1. Introduction
The use of supplements, which may be defined as multi-vitamins, single vitamins, single minerals, herbal supplements, oil supplements and any other dietary supplementation [
1] varies among populations. However, it is highest in countries such as the United States, United Kingdom and Denmark [
2], where supplement use is between 35 and 60% of adults [
1,
2,
3,
4,
5]. On a per capita basis, Australians are some of the world’s largest consumers of dietary supplements, with vitamin and mineral sales totaling AUD
$646 million in 2013 [
3].
The Australian Dietary Guidelines recommend that individuals, with the exception of pregnant women, only take supplements if they are eliminating a food group from their diet [
6]. However, many people continue to take supplements. Supplement users may differ from non-users in regard to a range of characteristics. Previous research suggests that supplement users are more likely to be female, older and have a higher educational attainment than non-supplement users [
1,
2,
3,
4,
5,
7]. Supplement use is also higher in people who adopt healthier lifestyle behaviours, such as being more physically active, not smoking and consuming more fruit and vegetables and less alcohol [
1,
8,
9]. Several studies show that less healthy diets are common among supplement non- users, including greater consumption of diets high in fat, low in fibre or low in fruit [
8,
9].
Just as supplement users have been shown to maintain healthier lifestyle behaviours, individuals who take dietary supplements are more likely to have a better health status [
2,
8,
10] including better self-rated health and fewer cardiovascular risk factors, such as high blood pressure, than non-supplement users.
While supplement users may have more optimal dietary intakes compared to non-supplement users [
11], many supplement users have also been found to be exceed the upper recommended intake limit for some vitamins and minerals [
11], which is defined as the highest level of individual daily intake that does not pose a threat to health [
11]. For example, the Multiethnic Cohort study in the United States reported that, of people who take supplements, 50% of men and 40% of women consumed more than the upper limit for niacin and folate [
11].
Studies to date that have characterized supplement users, have lacked detailed dietary records and supplement information, and there has been a paucity of nationally-representative data on supplement use in the Australian population. Given the high prevalence of consumption and the limited understanding of the characteristics of these consumers, this study aimed to examine supplement use within the Australian population. Specifically, the present study aimed to explore the sociodemographic, lifestyle and health status characteristics of supplement users; the dietary intakes of supplement users and the most common types of supplements used among Australian adults.
3. Results
Overall, supplement use was reported by 34% of men and 47% of women (
Table 1). Supplement use was highest among women, among those aged 71–85 years, those with highest levels of education, those living in areas with the least socio-economic disadvantage, those who met physical activity guidelines and those who met fruit and vegetable guidelines (
p < 0.05). After adjustment for age (
Table 2), the prevalence of supplement use was higher in women and when adjusted for sex, the prevalence of supplement use was higher in older adults (71–85 years). When adjusted for sex and age, the prevalence of supplement use was higher in those in the least area level disadvantaged group, those with higher educational attainment, those who met the guidelines for physical activity (PR 0.79; 95% CI 0.71, 0.88;
p < 0.001) and fruit and vegetable intakes (PR 0.81; 95% CI 0.68, 0.97;
p < 0.05), and those with higher DGI scores (PR 1.32; 95% CI 1.20, 1.44;
p < 0.001). An association was found between being an ex-smoker (PR 1.56; 95% CI 1.31, 1.86;
p < 0.001) or having never smoked (PR 1.46; 95% CI 1.25, 1.70;
p < 0.001) and supplement use, when compared with current smokers. No association was found between alcohol intake or sedentary behaviour and supplement use. An inverse association (
Table 3) was found between being hypertensive (PR 0.87; 95% CI 0.77, 0.99;
p = 0.031) and waist circumference (PR 0.89; 95% CI 0.79, 1.01;
p = 0.007) and supplement use. No association was found between BMI, self-assessed health, chronic disease and supplement use.
The most commonly used supplements were single vitamins (19%), herbal supplements (16%), and multivitamins (16%) (
Table 4). The most commonly used supplements used varied by age group, with folic acid being most commonly consumed by the 31–50 year age group, while lipids (e.g., fish oil supplements, fish oil supplements with added nutrients, fish liver oil supplements, evening primrose oil supplements and other lipid supplements grouped together) were most commonly consumed by the 51–70 year age group. Females consumed a higher proportion of multivitamins (62%), single minerals (67%), single vitamins (60%), herbal supplements (63%), iron (62%) and folic acid (59%) than males.
When comparing nutrient intakes from food only, supplement users were found to have higher intakes of fibre and most vitamins and minerals (except, zinc and vitamin B12) compared to non-supplement users, although differences were small in many cases (
Table 5). Intakes of all vitamins and minerals were higher when the contribution from supplements was added into overall intakes. Intakes of folic acid from food alone were lower among supplements users, but the inclusion of supplement intakes (in the calculation of total intake from food and supplements) reversed the differences. Supplement users were found to be reaching the upper range of the recommended dietary intake (RDI) for magnesium and exceeding the RDI for zinc, vitamin C, vitamin E and B12, when total intakes were considered.
4. Discussion
A significant proportion of the Australian population reported supplement use (34% men and 47% women), with supplements users more likely to be female, older, more highly educated and to exhibit healthier lifestyle behaviours and have better health status than supplement non-users. Nutritional supplements are considered complementary medicines within Australia, and undergo less regulation than higher risk products, such as medicines [
30]. Therefore, less emphasis is placed on assessing the evidence of the claims being made by the products; these marketing claims may persuade people to consume more dietary supplements [
30,
31].
