Bone health: Osteoporosis, calcium and vitamin D

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The human skeleton is constantly being restored and replaced.  In growing children, bone formation exceeds bone loss.  The two processes balance out in adulthood, but with advancing age, bone mass starts to decrease.

Osteoporosis is a disease characterized not only by a loss of bone mass, but also by increased bone fragility and risk of fracture.¹  The condition primarily affects older people, particularly women, and is associated with 80% of fractures in people older than age 60.  Those fractures can result in reduced quality of life, long hospital stays, institutionalization and higher mortality.  The cost is high for the individuals involved and for the health care system.²

The prevention and treatment of osteoporosis usually entail special attention to the intake of two nutrients:  calcium, which is essential for bone health, and vitamin D, which improves the absorption of calcium.^3-5 Dairy products are the main dietary source of calcium, although it is also found in some fruits, vegetables and grain products.  Very few foods provide concentrated Vitamin D.  It is added to milk, which is the largest dietary source.  The human body also creates vitamin D through sun exposure.  Both calcium and vitamin D can be taken in the form of supplements.

This article profiles the population aged 50 or older who reported having been diagnosed with osteoporosis.  Variables associated with increased risk of diagnosis and differences between 2004 and 2009 are presented.  Intake of calcium and vitamin D from food and from supplements is analyzed by the presence or absence of osteoporosis.

Methods

Data sources

The data are from two Statistics Canada household surveys:  the 2004 Canadian Community Health Survey (CCHS)―Nutrition and the 2009 CCHS―Healthy Aging.  Both surveys excluded full-time members of the Canadian Forces and residents of the three territories, Indian reserves or Crown lands, selected remote areas, institutions and Canadian Forces bases (military and civilian).  Detailed descriptions of the design, sample and interview procedures of the surveys are available in published reports.^6-8

The 2004 CCHS―Nutrition used a 24-hour dietary recall to estimate food and nutrient intake.  A total of 35,107 people completed an initial recall, and a subsample of 10,786 completed a second recall three to ten days later.  The response rates were 76.5% and 72.8%, respectively.  To help respondents remember what they ate and drank the previous day, the automated multiple-pass method,^9,10 was used.  It consists of five steps:

• a quick list (respondents reported all foods and beverages consumed);
• questions about specific food groups and frequently forgotten foods;
• questions about the type of meal and when it was eaten;
• questions asking for more detail about the foods and beverages and the quantities consumed;
• a final review.

The 2009 CCHS―Healthy Aging had a response rate of 74.4% with a sample of 30,865 people aged 45 or older.

This study is based on data for 10,879 people aged 50 or older who completed the initial 24-hour recall in 2004, and for 28,406 people aged 50 or older who completed the 2009 CCHS.

Calcium and vitamin D requirements

In 2010, the Institute of Medicine released new dietary reference intakes for calcium and vitamin D.³  For calcium, the estimated average requirement (EAR) for men aged 50 to 70 is 800 mg a day.  The EAR is higher—1,000 mg a day—for women aged 50 or older and for men aged 71 or older.  The prevalence of inadequate intake can be estimated using the EAR as a cut-point.  At age 50 or older, the tolerable upper intake level (UL), above which the potential of adverse effects exists, is 2,000 mg a day.

The EAR for vitamin D at age 50 or older is 10 mg a day, and the UL is 100 mg a day.

Methods of analysis

On the basis of weighted data from the 2004 and 2009 CCHS, frequencies, averages and cross-tabulations were produced to estimate the prevalence of diagnosed osteoporosis, dietary intake of calcium and vitamin D, the use of supplements, and total calcium and vitamin D intake.  Associations between the risk of a diagnosis of osteoporosis and socio-economic, dietary and lifestyle factors were examined with multiple logistic regression.

The percentage of the population below the EAR or exceeding the UL for calcium and vitamin D was determined using the Software for Intake Distribution Estimation (SIDE),^11,12 based on estimates of usual intake from the 24-hour recalls in the 2004 CCHS.  To estimate total intake of calcium and vitamin D from both food and supplements, the dietary intake of respondents who did not take supplements was combined with the dietary and supplement intake of respondents who took supplements.  This method was explained in a published report.¹³

Confidence intervals were estimated with the bootstrap technique, which takes the complex survey design into account.^14-16  The significance level was set at 0.05.

