Big bad bones
The news recently reported on a study that claimed to dispel one of the most established “obesity paradoxes.” Its conclusion — “obesity is not good for bone health” — went unquestioned by the media. But it should have been.
First, the news:
Obesity bad for the bones: study
New
The investigators say when the mechanical loading effect of body weight on bone mass was adjusted for, fat mass was negatively associated with bone mass. This means in general, the greater the fat mass, the lower the bone mass. The research team says the results of the study reaffirm the beneficial effects of appropriate weight-bearing and mechanical loading on a healthy skeletal system.
“Our study found that increasing body fat mass decreases bone mass, for people of similar weight,” Dr. Deng told Reuters Health. ...
But not a single media story reported what should have been the rest of that sentence to give a more accurate picture. The researchers had compared thin Asians who were in their 20s to heavier Americans in their 60s! The key point to remember with any association with lower bone densities is — bone density decreases with age. Yet there’s still more to this story, as we’ll see...
This study provides an ideal illustration of one of the most common ways we can be misled in population studies looking for a correlation between lifestyle, physical or dietary characteristic and ill-health. You’ll often see it employed by those trying to make us believe that a certain diet — usually one of all-natural, wholefood, vegetarian, “good” fats, traditional fare — eaten by primitive cultures or in underdeveloped regions of the world is healthier, claiming as proof that those people had/have lower rates of obesity, heart disease, or chronic diseases. They often fail to account for age and reveal that life expectancies among those peoples are considerably less than we enjoy today in developed countries — they typically don’t live long enough to get the chronic diseases of advanced aging. As we’ve seen, incidences for chronic diseases spike noticeably with age. We have to be especially alert to confounding factors, such as socioeconomic and genetic factors, whenever an epidemiological data dredge is comparing different cultures or very different groups of people. The ability of a computer model to accurately account and adjust for confounding factors when groups are so dramatically different gets shaky. In this study, the researchers were looking at osteoporosis and correlations with body composition. They took measures of body weight, fat/lean mass and bone mineral density (BMD) that had been done on 1,988 Chinese adults and compared them with those on 4,489 Caucasians in the United States. BMD was measured at the lumbar spine and femoral neck using Hologic DXA scanners. The Caucasians were different genetically and nearly 40 years older than the Chinese cohort, and weight gain is a natural part of aging, making them taller and heavier. The Caucasian women and men weighed on average 50.38 pounds and 77.18 pounds more, respectively. Their most significant finding was that BMI and body weight were positively related to bone density among both cohorts. The higher the BMI, the greater the bone mass. This unreported finding is consistent with the entire body of evidence. The importance of heavier size in bone density proved so significant even in this study that, overall, the 60+ year-old Caucasians had comparable BMDs to the 20+ year-old Asians! As most people know, bone density naturally drops with aging. Women lose more than men and over their lifetimes, women lose about 30-50% of the bone mass they had at age 20. In 1994, the World Health Organization defined osteoporosis as a disease by using the average BMDs of 25 year old, white females. When values for older people reach 2.5 standard deviations below those of a 25 year old, they are considered “osteoporotic.” This pretty much ensures, like so many other ideal health indices based on young persons, we have another way to see aging as a disease. But fat people have long been shown to have less osteoporosis and better bone densities than thinner people. It’s been suggested to be due to better nutritional status, a role of estrogen and the greater weight-bearing mechanical stress on bones (as is also found with weight-bearing exercise). In fact, in other studies, BMI was the “best independent predictor of bone density,” with BMIs over 30 and 35 seeing an 80% and 90% reduction in osteoporosis, respectively. A small preliminary study in OB/GYN News by Dr. Coleman of Wilford Hall Medical Center, Lackland Air Force Base, TX, found no cases of osteoporosis among those with BMIs greater than 33. If this is confirmed in a larger study, he said, it could mean substantial savings by just screening those below a certain BMI; the $200-$300 per screening could be saved by not screening obese people who “would have an extremely low likelihood of having osteoporosis.” Yet, even the significance of body size still pales to our genes. The news also failed to mention that Deng’s genetic research has shown that bone density is even more determined by genes than by BMI. [Together with Dr. RR Recker, he also holds two provisional U.S. patents on gene mapping and identification of osteoporosis and obesity.] Despite common beliefs that lifestyle factors — exercise, calcium intake, protein, salt, etc. — hold the critical roles in bone density, none of the risk factors used to predict osteoporosis have been shown to make much difference. For instance, researchers evaluating the efficacy of various popular osteoporosis risk assessment tools reported in a recent issue of Archives of Internal Medicine, that the variance in BMD is so wide that most is explained by genetics and only 20% remained for all of the lifestyle factors combined, including weight, diet, exercise, etc. But being fat