Dietary supplements: characteristics of supplement users in New Zealand.
Objective: To determine the characteristics of adult New Zealand (NZ) supplement users.
Design: Secondary data analysis of a cross-sectional national survey.
Subjects: A total of 4636 adults aged 15 years and above.
Setting: Homes throughout NZ.
Methods: The National Nutrition Survey (NNS97), linked to the NZ Health Survey, had a multistage sampling design providing a representative sample of the NZ population aged 15 years and above. Secondary data analysis using logistic regression was used to determine characteristics of dietary supplement users (n = 2749) by a range of demographic, lifestyle and dietary variables.
Results: Income, marital status, Index of Deprivation (NZDep96 quartile) and place of residence were not independently associated with dietary supplement use. The odds of consuming dietary supplements over the previous year was increased in younger adults. Participants in the 'NZ European and others' ethnic group were the most likely to be supplement consumers compared with Maori and Pacific participants. Dietary intakes from food, for folate, riboflavin (males), vitamin C (females), niacin (females), dietary fibre (females) and calcium (males) were significantly higher in supplement users.
Conclusions: NZ supplement users are characteristically female, of 'NZ European and others' descent, well educated and relatively young. Nutrient intake from foods and beverages alone was adequate for those individuals who chose to include supplements in their diet.
Key words: dietary supplements, minerals, New Zealand, Nutrient intake, predictors, vitamins
(Nutr Diet 2005;62:123-129)
An increase in the prevalence of dietary supplement use is a worldwide trend, (1-4) particularly among females with education beyond high school, with higher incomes and of an older age. (5,6) Previous research has paradoxically shown the use of dietary supplements to be highest among people who are the least likely to need them. Kirk and coworkers have labelled this as the 'inverse supplement hypothesis'. (7)
There is much controversy over the necessity for and cost and safety of dietary supplements among both health professionals and the general public. In Australia, an estimated $621 million was spent on alternative medicines in 1993 (8) and this had escalated by 120% in 2000. (9) The use of dietary supplements and the growth and diversification of the dietary supplement market cannot be ignored, and it is important to health professionals to have an improved understanding of the characteristics of dietary supplement users. (10)
The purpose of the present paper is to explore the relationship between the demographic, lifestyle and dietary factors and the supplement practices of New Zealand (NZ) users.
The New Zealand 1997 National Nutrition Survey (NNS97) provides the most recent national data collected in Australasia. It was a voluntary cross-sectional survey of New Zealanders aged 15 years and above (n = 4636) and was a subsample of the 1996/1997 NZ Health Survey (NZHS). The methodologies and design of the NNS97 and NZHS have been fully documented. (11-13) Data collected included information on food intakes, dietary habits, anthropometry and use of dietary supplements (during the previous year). Data were collected between December 1996 and November 1997. (11,12)
Data on all types of supplements consumed were collected: vitamin and mineral supplements, including all single- and multinutrient preparations and 'other' supplements, for example herbal supplements, sports preparations and garlic products. The questions were interviewer-administered in the respondent's home and responses entered directly into a laptop computer. For each supplement, the brand name, the amount consumed (including units of the amount) were requested as well as the number of supplements taken per usage (when known). If the participant had the supplement container/bottle available, information on the brand name and amount consumed was taken directly from that.
For the use of supplements 'in the previous year' respondents were asked how often they consumed each type of supplement. They were given the following seven choices: less than once per month, one to three times per month, once per week, more than once per week, daily, episodic, don't know. 'Episodic' was the term used to describe regular supplement taking for a limited time period.
Secondary data analysis unique to the present paper included the use of logistic regression to determine characteristics of supplement users over a range of demographic, lifestyle and dietary variables.
A participant was classified as a supplement user if they reported that they had consumed at least one vitamin, mineral or other supplement in the previous year.
These included gender, age, ethnicity, socio-economic status (NZDep96), education, household income, place of residence and marital status derived from the NZHS. (12) Participants were classified into one of three ethnic groups based on self-identification. If participants stated that they belonged to more than one ethnic group, a single ethnic category was assigned on an hierarchical basis: Maori, then Pacific, then 'NZ European and others' (NZEO). The variable NZDep96 is an area-based Index of Deprivation based on a combination of variables including: income, transport, living space, ownership of home, employment status, qualifications, support (sole-parent families) and access to a telephone. (13) The scores were divided into four quartiles to give an index from I to IV. People classified in NZDep96 quartile I were living in the least deprived areas and those in quartile IV the most deprived areas.
Health and lifestyle variables
Health and lifestyle variables included self-reported health, smoking status, level of physical activity and alcohol intake derived from questions in the general health questionnaire (NZHS). Physical activity levels of participants were classified into four categories based on their self-reported leisure time physical activity in the previous seven days: sedentary--no activity; relatively inactive--less than 2.5 hours of activity; relatively active--at least 2.5 hours, but less than 5 hours of activity; highly active--5 hours or more of activity. Height and weight measurements were obtained by trained interviewers as part of the NNS97. Body mass index (BMI) cut-offs were used to classify participants into underweight, normal, overweight and obese categories of weight status. Different cut-offs for overweight and obesity were used depending on the participant's ethnicity. (14) Overweight: Maori/Pacific (26 kg/[m.sup.2] = BMI<32 kg/[m.sup.2]); NZEO: (25 kg/[m.sup.2] = BMI < 30 kg/[m.sup.2]). Obese: Maori/Pacific (BMI [greater than or equal to] 32 kg/[m.sup.2]); NZEO: (BMI [greater than or equal to] 30 kg/[m.sup.2]).
