTH, which accelerates bone resorption to release more calcium into the bloodstream.
Lack of Exercise
Weight-bearing exercise and muscle-strengthening exercise apply mechanical loading on bones that stimulates bone formation. Lack of exercise causes bone to become less dense over time. This effect is most pronounced during growth and adolescence.
Tobacco use is toxic to bone cells and can decrease calcium absorption. Postmenopausal women who smoke have lower bone density and a higher risk of fracture than nonsmokers. Smoking causes more rapid bone loss in aging adults.
Excessive Alcohol Intake
Heavy alcohol consumption reduces bone formation, and may also decrease calcium absorption and vitamin D levels. It can contribute to low bone mass and increased fracture risk. More than 2 drinks per day for men or 1 drink per day for women is considered excessive.
Glucocorticoids like prednisone are powerful anti-inflammatories, but long-term use raises fracture risk by suppressing bone formation. Other drugs that increase bone loss include aromatase inhibitors, anticonvulsants, proton pump inhibitors, SSRIs, thiazolidinediones, and GnRH agonists.
Many diseases increase osteoclast activity and bone loss, including hyperparathyroidism, hyperthyroidism, rheumatoid arthritis, diabetes, kidney disease, cancers, and genetic disorders.
Low Body Weight
Having a low body mass index is associated with increased risk of osteoporosis, especially in postmenopausal women. Estrogen levels are higher with greater body fat. Amenorrhea and low bone density are more common among underweight and anorexic women.
Foods and Nutrients that Promote Calcium Loss
Certain dietary factors can also tip the scales towards excessive bone resorption and calcium loss from your bones:
High Sodium Intake
Consuming excess sodium causes increased urinary calcium excretion. For every 2,300 mg of sodium (about 1 teaspoon of salt) excreted, you may lose 40-60 mg of calcium in urine. High sodium diets accelerate bone resorption markers and are linked to reduced bone density. The Tolerable Upper Limit for sodium is 2,300 mg per day.
High Caffeine Intake
Caffeine may modestly increase calcium excretion in urine. Older studies estimate that drinking 2-3 cups of coffee per day causes the loss of 3-5 mg of calcium. However, more recent research indicates caffeine’s effect on calcium excretion is small to negligible when calcium intake is adequate.
High Protein Diets
Eating excessive protein, especially from animal sources like meat and dairy, increases urinary calcium losses. However, the effect seems to be transient and normalize within a few months with calcium homeostasis. Moderately high protein intake (up to 20% of calories) doesn’t appear harmful to bone health.
High Phosphorus Intake
Phosphorus impairs calcium absorption when consumed in excess of calcium. A high phosphorus-to-calcium intake ratio correlates to lower bone density and higher fracture risk. Phosphorus additives in processed foods are a major concern.
Foods high in oxalates – like spinach, rhubarb, beet greens, okra, sweet potatoes – bind to calcium in your GI tract so less gets absorbed into your bloodstream. Over time, this induces a mildly negative calcium balance. However, oxalates only impair calcium status when calcium intake is low.
Phytic Acid in Grains and Legumes
Phytic acid in plant foods like whole grains, beans, seeds and nuts reduces calcium (and zinc and iron) absorption to a small degree. Food processing and leavening can lower phytate levels. Consuming vitamin C and fermented foods helps counteract phytic acid.
Diets providing inadequate intake of bone-supporting nutrients – like calcium, vitamin D, potassium, and vitamin K – promote an environment of poor bone health and increased resorption.
Tips to Minimize Bone Calcium Loss
Here are some tips to help maintain a positive calcium balance and strong, healthy bones:
- Meet recommended calcium intakes, particularly from dairy products & green leafy vegetables.
- Get sufficient vitamin D from sunlight, fatty fish, eggs and supplements.
- Reduce sodium intake and limit processed foods with added sodium.
- Engage in regular weight-bearing and muscle-strengthening exercise.
- Avoid smoking and heavy alcohol intake.
- Follow a balanced diet with adequate protein, fruits, vegetables and omega-3 fats.
- Maintain a healthy body weight.
- Limit caffeine to 400 mg per day at most.
- Consume high-oxalate foods with dairy for better absorption.
- Soak grains and legumes to reduce phytates.
- Take calcium supplements if needed (under medical supervision).
See your doctor if you have ongoing conditions or take medications that may cause excessive bone loss. Certain drugs can be adjusted or supplemented with calcium and vitamin D to prevent negative effects on your skeleton.
Aging, poor diet, inactivity, and some health conditions or medical treatments can disrupt the balance between bone breakdown and formation. These factors accelerate bone resorption by over-activating osteoclast cells in bone.
Excessive bone breakdown releases calcium from the bone mineral matrix into the bloodstream, leading to weak, porous bones prone to fracture. Getting adequate calcium, vitamin D and exercise can help tip the scales back towards healthy bone remodeling and prevent calcium loss from bones. Monitoring for risk factors and following prevention tips can help maintain strong bones over your lifetime.