isn’t supposed to be good for us. Dr. Deng and colleagues did a second analysis, adjusting bone mass for body weight in their computer model, and said they found lean muscle mass, but not the fat mass, correlated with bone mass. The role of lean mass isn’t anything new, as large muscles are thought to put more mechanical loading (impact stress) on bones. Fat people also have more muscle, not just fat mass. While the Deng researchers emphasized the role of muscle and “appropriate” weight-bearing stress on bone density, muscles proved to not be everything. Even the significantly fatter and older Americans in their study, who had only about 20% more lean body mass than their thinner counterparts by DXA measurements, still came out ahead in bone density. While there’s still that sticky age thing, the spin surrounding this finding — claiming obesity is bad for bones — certainly doesn’t accurately describe what they found. But many older women who suffer hip fractures do not have particularly low bone densities, doctors at the Bone and Mineral Research Unit at the Oregon Health and Science University in Portland found. They measured the hip BMDs and osteoporosis risk factors (exercise, strength, falls, etc.) for 8,065 elderly women and found during five years of follow-up, more than half of the women who had hip fractures hadn’t been osteoporotic. Most people who will develop fractures don’t have osteoporosis. It’s like heart attacks and strokes, where most events occur among those at moderate or low risk. It may come as a surprise to learn that in September, 2002, when researchers did a review of the evidence on postmenopausal screening for osteoporosis for the U.S. Preventive Services Task Force, they concluded: “The role of risk factor assessment and different bone density techniques, frequency of screening, and identification of subgroups for which screening is most effective remain unclear. No trials have evaluated the effectiveness of screening; therefore, no direct evidence that screening improves outcomes is available.” Instruments developed to assess clinical risk factors for low bone density or fractures have moderate to high sensitivity and low specificity, they said. The USPSTF’s latest Evidence Report (Number 28) “Osteoporosis in Postmenopausal Women: Diagnosis and Monitoring” found: “No marker was associated with increased fracture risk consistently across all studies. One study provides evidence that using markers in conjunction with densitometry may increase predictability, but this result has not been otherwise confirmed.” [Sidenote: Nevertheless, the USPSTF went on to recommend “that women 65 and older be screened routinely for osteoporosis. For women at high risk for fractures, the USPSTF recommends that screening begin at age 60.” But these recommendations, they say, “should not be used to make treatment or policy decisions.” I’m not even going to try to make sense of that.] While it’s popularly believed that bone density is a measure of bone strength, they are different. At the American College of Sports Medicine’s annual meeting last year, more than 40 papers on DXA technology to measure bone density were specifically reviewed. First off, the scientists found this instrument wasn’t the ideal way to assess bones, especially for measuring the bone’s response to mechanical load. The scientists also found that measuring density was not as important as measuring bone strength when it came to identifying people at risk for fractures. But density or bone mass does not equal strength, stressed Dr. Moira Petit, Ph.D., University of Minnesota in Minneapolis. DXA only measures area density, not volumetric density. For example, we used to say that obese children and adults had high bone density because of their high body weight. From our pQCT data, we now know that obese children (and likely adults) actually have a normal bone “density” (both cortical and trabecular), but high bone strength. Their high bone strength is adapted to their higher muscle mass, but is actually low for their body weight. It’s much the same as recent findings that younger runners have increased bone strength but equivalent bone density because of the distribution in mechanical load, he said. Muscles aren’t everything. We can’t underestimate the wonders of the human body. Even a fat one.
BMI and age
Genes and lifestyle
Osteoporosis screening and risk factors
Moving on, things get complicated and enter the realm of emerging science. But what has and has not been shown may counter some common beliefs about osteoporosis. It’s become popular to assume that bone density and osteoporosis risk factors predict our risks for fractures. All of those recommended BMD tests and osteoporosis screenings would be meaningless unless they help reduce fractures, especially hip fractures which is one of the most critical factors for morbidity and mortality as we age. “The rate of hip fracture increases with age, doubling each decade after age 50 years,” according to Dr. Gigi R Madore, M.D., emergency medicine physician at New York University/Bellevue Hospital Center. “Nearly half of all hip fractures occur in adults older than 80 years....Overall mortality rates of hip fractures is 15-20%, yet in older persons this can increase to 36% over the year following hip fracture.”
Bone density versus strength
While density is relevant, it’s not really the most important point when it comes to how bones respond to mechanical loading, he said. With mechanical loading, in animal studies there is a small increase in mass (less than 10%) but that leads to a substantial increase in strength (more than 70%), according to Petit. There are a number of new technologies for measuring bone strength (such as MRI, peripheral quantitative computed tomography, and QCT) which have made scientists rethink some of their old ideas, he said:
© 2007 Sandy Szwarc
<< Home