Definitions

Both the 2004 and the 2009 CCHS determined the presence of osteoporosis by asking respondents if a health professional had diagnosed them as having the condition.

The following socio-demographic variables were defined the same way in both surveys:  immigrant status, highest level of household education (less than secondary graduation, secondary graduation, some postsecondary, and postsecondary graduation), and household income.  Household income was total self-reported household income from all sources in the previous 12 months.  The ratio of total household income to the low-income cut-off for the relevant household size and community size was calculated for each household.  The ratios were adjusted by dividing them by the highest ratio for all respondents combined.  The adjusted ratios were divided into quintiles.

Aboriginal status differed slightly in the two surveys.  In 2004, "Aboriginal" was among the choices in the question on cultural and racial origins.  In 2009, respondents were asked if they were Aboriginal before the question on cultural and racial origins.

The lifestyle variables―smoking (smokers are defined as those who smoke every day or occasionally; former smokers as those who no longer smoke but used to do so daily or occasionally) and alcohol consumption in the 12 months before the interview (yes or no)―were the same in both surveys.

In 2004, the frequency of fruit and vegetable consumption was measured as the sum of the frequencies with which respondents reported consuming foods in six categories:  fruit juice, fruit excluding juice, green salad, potatoes (excluding fries, hash browns and chips), carrots, and other vegetables.  In 2009, respondents were asked how many servings of fruits and vegetables they consumed per day in general.  This question also contributed to the nutritional risk index.

High nutritional risk, which is specific to the 2009 CCHS―Healthy Aging, is defined as a nutritional risk index of less than 38.  The index consists of 10 components measuring weight change in the last six months, appetite, fruit and vegetable consumption, fluid consumption, meals, and meal preparation.

Body mass index (BMI) is weight in kilograms divided by height in metres squared.  It is used to classify participants as underweight (BMI less than 18.5 kg/m²), normal weight (18.5 kg/m² to 24.9 kg/m²), overweight (25 kg/m² to 29.9 kg/m²) or obese (greater than or equal to 30 kg/m²).¹⁷  In this study, BMI was used only with 2009 data.  Weight and height were self-reported.

In 2004, respondents were asked how many days in the previous 30 days they had taken supplements and how many they took on average.  In 2009, respondents were specifically asked how often they took vitamin D or calcium supplements in the previous month.  Respondents were identified as users if they had taken supplements at least once in the past month.  More information about these derived variables is available in the survey documentation.¹⁸

Data about calcium and vitamin D intake from food pertain to 2004; this information was not collected in 2009.  The calcium and vitamin D content of food was derived from Health Canada's Canadian Nutrient File (Supplement 2001b).¹⁹  Supplement composition was taken from the September 2003 Drug Product Database (DPD)²⁰ in the case of drug identification numbers (DINs) listed at the time of data collection, and from the spring 2005 DPD in the case of DINs that were missing or incorrect.

Results

Osteoporosis

In 2009, 19.2% of women and 3.4% of men aged 50 or older reported that they had been diagnosed with osteoporosis by a health professional; at age 71 or older, the percentages were much higher:  31.1% of women and 6.4% of men (Table 1).  These figures were unchanged from 2004 (data not shown).

Table 1 Percentage diagnosed with osteoporosis, by age group and sex, household population aged 50 or older, Canada excluding territories, 2009

In addition to age and sex, diagnosed osteoporosis was significantly associated with Aboriginal status, low household income, alcohol consumption in the previous 12 months, high nutritional risk, low body mass index and the use of calcium or vitamin D supplements (Table 2).  However, for some factors, whether they preceded or followed the diagnosis, or indeed, were a consequence of it, could not be determined.  For instance, people who took calcium and vitamin D supplements had significantly high odds of having been diagnosed with osteoporosis.  But taking such supplements is a component of osteoporosis treatment, so it is possible that the diagnosis triggered their use.  By contrast, low body mass index, a known risk factor, probably predated the diagnosis.

