Calcium: No bones about it - part 2
Matthew Barreau continues his review of calcium and the potential benefits of calcium supplementation.
Elderly individuals are usually the focus of calcium supplementation because they are the ones who suffer more bone injuries. However, it is the bone mass built up in the pre-pubertal years, and it is during this time that proper calcium intake will be more beneficial in preventing osteoporosis. Calcium intake, vitamin D, nutrition, and exercise are important modifiable factors that optimize bone accretion during adolescence. In females, less than 90% of the total body bone mass is achieved by age 16.9, 95% by age 19.8, and 99% by age 26.2 giving even more credence to the adage that an ounce of prevention is worth a pound of cure.
Aside from getting adequate calcium to ensure bone health, activity levels have also been linked. Many studies have shown that women are more at risk than men, due to their decreased activity levels as a youth. A study of 800 7 to 9-year-olds in the British Journal of Sports Medicine shows that girls are almost half as physically active as boys when it comes to regular vigorous exercise.
Another study showed that boys were 25% more active than girls. The same study showed that boys were found to have a 25% higher calcium intake than girls, though they only just met the national guidelines for 8-year olds while falling short of the guidelines for 10-year-olds. The boys' bone density was 12% higher than the girls. Body size, sex, and calcium intake were all taken into consideration when giving the results of the study. One interesting point made was that children from more disadvantaged backgrounds expended significantly more energy than children from more privileged backgrounds. This puts them at a bone-density advantage, provided their calcium intake levels are adequate. As stated elsewhere in this paper, 35 minutes of walking can mean the difference between low and optimum levels of bone density. This is typically the equivalent of walking to and from school or replacing one television show with outside activities.
There is also considerable evidence that children's bone health can be attributed to the habits they see their parents undertake. Mothers' weight concerns and self-esteem issues got passed along to their daughters, even at the age of five. A survey about milk drinking habits indicates that the amount of milk a mother drinks strongly influences the amount her child drinks.
It has been preached since the beginning of time that exercising is healthy for the heart, but not many know that it is very healthy for the bones as well. Physical activity, specifically weight-bearing or repetitive exercise, is associated with higher bone mass or bone mineral density[13,29,35]. Physical activity itself will not promote bone health, however. Many lifestyle factors play a role; most importantly, it is still necessary to take adequate levels of calcium (>1g/day). Exercise, along with adequate calcium levels, was more beneficial than exercise alone at the femoral neck. Not only will physical activity be beneficial, but physical inactivity can increase calcium losses from the body.
Most recommendations related to the amount of exercise needed to maintain bone health or minimize losses are very similar to that prescribed for general health. One study says that one hour, three times a week is adequate, while another suggests that 20 to 25 minutes of resistance training accompanied by 7 to 10 minutes of cardiovascular weight-bearing activity is a good amount. Still, another noted that walking 45 to 60 minutes three times a week was associated with a reduction in bone loss in the lumbar spine and that the brisker the pace, the greater the protective effect. The amount of exercise that differentiates between those with low bone density and those with optimum bone density is as little as 35 minutes of walking or 25 minutes of energetic exercising spread throughout the day.
There are a couple of things to note when trying to determine the amount of exercise one should undertake. First is that the number of hours of the weekly exercise was found to be directly correlated with the total body, lumbar, and hip bone mineral density. But while this suggests non-stop exercising, there is a limit. The most important limit is the amount that will not make anybody quit their regimen of exercise because it is pointed out that the benefits of the exercise program only lasts as long as the program continues.
Several studies back the importance of exercise, along with adequate calcium intake, to promote bone health.
The US National Institute of Arthritis and Musculoskeletal and Skin Diseases of the National Institute of Health found that weight-bearing resistance exercises combined with calcium citrate supplementation over one year provided a significant improvement in bone mineral density of postmenopausal women at the hip and spine. This was found regardless of whether the women were on HRT or not.
In women who have recently become postmenopausal, and who are in good health and are felt to have a fairly low fracture risk, optimizing the daily calcium intake and faithfully adhering to a program of athletic activities or workouts can help to maintain a satisfactory bone mass.
