[icon name=”user” class=”” unprefixed_class=””]  Dr. L. Claudia Pop, MD

Throughout life, there is a strong relationship between body weight and bone density.Low body weight results in low bone density and increasesthe risk of hip fractures by twofold at older age.^1,2 Reasons for that are: decreased weight bearing, low bone mass attained at skeletal maturity and/or higher rates of bone with aging in in underweight when compared to healthy weight individuals.³

Conversely, obesity has been associated with higher BMD and reduced fracture riskdue to greater mechanical-loading at higher body weight on bone.^{4, 5} In fact, the absence of mechanical loading which occurs during spaceflights resulted in 12% bone loss over a six-month mission compared to 0.5% annual bone loss that occurs in load-bearing bones of postmenopausal women with aging.^{6, 7} Additionally, hormonalimbalance in obesity (i.e. higher estradiol or altered adipose-derived hormones such as adiponectin or leptin) and lower bone turnover can increasebone mineral density.⁸

On the other hand, although reducing excess body weight improves health status, it also causes bone loss and increases the risk of fracture in middle-aged and older adults.⁹ Bone is being constantly remodeled during life in response to changes in biomechanical forces. Old, damaged bone has to be removed and replaced with new, mechanically stronger bone, able to resist stress.Due to hormonal changes with weight loss,bones are being remodeled at an accelerated rate, but bone breakdown is much higher compared to bone formation. This imbalance between breakdown and formation weakens bone and increases fragility.³ Moreover, inadequate nutrient intakes with dieting, particularly vitamin D and minerals (calcium, phosphorus) can alter bone mineralization and form fragile bone.⁹

It appears that higher body weight protects against boneloss while weight loss increases bone loss. Accumulating evidence, especially regarding obesity, suggests that excess body weight is actually detrimental for bone health.

BONE HEALTH EVALUATION

BONE MINERAL DENSITY

The gold standard method in the field is DEXA (Dual-energy x-ray absorptiometry) which is used to measure the amount of minerals (i.e. calcium) per square centimeter of bones (grams/cm²). Results are then compared against a young healthy reference population of the same gender and expressed as T-scores (standard deviations above and below the average). You have normal bone mass if T-score is greater than -1.0; low bone mass (osteopenia) if T-score is between –1.0 and –2.5 and you are diagnosed with osteoporosis if T-score is -2.5 or below.¹⁰ In the obese, the average T–score is 1.45 units higher compared to normal weight individuals.¹¹ Although, being underweight still constitutes the major risk factor for fractures, research has shown that despite a higher BMD, fracture burden in the obese is not negligible.¹²

Moreover, fracture rates differ significantly across T-score ranges depending on individuals’ age and younger bones are stronger even at similar T-score values.¹³ In addition, in many populations fracture risk is only partially explained by changes in bone mineral density.^{14, 15} Furthermore, epidemiological studies and clinical interventions with osteoporotic drugs yielded results that challenged the idea that denser bones mean stronger bones.¹⁶ Hence, the concept of bone strength, more complex than bone density, has emerged and has expanded to include several properties of bone that are in sum referred to as bone quality.

BONE QUALITY

Bone quality is a combination of interdependent factors including bone mass, microarchitecture, tissue composition (accumulated microscopic damage, the quality of collagen, mineral crystal size) and bone turnoverthat make bone resistant to fracture.¹⁷ From childhood to adulthood, in response to physiological and pathological stress factors (i.e. growth, menopause,and aging) material and structural properties of bone change in an attempt to maintain tissue integrity and strength.³ In the absence of these adaptive changes, bones become fragile and fracture.

BONE STRENGTH

Bone strength is defined as the maximal stress tolerated before structural failure (i.e., fracture) occurs.¹⁸ As mentioned before, a BMD score (T-score) lower than -2.5 standard deviations than the average bone mass of healthy young adults indicates that you have osteoporosis.¹⁰ However, referring to its definition, osteoporosis is a progressive skeletal disease with reduced bone strength and increased risk of fracture. But fragility fractures frequently occur in individuals who do not have osteoporosis according to T-score value.¹⁹ Therefore, although BMD is considered an indicator of osteoporosis, it is only a quantitative tool and an insufficient measure of bone quality. Other bone properties contributing to bone strength should be evaluated for a correct assessment of skeletal health. Reduced bone mass always results in compromised bone strength and fragility fractures. However, fragility fractures may occur due to impaired bone microarchitecture or structural properties of the bone tissue without changes in bone mass. Many times, reduced bone mass and compromised bone quality simultaneously occur.In fact, several drugs used in the treatment of osteoporosis and physical exercise seem to improve bone strength and resistance in the absence of noticeable BMD changes.¹⁶

SHORT REVIEW OF THE EVIDENCE

Epidemiologic studies have found that both low body weight and weight loss are related to reduced bone mass and increased fracture risk.¹ Moreover, repeated attempts to lose weight through dieting reduces bone density in young women.²⁰ Similarly, even modest weight loss induced in older women results in bone loss.^{21, 22}Observational studies in older men showed that weight loss predicts bone loss.²³ So far, nutrient supplementation on bone during weight reduction (calcium, protein, and/or vitamin D) and increased physical activity have proven effective at reducing bone loss with weight loss.^{21, 24}

In numerous studies obesity has been associated with higher bone mass, reduced bone turnover and loss.⁹ Research conducted over the past decade has shown that bone quality is compromised in the obese.²⁵ Several factors,including hormonal dysregulation in obesity have been shown to alter bone quality. In addition, low mobility and reduced muscular strength put obese individuals at greater risk of falling. Moreover, the impact force with the ground is much higher in the obese due to heavier body weight increasing the risk of fracture.²⁶ However, the complex interaction between fat and bone, as well as the implications of excess adiposity on bone health are not completely understood and require more research. Limited data from weight loss interventions suggest a beneficial effect of weight reduction on bone quality but this may be gender specific.²⁷ In addition, obesity in children, found at a critical stage for bone development may affect bone health later in life.²⁸ Observational studies and clinical interventions have shown that physically active individuals and increased levels of exercise training maintain and improve bone quality.¹⁶

CONCLUSION

Body weight and changes in body weight are strong predictors of bone mineral density. Being underweight increases bone loss and is a major risk of fracture. Similarly, weight loss results in bone loss and greater risk of fracture. However, bone health goes beyond bone mineral density. Despite higher BMD in obesity, altered bone quality due to excess weight increases bone fragility as well.

KEY POINTS

1. A healthy body weight is essential for your bone health.
2. Adequate diet and physical activity maintain bone health.

REFERENCES:

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