Influence of Obesity on Vertebral Fracture Prevalence and Vitamin D Status in Postmenopausal Women

A. El Maghraoui; S. Sadni; A. El Maataoui; A. Majjad; A. Rezqi; Z. Ouzzif; A. Mounach

Disclosures

Nutr Metab. 2015;12(44) 

In This Article

Discussion

This study is a large descriptive evaluation of VFA in a population of asymptomatic postmenopausal women and documents that VFs are significantly related to age, hypovitaminosis D and years of menopause while the influence of BMI was only evident on BMD.

About 18 % of asymptomatic healthy women over 50 had a previously undiagnosed grade 2/3 VFs. This prevalence varied according to BMI and was higher in lean and overweight groups (22 and 21 % respectively compared to obese women (13 %). However, among obese women this prevalence was 23 % in osteoporotic women.

Obesity, defined by the World Health Organization (WHO) as a body mass index (BMI) ≥30 kg/m2, has become a widespread disease. According to WHO, worldwide prevalence has been steadily increasing to epidemic proportions and it has more than doubled worldwide since 1980.[22] Obesity is associated with high BMD,[23] while weight loss is associated with bone loss.[24] The mechanisms responsible for this fat-bone relationship are not fully clarified, but weight change may act on the skeleton through changes in mechanical loading and changes in hormone regulation on bone metabolism. Several studies described the association of fat mass with the secretion of bone active hormones from the pancreatic beta cell (including insulin, amylin, and preptin) and from the adipocyte (e.g., estrogens and leptin).[25–27] These factors alone probably do not fully explain the observed clinical associations, and study of the actions on bone of novel hormones related to nutrition is an important area of further research. Weight changes are also related to changes in lifestyle such as physical activity and intake of nutrients.[28]

It is still not clear whether obesity-related higher BMD increases bone strength and thereby protects against fractures and osteoporosis. The effect of obesity on fractures prevalence may vary by fracture type. Two studies demonstrated increased risk of lower limb fractures among obese women, but no increased risk with fractures of other types.[5,29] According to a large Spanish study, obesity increased the risk of proximal humerus fractures, whereas hip and pelvic fractures were found to be less prevalent.[30] These differences may not be related to BMD but to weight determined higher impact of falls on the peripheral skeleton, whereas other skeletal regions are protected by subcutaneous fat deposits. Some studies support this site-dependent fracture risk both in obese pre- and postmenopausal women and men.[31,32] Concerning VFs, Laslett LL et al. showed a dose–response relationship between both prevalence and number of vertebral deformities and multiple measures of body fat in women.[33] Ferrar et al. suggested that some types of mild deformities may be "non-fracture variants" rather than vertebral deformities. These include short vertebral height, which they report as being more common in older, heavier women, unrelated to osteoporosis equally prevalent in pre and postmenopausal women and therefore unimportant.[34] The assessment of prevalent mild VFs is still problematic, as such deformities may reflect old trauma or are sometimes considered as an expected effect of aging.[35] Actually this point is raised when such deformities are used to discriminate patients with or without osteoporosis.[36] However, mild VFs (grade 1) have been shown to be a risk factor for subsequent vertebral and non-vertebral fracture in postmenopausal women with osteoporosis.[37]

Vitamin D insufficiency may lead to secondary hyperparathyroidism resulting in greater bone turnover, bone loss, and increased risk of fractures. This insufficiency may result from decreased cutaneous synthesis, decreased dietary intake, impaired hepatic and/or renal activation, or resistance to vitamin D action.[38] Serum 25-hydroxyvitamin D (25 (OH) D) level is considered to be the best indicator of vitamin D nutritional status and it has been suggested that serum 25 (OH) D values <10 ng/ml indicate vitamin D deficiency and <20 ng/ml indicate vitamin D insufficiency.[39] Although this was not the case in our study, recent evidence indicates that obesity is linked to lower serum levels of 25 (OH) D,[14,40,41] which may be due to poor exposure to sunlight and/or decreased vitamin D bioavailability (due to the sequestering effect of high quantity of subcutaneous fat on circulating vitamin D).

Our study has strengths and limitations. The assessment of fracture was carefully conducted using standard procedures of acquisition, and standard reading of all VFAs. All the morphometric assessments were made by an experienced investigator. Before diagnosis of fracture, a non-osteoporotic origin was considered for each deformity. The main limitation lies in the procedures used to select subjects, who were all volunteers and ambulatory, and presumably not representative of the general population. However, the population of Rabat is a mixture of the Moroccan population and we believe that there is a little impact on the prevalence of obesity, osteoporosis or VFs or hypovitaminosis D.

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