4.6 Exercise, Nutrition, Hormones and Bone Tissue
Exercise and Bone Tissue
Lack of mechanical stress causes bones to lose mineral salts and collagen fibres, and thus strength. Similarly, mechanical stress stimulates the deposition of mineral salts and collagen fibres. The internal and external structure of a bone will change as stress increases or decreases so that the bone is an ideal size and weight for the amount of activity it endures.
For example
During long space missions, astronauts can lose approximately 1 to 2 percent of their bone mass per month. This loss of bone mass is thought to be caused by the lack of mechanical stress on astronauts’ bones due to the low gravitational forces in space.
People who exercise regularly have thicker bones than people who are more sedentary.
A broken bone in a cast atrophies while its contralateral mate maintains its concentration of mineral salts and collagen fibres. The bones undergo remodelling because of forces (or lack of forces) placed on them.
Nutrition and Bone Tissue
The vitamins and minerals contained in all the food we consume are important for all our organ systems. However, there are certain nutrients that affect bone health.
Calcium and Vitamin D
You already know that calcium is a critical component of bone, especially in the form of calcium phosphate and calcium carbonate. Since the body cannot make calcium, it must be obtained from the diet. However, calcium cannot be absorbed from the small intestine without vitamin D. Therefore, intake of vitamin D is also critical to bone health. In addition to vitamin D’s role in calcium absorption, it also plays a role, though not as clearly understood, in bone remodelling.
Milk and other dairy foods are not the only sources of calcium. This important nutrient is also found in green leafy vegetables, broccoli, and intact salmon and canned sardines with their soft bones. Nuts, beans, seeds, and shellfish provide calcium in smaller quantities.
Except for fatty fish like salmon and tuna, or fortified milk or cereal, vitamin D is not found naturally in many foods. The action of sunlight on the skin triggers the body to produce its own vitamin D, but many people, especially those of darker complexion and those living in northern latitudes where the sun’s rays are not as strong, are deficient in vitamin D. In cases of deficiency, a doctor can prescribe a vitamin D supplement.
Other Nutrients
Vitamin K also supports bone mineralisation and may have a synergistic role with vitamin D in the regulation of bone growth. Green leafy vegetables are a reliable source of vitamin K.
The minerals magnesium and fluoride may also play a role in supporting bone health. While magnesium is only found in trace amounts in the human body, more than 60 percent of it is in the skeleton, suggesting it plays a role in the structure of bone. Fluoride can displace the hydroxyl group in bone’s hydroxyapatite crystals and form fluorapatite. Like its effect on dental enamel, fluorapatite helps stabilise and strengthen bone mineral. Fluoride can also enter spaces within hydroxyapatite crystals, thus increasing their density.
Omega-3 fatty acids have long been known to reduce inflammation in various parts of the body. Inflammation can interfere with the function of osteoblasts, so consuming omega-3 fatty acids in the diet or in supplements may also help enhance production of new osseous tissue. Table 4.4 summarises the role of nutrients in bone health.
Table 4.4 Nutrients and bone health
| Nutrient | Role in bone health |
| Calcium | Needed to make calcium phosphate and calcium carbonate, which form the hydroxyapatite crystals that give bone its hardness |
| Vitamin D | Needed for calcium absorption |
| Vitamin K | Supports bone mineralisation; may have synergistic effects with vitamin D |
| Magnesium | Structural component of bone |
| Fluoride | Structural component of bone |
| Omega-3 fatty acids | Reduces inflammation that may interfere with osteoblast function |
Hormones and Bone Tissue
The endocrine system produces and secretes hormones, many of which interact with the skeletal system. These hormones are involved in controlling bone growth, maintaining bone once it is formed, and remodelling it.
Hormones that influence Osteoblasts and/or Maintain the Matrix
Several hormones are necessary for controlling bone growth and maintaining the bone matrix. The pituitary gland secretes growth hormone (GH), which, as its name implies, controls bone growth in several ways. It triggers chondrocyte proliferation in epiphyseal plates, resulting in the increasing length of long bones. GH also increases calcium retention, which enhances mineralisation, and stimulates osteoblastic activity, which improves bone density.