Weight-bearing exercise is an important contributor to the establishment of peak bone mass and may have the greatest impact on bone mineral density when initiated before the completion of puberty. A retrospective study of young adult women noted that those who participated in high school sports exhibited a 7% increase in femoral neck bone mineral density. Interestingly, sports participation before the onset of menses appears to increase bone mineral density significantly more than when started after the initiation of menses.
A study of Canadian children showed that physically active girls had a 17% higher total body bone mineral density compared with that of their sedentary peers. If excessive weight-bearing exercise leads to estrogen deficiency and amenorrhea, however, the benefits of activity are lost, and the activity may be detrimental to bone.
The strongest evidence that physical activity plays an important role in bone mineral accrual in growing children comes from unilateral control studies in which the physically active or dominant limb has a higher bone mineral density or bone mineral content than the opposite limb in the same individual.
A retrospective study showed that participation in high school sports could predict the bone mineral density of the hip in 204 young women aged 18 to 31 years old. However, the spine, radius, and total body bone mineral density could still be influenced by current energy expenditure. Intervention studies showed only small changes in bone with resistance training. In pre-menopausal women, an eight-month weight training or jogging program resulted in a significant increase in lumbar spine BMD. Both weight training and jogging resulted in similar increases (1.2% compared with 1.3%, respectively) in bone mineral density, whereas a slight decrease was observed in the control group.
In a study of three age groups of women (25 to 30, 40 to 45, 60 to 65), both high physical activity and high calcium intake were associated with a higher total body bone mineral content and larger femoral and radial shafts.
The interaction between physical activity and calcium intake is less conclusive in growing children. One study of 1359 Dutch boys and girls 7 to 11 years old reported a positive effect of high physical activity but no association between calcium intake and bone mineral content. Another study hypothesized that a calcium intake of up to 1g/d is helpful but that higher intakes afford no additional advantage in improving bone mineral density.
Physical activity was reported to have a greater role in affecting bone mineral density before puberty. For example, the benefit of playing tennis or squash to bone mineral content was twice as great if females started playing at or before compared with the onset of menarche. A 10-year-old female gymnasts total body bone mineral density was 5% higher than that in control subjects and annual gains were 2% greater. In a three-year prospective study of 90 white children age 6 to 14 years of age, pre-pubertal children in the highest quartile of weight-bearing physical activity had 4 to 7% greater rates of bone mineralization than did children in the lowest quartile of activity. In a 15-year prospective study of 84 males and 98 females studied from ages 13 to 28, physical activity was the strongest predictor of spine bone mineral density in men at age 28. Weight was the strongest predictor in females. Similarly, in 581 children aged 7 to 9, physical activity, but not calcium intake, correlated significantly with radius bone mineral density measured 14 years later in the same subjects. In 53 girls and 50 adolescent Canadian boys, active boys and girls had 9% and 17%, respectively greater total body bone mineral content compared with that in sedentary peers one year after achieving peak bone mineral content velocity.
There is still some debate regarding whether there is an increased need for athletes when it comes to calcium. Conventional wisdom would point to additional requirements for athletes due to the continual stresses their bones have to deal with, especially with high-impact sports like running and gymnastics. In the case of women athletes, it is even more of a question of whether the RDA's are adequate to meet the needs of women athletes.
The increased load in athletes causes an increase in micro-fractures to the bone that must be replaced or repaired[13,22]. From training, the damaged cells are decomposed and calcium is deported by the osteoclasts. These micro-fractures show up quite often in the form of shin splints or, even worse, stress fractures. Too much stress on weakened bones, caused by inadequate calcium intake, can make an athlete more susceptible to stress fractures. However, the increase in training athletes undergoes increases their ability to make stronger bones, specifically by depositing more calcium and becoming increasingly dense in the areas of stress.
