2 major minerals in the body
calcium and phosphorous
RDA of calcium for ages 19-50?
1000 mg
What is the RDA of calcium based off of?
bone metabolism
RDA of calcium for ages 51+ ?
1200 mg
Best source of calcium?
dairy products (almond milk comparable as well)
Absorption efficiency of calcium
30%
Average consumption of calcium
600 mg
3 main absorptive processes of calcium
- calbindin system
- paracellular pathway
- endosomal/lysosomal pathway
Outside of the 3 main absorptive processes, calcium can also be absorbed via...
the large intestine
- bacterial fermentation
- only 4-10% of dietary calcium absorbed this way
The calbindin absorptive system is regulated by ________. What does it do?
calcitriol
- increases calbindin levels and increase levels of calcium pump
Describe the calbindin system
- Ca enters through Ca channel of BBM
- Calbindin binds and shuttles Ca to basolateral membrane where there is a Calcium pump
- Calcium pump is energy dependent and actively pumps Ca through to the portal vein
- saturable (120 mg), at which point paracell
Describe the paracellular pathway
- diffusion-dependent and nonsaturable
- takes over once calbindin saturated by 120 mg+
- transports between enterocytes
- not well understood
Describe the endosomal/lysosomal pathway
- calcium enters via endocytosis
- endosome fuses with lysosome to form endosomal-lysosomal component
- transport via lysosomes to basolateral membrane
- exits via exocytosis
least active mechanism
Most effective/common supplemental form of calcium
calcium citrate
what 3 things improve calcium absorption?
sugars, protein, vitamin D
How much calcium does it take to saturate the calbindin system? What happens?
120 mg. Paracellular pathway becomes more significant
What is the rate limiting step of the calbindin system
calbindin d-mediated component
Describe the paracellular pathway
- goes through gaps of enterocytes
- becomes more significant after 120 mg saturates calbindin system
what does percent of calcium absorbed depend on?
source and other factors
What are the different forms of supplemental calcium and their rates of absorption?
- Calcium carbonate = 39%
- Calcium citrate = 30% (most common)
- Calcium gluconate = 27%
Calcium carbonate is known for
being contaminated with lead and poor bioavailability
What can increase calcium excretion?
sodium, protein, caffeine
What substances can increase calcium absorption?
vitamin D, sugars, protein
Nutrients whose absorption may be inhibited by excess calcium intake
Iron and FA's
Nutrients/substances that inhibit calcium absorption?
- fiber
- phytate
- oxalate
- unabsorbed fatty acids
5 functions of calcium
- nerve function
- blood clotting
- muscle contraction
- cellular metabolism (insulin secretion)
- bone metabolism
Most abundant protein in bone
collagen
Transport of calcium in blood occurs in 3 ways
- 40% bound to proteins (albumin and prealbumin)
- 50% free (ionized)
- 10% complexed with sulfate, phosphate, or citrate
Measuring....tends to be the best measurement of true calcium status
Measuring free (ionized) calcium in the blood tends to provide true/best calcium status
How is calcium distributed (concentrated) throughout the body?
1% = in blood
99% = in bone and teeth
60-66% of bone weight is from
minerals such as calcium, phosphorous, and others
30-34% of bone weight is from
water and organic molecules like proteins
- collagen, osteocalcin, matrix Gla protein
3 forms of proteins found in bone
- collagen, osteocalcin, matrix Gla protein
2 functions of bone
- serve as calcium bank
- lends strength and rigidity to body frame
Calcium found in bone is in the form of
Hydroxyapatite
- Ca10(PO4)6(OH)2
2 main types of bone
- Cortical = dense outer shell (75% of bone)
- Trabecular = spongy inner network (25% of bone)
Which form of bone tends to degrade quicker?
Trabecular because there is less of it, making it metabolically susceptible
When does bone density reach its maximum?
in late 20's, good to maximize while we still can before decline
Describe the dynamicness of bone material
- undergoing constant turnover
- growth, breakdown, regrowth
3 main types of bone cells
- osteoblasts
- osteoclasts
- osteocytes
osteoblasts
bone-building cells
osteocytes
osteoblasts incorporated into bone matrix
osteoclasts
bone breakdown cells
How do osteoblasts facilitate bone formation?
- synthesize collagen and osteocalcin
- facilitate mineralization of bone matrix
Osteoblasts have what 3 types of receptors? Functions?
- D3 = promotes osteocalcin production
- Estrogen = stimulates synthesis of bone proteins
- PTH = stimulates osteoblasts to signal osteoclast precursor cells
Osteoclasts promote
Bone degradation via enzymes and acid secretion
Osteoclasts have what type of receptors?
Calcitonin receptors
How is osteoclast activity effected by calcitonin concentration?
As calcium concentration increases, calcitonin increases, and osteoclast activity decreases.
What is osteopenia?
too little bone mass at any stage in life
What is osteoporosis?
loss of bone tissue to extent that skeletal site unable to withstand ordinary strains
Most common type of bone fracture?
Vertebral. most go unreported
How does parathyroid hormone effect bone formation?
stimulate osteoblasts to signal osteocyte precursor cells
How does estrogen hormone effect bone formation?
stimulates synthesis of bone proteins
How prevalent is osteoporosis?
Major public threat to 55% of 50+ year old population
4 risk factors for osteoporosis
-
Age
- low calcium intake
- inactivity
- female (less dense bones than men)
Why do females tend to have higher risk of osteoporosis?
Menopause causes a decrease in estrogen, and therefore decreased bone protein synthesis
At what age is bone loss higher than bone growth?
30+
What type of exercise increases bone density?
weight-bearing exercise
- walking, running
What should be used to assess calcium status?
bone mass density using DXA
- cant test blood levels cause constantly fluctuates
Describe how low blood calcium is regulated
- blood calcium drops, caused PTH secretion from parathyroid gland
- PTH signals to kidney to synthesize calcitriol
- calcitriol increases bone reabsorption in kidney, bone, and intestine
Calcium UL
2500 mg
Where and when is calcitonin secreted?
Secreted from thyroid gland when blood calcium levels are high or adequate
3 functions of calcitonin
- stimulates osteoblasts bone-building activity
- inhibits activity of degenerative osteoclasts
- inhibits synthesis of calcitriol
Calcium also functions as...
a second messenger, and activates proteins
3 routes of calcium excretion and how much lost?
- urinary: 100-240 mg
- fecal: 45-100 mg
- sweat: 60 mg
Excessive calcium deficiency can lead to what 5 conditions?
- osteoporosis
- rickets
- hypertension
- colon cancer
- obesity
Describe how high blood calcium or adequate levels are regulated
- calcitonin synthesized by thyroid gland
- calcitonin stimulates bone-building osteoblasts
- inhibits osteoclast activity and calcitriol synthesis