All About Bones
by Kathleen Moltz, MD Pediatric/Adolescent/Young Adult Endocrinology, New England Diabetes and Endocrinology Center, Waltham, MA & Tufts University
Our bones, like our skin and hair, are parts of our body which continue to develop and change throughout our lives. There are a number of factors, including genetics, nutrition, disease, medication and activity which play a role in the overall health of bones. In this article I will review the development of bones, the role that nutrition and activity play, and the challenges in bone health for patients with inflammatory bowel diseases.
Bone Background
To begin, we are born with over 200 bones, all the bones we will ever have. Bone serves three essential functions: mechanical, protective and metabolic, Mechanically, bone provides physical support, allowing for locomotion and giving a form to the body. Bones protect both the organs inside the body (such as the heart, lungs and intestines) and the bone marrow inside of some bones. The metabolic aspects of bone include the storage of calcium, phosphate and other minerals, the regulation of blood levels of these minerals, and interaction with a number of hormonal systems including the parathyroid/calcitonin, vitamin D, growth hormone/IGF, estrogen, cortisol and thyroid hormone systems. I will review these in the following section. Structurally there are two types of bones: flat bones and long bones. Each type has its own strengths and weaknesses. Each develops differently, and each bone plays a role in the overall function. Flat bones “grow” outward from their edges; long bones primarily grow from the ends, at the area of the growth plate. Flat bones, such as the skull bones shoulder blades (scapula) and hip bones (ileum), develop without any cartilage and therefore are more stable. Long bones (like the femur and hand bones) first contain cartilage, which over many years obtain calcium. This process is called calcification. Calcification maximizes in the twenties, after which bone calcium, commonly referred to as “bone mineral density, ”remains relatively steady until later in life. Our society has recently recognized the importance of adult onset osteoporosis, more common in women but also a problem for older men. The health of bone in children and teenagers has become more of a concern lately because research suggests that the maximum bone density reached has an impact on future bone health
Bone is made of three types of cells: osteoblats, osteocytes and osteoclasts. Osteoblasts are the primary bone forming cells, osteocytes are the mature bonecells involved in slower bone formation and bone turnover, and osteoclasts are the bone resorption cells. Bone is in a constant state of turnover, constantly responding to the physical stresses to which the body is exposed. Bone turn-over is important for maintaining bone strength, and the balance between bone formation and bone resorption is critical. If bone resorption exceeds bone formation, bone density decreases and bone weakens and breaks. If bone formation surpasses bone resorption, bone becomes disorganized and may cause deformity or pain. All in all, the balance is fairly stable, but severe imbalances do occur.
Bones and Hormones
As mentioned above, a number of hormones are involved in bone function (see Table 1). Five of these (PTH, Calcitonin, Vitamin D, GH and E) are involved in normal bone physiology). The other two (T4 and cortisol) only have importance when present in excessive amounts.
Genetics and Bone
Several genetic factors influence bone health and strength. Ethnic populations have different “standards” for normal, but generally Caucasians and Asians have lower bone density than Africans and African-Americans. Smaller adults have lower bone density than larger adults. There are some specific genes which play a role in determining bone health (the vitamin D receptor for example) but researchers have not found a way to make this genetic information useful in clinical practice.
Nutrition, Activity, Lifestyle and Bone
Numerous nutritional issues impact bone health. The most important issue is calcium intake. Calcium is necessary for bone growth and strength. Calcium balance is maintained in the body by a number of hormones, but one requirement is that enough new calcium enters the body from food. There are a number of foods rich in calcium (see Table 2). The amount of calcium needed changes throughout life. Teenagers and pregnant or nursing women need the most calcium, however, infants and children need a lot compared to their size. Recommendations on intake are listed in Table 3, and are based upon elemental calcium in food or supplements.
Activity and lifestyle also influence bone density. Exercise helps to build and maintain bone density, although excessive exercise (which in teen or adult women leads to absent menstrual periods) can actually reduce bone density. Low gravity (like in space) reduces bone density. Swimming, which supports the body weight, is very good for overall health, but does little for bone strength. Lifestyles which include smoking, excess alcohol and some high protein diets also reduce bone density. The best advice is a good, steady exercise program involving a variety of fun activities.
Bones and Bowel: How does my (my child’s) disease affect bone?
People with bowel disease often have problems with their bones. This is not surprising, given that the intestines are the major route of nutrition absorption. Calcium and vitamin D are important for bone health, and are absorbed in the intestines. Calcium absorption occurs primarily in the duodenum and jejunum (the first and second segments of the small intestine).