Osteoblasts are required to make more bone-matrix to meet the increased demands on the bone. However, there is a limit to which the body can make new bone, and if it does not keep up with the breaking down of old bone, problems can occur; bone mass declines, bone density thins out, and there is an increased risk of a bone-related injury. Diets lacking in calcium and Vitamin D (to help calcium absorption) accentuates this problem, especially due to the need for calcium to regulate vital functions.
At the same time that taking too little calcium is a major concern of athletes, at least one article made a point that taking too much calcium can be equally damaging to bones. When calcium is absorbed in the gut, it then enters the bloodstream for transportation around the body. After the vital functions have their required levels of calcium, the remaining amounts need to be transported to the bones for temporary storage before excretion. This is because excessive calcium in the blood can block respiration by blocking muscle functioning. Regulating blood calcium level has top priority.
The problem with this is that to temporarily store the extra calcium, osteoblasts must be activated for this purpose, meaning they cannot perform their role of repairing damaged bone areas. Redundant calcium - that which is stored only temporarily before excretion, and solely to prevent elevated blood calcium levels - inhibits bone reformation and the repairing of microfractures is incomplete.
An additional concern to the female athlete is the reduction in the production of estrogen caused by intense physical exercise. Because estrogen inhibits both the uptake of calcium into the bones deportation of calcium, an increase in exercise for women can cause an even more devastating effect if excessive calcium is consumed.
Because of this, it is recommended that athletes make no special increases in their calcium intakes. Only that which is sufficient to maintain bone health is recommended. In addition to the total daily intake, it is important to watch the amount of calcium consumed at any one sitting, whether it is from a meal or supplement. This may be particularly true at dinner because bone absorption is greater at night than during the day. This would suggest that night would also be the time at which osteoblasts are most needed for bone repair and should not be preoccupied with storing redundant calcium. The more calcium consumption is spread throughout the day, the less redundant calcium will need to be temporarily absorbed in the bones.
Just how much calcium is required? As can be assumed, there is no clear-cut magic dosage that will solve all the problems. What is known, however, is that Americans are not consuming enough, that everybody should consider increasing their calcium intake, and that doing so daily, will provide the greatest benefit[5,6,10].
Instead of what has been written earlier regarding taking too much calcium, it is very difficult to consume too much. The Handbook of Non-prescription Drugs notes that daily consumption of as much as 8000mg of elemental calcium daily causes few side effects in healthy people[10,11]. The most recent Institute of Medicine report sets a "tolerable upper intake level" for calcium at 2500mg per day. The 1994 National Institutes of Health Consensus Conference on Optimal Calcium Intake noted that levels up to 2000mg per day should be safe and that calcium supplementation was desirable for people unable to increase their daily intake through diet alone. If taken excessively for short periods, harm is unlikely to occur. However, long-term overdosages can produce adverse effects.
Currently, the RDA is set at 1000mg, and any percentage seen on packages containing calcium should be calculated accordingly. For example, for a glass of milk containing 30% calcium per serving, the corresponding amount would be 300mg.
Certain groups of people need greater calcium intakes than others. These may include pregnant or lactating women, postmenopausal or amenorrhoeic women, athletes, and children or other developing individuals. 1000mg should be seen as a base from which to develop additional calcium needs. The greatest amount of calcium that was found to be safe for all groups was around 2000mg.
Calcium is best absorbed when taken in doses of 500mg or less. Smaller percentages of calcium are absorbed when taken at higher doses. The typical amount consumed during any meal in a Western diet (200 to 400mg) is within this recommended amount. Because of this, it is essential to spread calcium intake throughout the day, preferably in four or five doses of up to 500mg.
When to take calcium supplements
Take the calcium supplement with meals. For example, calcium carbonate is most effective with meals, and studies have shown that it may be better absorbed with food. Take your calcium supplement in divided doses throughout the day. The body can absorb only so much calcium at one time, so try taking a supplement with two or three meals each day.
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About the Author
Matthew Barreau is assistant Cross Country and track coach in charge of distances at Portland State University, a USATF Level II certified Endurance Coach and a USATF Level II certified Sprints/Hurdles/Relays Coach.