Table 2 Adjusted odds ratios relating osteoporosis diagnosis to selected characteristics, household population aged 50 or older, Canada excluding territories, 2009

The high odds of having been diagnosed with osteoporosis among members of households in the lowest income quintile largely reflected women aged 50 to 70 (Figure 1).  At age 71 or older, the percentage of women with osteoporosis did not differ significantly by household income.

Figure 1 Percentage diagnosed with osteoporosis, by age group and household income quintile, female household population aged 50 or older, Canada excluding territories, 2009

Findings for 2004 provide much the same picture; low household income, Aboriginal descent and underweight were significantly related to having been diagnosed with osteoporosis (data not shown).

Calcium

In 2004, Canadians older than age 50 obtained an average of 771 mg of calcium a day from what they ate and drank.  Milk, cheese, bread, vegetables (except potatoes) and yogurt were the main dietary sources.  Based on the dietary reference intakes of the Institute of Medicine, about half of men aged 50 to 70 did not obtain adequate calcium from food alone;  for women aged 50 or older and for men aged 71 or older, the percentage with inadequate calcium intake from food was 80% (Table 3).

Table 3 Calcium and vitamin D intake, by age group and sex, household population aged 50 or older, Canada excluding territories, 2004

However, 28% of men and 48% of women aged 50 or older reported taking supplements containing calcium (Table 3).  Among those with osteoporosis, the percentages taking calcium supplements were higher—36% of men and 59% of women (data not shown).

Total daily calcium intake from food and supplements combined averaged 969 mg for people aged 50 or older in 2004.  Depending on age group and sex, 45% to 70% had inadequate intake (Table 3).  The average total calcium intake of those who took supplements was 1,303 mg, 515 mg of which came from supplements.  Even so, 25% to 40% of them had inadequate total intake.  On the other hand, a substantial share of supplement users, particularly women, consumed more calcium than the tolerable upper intake level threshold of 2,000 mg (Table 3).

The amount of calcium obtained from food and beverages did not differ significantly between people who had and had not been diagnosed with osteoporosis (Figure 2).  However, those with osteoporosis derived more calcium from supplements, which resulted in significantly higher total intake, compared with people who did not have osteoporosis.

Figure 2 Average daily calcium consumption, by source, osteoporosis diagnosis and sex, household population aged 50 or older, Canada excluding territories, 2004

Vitamin D

Milk, fish, margarine, eggs and beef are the main dietary sources of vitamin D.  In 2004, the food and beverages that Canadians aged 50 or older consumed gave them an average of 5.9 mg of vitamin D a day, well below the Institute of Medicine's EAR of 10 mg.  Based on diet alone, more than 80% of people in this age range were below the EAR. 

In 2004, 27% of men and 44% of women took vitamin D supplements (Table 3).  For people with osteoporosis, the percentage using vitamin D supplements was 38% among men and 57% among women (data not shown).

At age 50 or older, total daily vitamin D intake from diet and supplements combined averaged 10 mg.  However, 54% to 66% of people in this age range were below the EAR.  For supplement users alone, total vitamin D intake averaged 17.5 mg a day, 11.3 mg of which came from supplements; 12% to 17% of this population were below the EAR (Table 3).  Fewer than 1% of people aged 50 or older had vitamin D intake above the tolerable upper intake level of 100 mg (data not shown).

Men diagnosed with osteoporosis obtained less vitamin D from their diets than did men who did not have the condition;  when supplements were included, total vitamin D intake did not differ between men who did and did not have osteoporosis (Figure 3).  Among women, vitamin D intake from food and beverages was similar whether or not they had osteoporosis.  However, when supplements were included, women with osteoporosis had significantly higher total vitamin D intake.