Vitamin D is absorbed in the ileum (last segment of the small intestine). Vitamin D levels in the body are very important for calcium absorption. If there is a deficiency of vitamin D, less calcium will be absorbed from the intestines. Very high levels of vitamin D can be troublesome, but this is very rare.
Magnesium and phosphate are also important for bone health. Magnesium absorption is a problem only in diffuse intestinal disease, or in combination with certain laxatives, and is not affected by vitamin D. Phosphate absorption is primarily regulated by the amount in the diet, but is also affected by vitamin D.
Increased stool losses of these nutrients can occur whenever the food “transit” through the intestines is very rapid. Increased stool losses means less mineral or vitamin absorbed, leading to the same problems.
Some of the medications used to treat inflammatory bowel disease complicate the problem of bone health. The largest problem occurs from one of the best medications, corticosteroids. Corticosteroids are anti-inflammatory medications used to reduce tissue inflammation. Examples include prednisone, prednisolone, dexamethasone, and intravenous solumedrol. Corticosteroids have four unfortunate effects on calcium and the bone [see also Table 1, cortisol, which is the natural corticosteroid made by the body.] Corticosteroids can: 1) block intestinal absorption of calcium, 2) increase PTH secretion, 3) increase loss of calcium in the urine and 4) directly block bone formation. All of these effects reduce bone density. The fastest loss of bone density actually occurs in the first six months on corticosteroids. The bone loss is cumulative, and most people who have been on corticosteroids for an extended period of time have reduced bone density. Some will develop problems with fractures. Of greater concern in children is the possibility that a child will never reach his “normal” maximum bone density, and will therefore be at increased risk for bone problems as an adult.
What people with bowel disease can do to help maintain good bone health.
Unfortunately, there are no specific treatments which are proven to be useful for the prevention or treatment of bone disease caused by bowel problems or the medications taken to treat bowel problems. Current recommendations include four areas: calcium, vitamin D, activity, and lifestyle.
CALCIUM: Everyone with bowel disease should be certain to eat enough foods with calcium, up to the daily requirement. There is no evidence that taking in “twice” or “three times” as much calcium is helpful. If your diet is not providing sufficient calcium, a supplement is helpful. Try to divide your calcium supplement into two or three doses a day, because too much calcium at one time will not be absorbed. Calcium comes in many forms, tablets and liquids. Avoid oyster shell sources, as there have been reports of contamination with lead! Also, read the label carefully: the daily recommendations are based upon milligrams (mg) of elemental calcium daily, not calcium carbonate or calcium citrate [See Figure 1.] Sources of calcium carbonate (like Tums®) should be taken with food, as stomach acid is needed to break down the carbonate. Other forms of calcium (like calcium citrate) can be taken at any point in the day. Table 4 gives some common sources of supplements, but is not all that are available. Check with your doctor if you have a question.
VITAMIN D: Vitamin D is also very important, and unfortunately is available only from a couple of sources “naturally”. The body can make some vitamin D with exposure of the skin to sunlight (but be careful about too much exposure, sunburn, etc.!) Vitamin D is also found in fortified milk, and in most of the “food supplements” like Vivanex and Pediasure. Children with bowel disease or any child on long term corticosteroids need 400 IU vitamin D daily. Older adolescents and adults need 800 IU daily. Some calcium supplements also include vitamin D.
ACTIVITY: One of the best things to do is to continue a good exercise program. This does not need to be vigorous, only something where gravity is involved [not swimming]. Walking, playing outside, team sports, and bicycle riding should all be encouraged, to the extent that the child/adult feels able. Flexibility exercises, such as yoga and tai-chi are also very good for reducing the rate of injury. And exercise is good for the spirit, and the weight!
LIFESTYLE: Obvious exclusions include smoking and alcohol. While no studies have been done to examine the effect of second-hand smoke, my feeling is that exposure should be limited wherever possible. Another suggestion is the avoidance of “cola’”, like Pepsi®, Coke®, and other phosphoric acid containing soda. Some studies have shown lower bone density in adults who have increased consumption of these sodas. Excessive caffeine avoidance is also reasonable, but an occasional cola or cup of tea/coffee is OK.
There is some research ongoing into treatment of corticosteroid bone problems with hormones or other medications (currently used for adult onset osteoporosis). Results are promising, however, no trials in children have yet been attempted.