Our findings, in relation to the sociodemographic characteristics of supplement users, are consistent with the literature [
1,
2,
4,
5,
7,
9,
31,
32,
33]. Many previous studies have found that supplement users are more likely to be female, older in age and more highly educated [
1,
2,
4,
5,
7,
9,
31,
32,
33]. This is reflected within the current study, as the largest percentage of supplement users were older in age. Females have been shown to be more health conscious and therefore may take more dietary supplements to prevent illness [
34].
Previous studies have also found that people of a lower socio-economic position are less likely to use supplements, which is reflected within the current study [
9,
31]. Previous studies have shown that people with a higher socio-economic position are more likely to be more health conscious, which may motivate them to take more dietary supplements [
5].
Previous studies suggest that supplement users are more likely to engage in a range of health behaviors and are more likely to meet recommendations for physical activity and fruit and vegetable consumption. Previous research has shown that supplement users are more likely to be physically active [
1,
2,
4,
9,
35]. The current study found a relationship between supplement use and meeting the physical activity guidelines; however, there was no significant difference found between supplement users and supplement non-users, with regard to sedentary behaviour. It is consistently shown that supplement use is associated with health conscious individuals; however, the lack of association may be due to the inability to modify sedentary behaviour, as much of it occurs in the workplace [
36]. Meeting the guidelines for fruit and vegetable consumption was positively associated with dietary supplement use in the present study. This is consistent with previous studies, which found an association between higher consumption of fruit and vegetables and dietary supplement use [
2,
9,
37]. This introduces the question of the need for additional nutritional supplementation in those who already obtain nutrients from a healthy diet, as supplement users’ dietary intakes were generally better and met the RDI better than non-supplement takers [
2].
The finding that smoking status was associated with supplement use, is consistent with previous research [
1,
2,
5]. The NHANES 1999–2000 found that former smokers and people who have never smoked were more likely to be supplement users than current smokers [
1,
5]. The NHANES 1999–2000 reported that 61% of former smokers were dietary supplement users, while 52% of people who had never smoked reported using dietary supplements and only 43% of current smokers reported using dietary supplements [
1].
Previous research had mixed findings with regard to alcohol consumption and its association with dietary supplement use [
1,
5,
9,
35]. Many studies found that people who consume less alcohol were more likely to take supplements [
1,
5,
35]; however, a study from Canada did not find an association between alcohol consumption and dietary supplement use [
9]. The current study did not find an association between alcohol consumption and supplement use. These discrepancies may be due to the type of alcohol consumed, as previous studies have found positive associations for wine consumption, but no associations for beer consumption [
1,
5,
38]. Dietary supplement users are more likely to be those who are more health conscious and therefore adopt health behaviours, such as not smoking and consuming less alcohol, which may motivate them to take dietary supplements [
39].
Supplement users are also more likely to have a better health status. Previous research has reported varied conclusions in relation to BMI and its relationship to supplement use; however, the current study did not find an association between BMI and supplement use and found an inverse association between waist circumference and supplement use. A study on supplement use in Taiwan, which focused on the use of multivitamin and mineral supplements, calcium, vitamin E and fish oil, found no association between BMI and supplement use [
40]. Similarly, NHANES 2007–2008 did not find an association between supplement use and BMI [
10]. On the contrary, a study conducted in the United States on herbal supplements found that herbal and specialty supplement users had lower BMIs [
37]. Most studies focus on BMI as an indicator of good health status, with regard to supplement use; however, waist circumference has been regarded as a more accurate indicator of metabolic index, with this measure being used when defining metabolic syndrome [
41]. The results of a Danish study showed that people who scored lower in the health index—meaning they had lower blood pressure, a lower waist circumference and did not test positive to the urine glucose test—had higher supplement use [
2,
5]. In line with our findings, a study conducted in the United Kingdom found that people who had a history of high blood pressure were less likely to be taking dietary supplements [
2]. Previous studies have reported an association between self-reported health and supplement use; however, the present study did not find this association [
2]. The discrepancy may be due to variations in the populations under study, as our study included a wide age range of participants [
42].
Supplement users have been shown to have dietary intakes that tend to be healthier, and higher in a range of nutrients, when considering food intake alone, compared to supplement non-users. Supplement users were reported to have a higher fibre consumption from food only compared to supplement non-users, which is consistent with previous research [
43,
44]. This is reflective of many nutrients, as the diets of those using dietary supplements are higher in nutrients than non-supplement users. Not surprisingly, when the contribution from supplements is taken into account, supplement users have higher vitamin and mineral intakes, compared to supplement non-users. Previous research has suggested that many people are reaching the upper limit for vitamins and minerals, such as niacin, folate, iron and zinc [
11]. In the current study, some nutrients intakes may actually be exceeding the RDI—for example vitamin E, vitamin B12 and zinc—which may result in toxicity [
9,
45]. It is unclear from the current data how long participants had been consuming the reported supplements, and therefore whether these intakes are higher than the RDI is of concern. Further research focusing on biomarkers of nutritional status may provide insight into understanding the impact of these levels of supplementation.
The strengths of the present study include the nationally-representative data, which make our findings generalizable to the wider population. A limitation to the current study is its cross-sectional design, which did not allow for an investigation of any causal relationships. A further limitation is the differing definitions of supplement users across studies [
1,
4,
5,
9,
31]. The lack of standardization, in regard to the definition of supplement users, makes it difficult to compare the prevalence between populations. However, despite the methodological differences, similar results, with regard to the demographics, lifestyle habits and health status of supplement users, were identified in a number of studies. Given the widespread use of supplements, further investigation on the social, psychological and economic determinants that motivate the use of supplements is required, to ensure the appropriate use of supplements and to minimise the potential harm which can be caused through excessive use of dietary supplements [
32].