Figure 3 Average daily vitamin D consumption, by source, osteoporosis diagnosis and sex, household population aged 50 or older, Canada excluding territories, 2004

Discussion

The self-reported prevalence of diagnosed osteoporosis and the characteristics associated with it did not change between 2004 and 2009.  Many of the characteristics identified in this study have been observed previously or are established risk factors.  Low BMI, for example, is a well-documented risk factor for fractures²¹ and osteoporosis.²²  The nutritional risk variable used in this analysis is based, in part, on weight loss, which is also associated with fractures and osteoporosis.^23-25  As well, higher fracture risks for Aboriginal Canadians have been reported.²⁶

Clinical practice guidelines recommend that those with osteoporosis consume sufficient calcium and vitamin D.²⁷  Therefore, it is no surprise that in this study, people with the condition were more likely than those without it to have taken supplements and to have derived larger amounts of calcium and vitamin D from supplements.  In fact, those with osteoporosis had the same dietary calcium intake as people without the condition.  Moreover, men with osteoporosis actually obtained less vitamin D from dietary sources alone.  Taking supplements offset the difference in vitamin D intake among men and gave those with osteoporosis an advantage in calcium intake.

The link between osteoporosis and household income has received relatively little attention, and the results of the research that has been conducted are not definitive.  A comprehensive review of articles published between 1966 and 2007 on the association between socio-economic status and osteoporosis fracture uncovered only three studies that found a higher risk of fracture in lower-income people.²⁸  A study of American women older than age 50 reported no correlation between osteoporosis diagnosis and household income.²⁹  By contrast, an association between low bone density and low income has been reported,³⁰ and according to a Canadian study,²⁶ fracture risks were higher among low-income people.  However, because these studies were cross-sectional, the osteoporosis diagnosis itself may have affected household income—for example, by restricting the ability to work.  Supplementary analyses of the 2004 CCHS data showed a significant correlation between household income and supplement use, but not between household income and total intake of calcium or vitamin D (data not shown).

Measured concentrations of vitamin D in the blood (25-hydroxyvitamin D [25(OH)D]) and reported vitamin D intake differed.  According to recent data from the 2007 to 2009 Canadian Health Measures Survey, an estimated 22% of 50- to 79-year-olds had measured blood concentrations below 50 nmol/L, the level targeted by the EAR.³  However, results from the 2004 CCHS show that the prevalence of inadequate vitamin D intake was around 60%.  Sun exposure might account for this difference, because the EAR assumes that it is minimal.  Underreporting of intake is another possible explanation.

Limitations

The main limitation of this study is that it is based on cross-sectional data.  Characteristics in childhood or even before birth may affect the risks of developing osteoporosis in adulthood.³¹ Such longitudinal factors could not be taken into account.

In addition, the osteoporosis diagnosis is self-reported, and therefore, prevalence is likely underestimated because some people who have the condition may not have been diagnosed.

Nutrition surveys are subject to underreporting of energy intake, and by extension, of the intake of nutrients such as calcium and vitamin D.  Earlier studies of the collection instrument used by the CCHS estimated average energy underreporting at 10%³² or 11%.³³

No nutritional data for calcium and vitamin D were available for 2009.  The 2004 data on dietary and supplement intake are the most recent and comprehensive available.

Conclusion

According to the 2009 CCHS―Healthy Aging, 3% of men and 19% of women aged 50 or older reported having been diagnosed with osteoporosis.  A diagnosis of osteoporosis was significantly associated with age, sex, Aboriginal origin, high nutritional risk and underweight.  The odds were also high for people in lower-income households, notably women aged 50 to 70.

Physicians often recommend increased calcium and vitamin D consumption for people with osteoporosis.  And in fact, those with osteoporosis were more likely to take supplements, and so had higher total calcium and vitamin D intake than did people who did not have the condition.  Yet household income was not significantly related to the total intake of calcium and vitamin D.  While sufficient calcium and vitamin D are required to promote bone health, other nutrients are also involved.³⁴  As well, smoking and excessive sodium, caffeine and alcohol consumption can increase the risk of osteoporosis,³⁵ and a balanced diet and physical activity, especially weight-bearing exercises, can reduce it.  More detailed studies might provide a clearer understanding of the associations between osteoporosis and demographic, socio-economic, dietary and lifestyle factors.