Thyroxine, a hormone secreted by the thyroid gland promotes osteoblastic activity and the synthesis of bone matrix. During puberty, the sex hormones (oestrogen in girls, testosterone in boys) also come into play. They too promote osteoblastic activity and production of bone matrix, and in addition, are responsible for the growth spurt that often occurs during adolescence. They also promote the conversion of the epiphyseal plate to the epiphyseal line (i.e., cartilage to its bony remnant), thus bringing an end to the longitudinal growth of bones. Additionally, calcitriol, the active form of vitamin D, is produced by the kidneys and stimulates the absorption of calcium and phosphate from the digestive tract.
Case study
A 12-year-old domestic shorthair cat, Misty, presented with reduced mobility and reluctance to jump. Diagnostic imaging revealed osteoporosis, characterised by decreased bone mass due to an imbalance between bone resorption and formation—common in ageing animals. Unlike Paget’s disease, which involves disorganised new bone formation and elevated alkaline phosphatase levels, Misty’s bloodwork was normal.
Osteoporosis is managed with nutritional support, vitamin D and calcium supplementation, and environmental modifications to reduce injury risk.
Tricolor domestic cat in Suomenlinna, Helsinki by Htm via Wikimedia Commons, CC BY SA 4.0
Hormones That Influence Osteoclasts
Bone modelling and remodelling require osteoclasts to resorb unneeded, damaged, or old bone, and osteoblasts to lay down new bone. Two hormones that affect the osteoclasts are parathyroid hormone (PTH) and calcitonin.
PTH stimulates osteoclast proliferation and activity. As a result, calcium is released from the bones into the circulation, thus increasing the calcium ion concentration in the blood. PTH also promotes the reabsorption of calcium by the kidney tubules, which can affect calcium homeostasis (see below).
The small intestine is also affected by PTH, albeit indirectly. Because another function of PTH is to stimulate the synthesis of vitamin D, and because vitamin D promotes intestinal absorption of calcium, PTH indirectly increases calcium uptake by the small intestine. Calcitonin, a hormone secreted by the thyroid gland, has some effects that counteract those of PTH. Calcitonin inhibits osteoclast activity and stimulates calcium uptake by the bones, thus reducing the concentration of calcium ions in the blood. As evidenced by their opposing functions in maintaining calcium homeostasis, PTH and calcitonin are generally not secreted at the same time. Table 4.5 summarises the hormones that influence the skeletal system.
Table 4.5 Hormones That Affect the Skeletal System
| Hormone | Role |
| Growth hormone | Increases length of long bones, enhances mineralisation, and improves bone density |
| Thyroxine | Stimulates bone growth and promotes synthesis of bone matrix |
| Sex hormones | Promote osteoblastic activity and production of bone matrix; responsible for adolescent growth spurt; promote conversion of epiphyseal plate to epiphyseal line |
| Calcitriol | Stimulates absorption of calcium and phosphate from digestive tract |
| Parathyroid hormone | Stimulates osteoclast proliferation and resorption of bone by osteoclasts; promotes reabsorption of calcium by kidney tubules; indirectly increases calcium absorption by small intestine |
| Calcitonin | Inhibits osteoclast activity and stimulates calcium uptake by bones |
Reflective question
How do mechanical stress, nutritional intake and hormonal regulation work together to maintain bone integrity in animals?
Section Review
Mechanical stress stimulates the deposition of mineral salts and collagen fibres within bones. Calcium, the predominant mineral in bone, cannot be absorbed from the small intestine if vitamin D is lacking. Vitamin K supports bone mineralisation and may have a synergistic role with vitamin D. Magnesium and fluoride, as structural elements, play a supporting role in bone health. Omega-3 fatty acids reduce inflammation and may promote production of new osseous tissue. Growth hormone increases the length of long bones, enhances mineralisation, and improves bone density. Thyroxine stimulates bone growth and promotes the synthesis of bone matrix. The sex hormones (oestrogen in women; testosterone in men) promote osteoblastic activity and the production of bone matrix, are responsible for the adolescent growth spurt, and promote closure of the epiphyseal plates.
Calcitriol stimulates the digestive tract to absorb calcium and phosphate. Parathyroid hormone (PTH) stimulates osteoclast proliferation and resorption of bone by osteoclasts. Vitamin D plays a synergistic role with PTH in stimulating the osteoclasts. Additional functions of PTH include promoting reabsorption of calcium by kidney tubules and indirectly increasing calcium absorption from the small intestine. Calcitonin inhibits osteoclast activity and stimulates calcium uptake by bones.
Review Questions
Critical Thinking Questions
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