You have probably been told to drink your milk because it will build your bones. The main nutrient in milk and dairy products that is responsible for bone building is calcium. Most of the calcium (99%) in our body can be found in our bone. The other 1% is in our blood. Calcium levels must be carefully regulated in the body since it is responsible for several essential functions including:
- Bone building
- Muscle contraction
- Maintaining blood pressure
- Blood clotting
- Hormone and enzyme regulation
How Calcium Is Regulated?
When blood calcium is low, the parathyroid gland secretes parathyroid hormone (PTH) which accomplishes the following:
- Increase osteoclast activity, hence breaking down more Ca for release into blood
- Increases resorption of Ca by the kidneys
- Activating Vitamin D which acts not just as a nutrient but as a hormone, to stimulate increase Ca absorption in the intestine. Vitamin D also increases both Ca release from the bone and Ca resorption from the kidneys
When blood calcium is high, the thyroid produces calcitonin which inhibits the production of PTH. Inhibition of PTH accomplishes the opposite of the above i.e.
- Decrease osteoclast activity, hence decreasing Ca release into blood
- Decrease resorption of Ca by the kidneys
- Inhibiting Vitamin D which reduces Ca absorption in the intestine
We do not absorb all the calcium that we consume. The amount we absorb is dependent on Ca bioavailability which is affected by a number of factors:
- Our need for calcium: If our body needs less, then less will be absorbed
- Amount in our food: Absorption drops if calcium concentration is high (the maximum calcium that can be absorbed at any one time is 500 mg). Conversely, when the dietary concentration is low, bioavailability goes up
- Age: Ca absorption declines with age e.g. 60% absorption for infants, children and adults, and about 30% for young adults
- Presence of antinutrients e.g. phytates and oxalates in our food
- Consuming calcium with certain minerals like iron, zinc, manganese and phosphorus may interfere with absorption
Like calcium, vitamin D is essential for bone building. We are able to make vitamin D using sunlight. A form of cholesterol called 7-dehydrocholesterol in our skin is converted to cholecalciferol on exposure to sunlight. Cholecalciferol is converted to Calcidiol in the liver and then to the active Vitamin D form, calcitriol in the kidneys.
People living in tropical regions closer to the equator can make enough vitamin D to supply their needs as long as they spend enough time outdoors. However populations 37 degrees latitude north or south of the equator do not get enough radiation from the sun during winter times to allow them to make enough Vitamin D. Therefore, they must get the balance from food. Few foods have vitamin A in its active form apart, from fatty fish that Americans do not eat enough of. Therefore the majority of vitamin D that you will get from food comes from those that have been fortified with vitamin D either as vitamin D2 or Vitamin D3 which can be converted to active vitamin D in the body. Vitamin D2 is ergocalciferol, originating from plants and fungi. Vitamin D3 is cholecalciferol, which comes from animal sources.
Factors Affecting Vitamin D Synthesis
- Time spend outdoors in the sun: More sun exposure = more vitamin D production
- Time of day: 10 am – 3 pm is the best time since this is when sun rays are the strongest
- Skin pigmentation: Darker skin blocks more sunlight, making it more difficult to make vitamin D
- Age: People 65 and over have less capacity to synthesis vitamin D
- Obesity: Decreases the body’s capacity to synthesize vitamin D
We learned in the previous lesson that vitamin D is made in the body. Another vitamin that is made in the body and supports bone health is vitamin K. Vitamin K can be made by bacteria in the gut although it can also be obtained from food. Vitamin K made in the body by bacteria is called menaquinone or K2, while vitamin K in plants is called phylloquinone or K1. Vitamin K supports bone health by serving as a co-enzyme to produce certain proteins that supports bone health. These are namely osteocalcin and matrix gla protein (MGP). These are involved in the bone building work of the osteoblasts. Vitamin K deficiency may play a role is osteoporosis although more research is being done in this area. For example, low vitamin K intake in women has been associated with increased risk of hip fractures. Toxicity of vitamin K is not known. When trying to meet your vitamin K needs, think green fruits and vegetables.
Phosphorus is the second most abundant mineral in our body; second only to calcium. Up to 85% of the phosphorus in our body is stored in our bone where it is combined with calcium as hydroxyapatite. It is also an integral part of our cell membrane where it forms part of the phospholipid bilayer. You also saw in earlier lessons that phosphorus is a part of ATP which is the body’s form of energy. It is also part of DNA and RNA. Sources for phosphorus include meat and meat products like milk and other dairy foods, as well as plant sources e.g. peanuts, peas and beans. If you drink carbonated beverages that’s another source. Phosphoric acid is put in these beverages to modify acidity and hence improve taste. Drinking soft drinks however is not the best way to get your phosphorus, as carbonated beverages have been associated with poor bone health including more frequent bone fractures. High phosphorus intake coupled with low calcium intake leads to loss of calcium from the bone and hence decreased bone mass and increased risk of osteoporosis (porous bone). Too much phosphorus is also associated with calcification of tissue in the body. Deficiency (rare) causes muscle weakness, joint pain and rickets (softening and weakening of bone in children).
Along with calcium and phosphorus, magnesium can also be found in the bone matrix. Half of the magnesium in our body is in our bones, while the rest is contained in our cells (49%) and blood (1%). In our cells, magnesium function as cofactors for more than 300 enzymes including enzymes involved in metabolism, protein synthesis, muscle, nerve, heart and bone cell (osteoclasts and osteoblasts) function. Magnesium is abundant in green fruits and vegetables since magnesium is a component of chlorophyll. It is also abundant in the bran layer of grains, so eating whole grain will increase your magnesium intake. We usually do not get too much magnesium from eating regular food but megadose in supplement form can cause diarrhea. Deficiency is associated with muscle weakness, seizures, depression, irregular heartbeat, and increased risk of osteoporosis.
Ninety nine percent of the fluoride stored in the body is found in the bone and teeth. Fluoride combines with calcium and phosphorus to form fluorohydroxyapatite, (Ca10[PO4]8[OH]2 or fluoroapatite, (Ca10[PO4]6F2 which is stronger and more resistant to demineralization than hydroxyapatite. In teeth, fluorohydroxyapatite reduce activity of bacteria responsible for dental carries. Too little fluoride can cause dental carries but too much can cause pitting in the teeth; a condition called fluorosis. Fluorosis in the bone may also occur, leading to osteoporosis, stiffness or pain in joints, and calcification of ligaments. The best source of fluoride is fluoridated water and dental products. Municipal water in many parts of the US is treated with fluorine, so drinking tap water and cooking with it helps in meeting your fluoride needs. Do you want to know if your water is fluoridated? Contact your local water utility provider. If your water is not fluoridated, you can purchase bottled fluoridated water from the grocery store.
Reference: Thompson,& J., Manore, M., Vaughan, L. (2020). The science of nutrition (5th ed.). New York. Pearson