The nutrient intake of participants was assessed on the basis of a 24-hour diet recall. (11) The mean intake of energy, total fat, protein, carbohydrate, dietary fibre, folate, vitamin C, vitamin [B.sub.12], iron, zinc, [beta]-carotene, vitamin A equivalents, riboflavin and niacin of supplement users, was compared with non-supplement users. The per cent energy from fat, carbohydrate and protein was also compared.
The NNS97 was carried out using a multistage sampling design including stratification, clustering and unequal probabilities of selection. The survey commands of the statistical package Stata Statistical Software release 5.0 (Stata Corporation, College Station, TX, USA) were used to allow for the complex sampling design. All proportions reported have been weighted to provide population estimates. A logistic regression model was used to determine possible predictors of supplement use using variables listed in Table 1. From this a reduced model was created including only the variables from the full model that had P-values less than 0.20. The exponential of the coefficient for each predictor can be interpreted as an odds ratio adjusted for all other predictors.
Multiple linear regression was used to determine whether there was a difference in the nutrient intakes of supplement users and non-users after adjusting for potential confounding factors. The independent variables included in the model were gender, ethnic group, education level, deprivation level (NZDep96 quartile), smoking status and age groups, with interactions between gender for each of these variables.
Nutrient intakes that were positively skewed were transformed using natural log transformation to normalise them. For these models the coefficients can be back transformed providing an estimate of per cent change in nutrient intake for those using supplements compared with those not using supplements. If the confidence interval surrounding this estimate includes one then there is no evidence that nutrient intake differs between users and non-users.
A total of 59% of participants in the NNS97 (n = 2749) reported using at least one supplement in the previous year. Among females the prevalence of supplement use in the previous year was significantly higher (P<0.05) than in males (68% vs 50%). A lower proportion of both males and females reported using non-vitamin and mineral supplements compared with vitamin and mineral supplements.
For the majority of supplements used in the previous year participants reported using them on either a daily (43%) or an episodic basis (38%) (Table 2). Few participants (3%) reported that they consumed supplements less than once per month, one to three times per month (3%) or once per week (2%). For 'other' supplements a higher number of supplements were reported as being used daily compared with vitamin and mineral supplements (47% vs 41%, P<0.05).
The intake of dietary fibre from food was significantly higher in female supplement users compared with nonsupplement users by 1.28 g per day (Table 3). The 24-hour dietary intake of male supplement users was 12% higher for folate, 7% higher for riboflavin and 21% higher for calcium compared with that of male non-supplement users (Table 4). Female supplement users had a significantly higher dietary intake of vitamin C (12%) compared with non-users and also had significantly higher dietary intakes of niacin (5% higher) and folate (7% higher) (Table 4).
All socio-demographic, health and lifestyle and dietary variables (Table 1) were included in a logistic regression model to determine the best predictors of dietary supplement use in the NZ population. Variables not found to be associated with dietary supplement use were eliminated from the model and Table 5 gives the resulting odds ratios for those variables left.
Income, marital status, NZDep96 quartile and place of residence were not found to be independently associated with dietary supplement use. The odds of being a supplement user were more than twofold higher in females compared with males. The odds of consuming dietary supplements in the last year, decreased with increasing age. Participants aged 15-24 years were twofold as likely to consume dietary supplements compared with those aged 65-74 years and fourfold as likely as those aged more than 75 years. Maori and Pacific people had significantly lower odds of being a supplement user compared with NZEO people. People with school and post-school qualifications were 30% more likely to consume dietary supplements compared with those with no qualifications.
After adjustment for other variables (socio-demographic and dietary habits listed in Table 1), BMI, physical activity and self-reported health status were not found to be associated with dietary supplement use. Current smokers were significantly less likely to take dietary supplements (odds ratio: 0.68) compared with people who never smoked (odds ratio: 1.00). People who drank alcohol less than once per month (odds ratio: 1.02), two to four times per week (odds ratio: 1.44) and more than or equal to four times per week (odds ratio: 1.45) were significantly more likely to consume dietary supplements than non-drinkers.
Consuming five or more servings of fruit and vegetables per day was not found to be associated with supplement use. People who followed an alternative diet were 70% more likely to consume supplements compared with those who followed a normal diet. Dietary supplement use was associated with increased consumption of serves of cereal, and people who consumed seven or more serves of cereal per week were 20% more likely to be a dietary supplement user compared with people who consumed less than seven servings of cereal per week.
New Zealand supplement users have some, but not all, characteristics consistent with those observed in other countries.
New Zealand females were more than twofold as likely to be dietary supplement users compared with males, which has consistently been shown in other studies. (2,3,15-19) Reasons for the higher use of supplements by females could include the higher requirement for some nutrients such as iron and calcium, (20) which are consequently more likely to be chosen voluntarily or to be prescribed.
Adults of NZEO descent were twofold as likely to be dietary supplement users compared with Maori and eight-fold more likely to be supplement users compared with Pacific adults. This is similar to US data where clear ethnic differences have been shown in the prevalence of dietary supplement use with white people being more likely to consume dietary supplements than African Americans and Hispanics. (1,17,21) Although in these studies it has been suggested that ethnic differences were a reflection of socioeconomic status, in the present study income and education were controlled and ethnicity was an independent predictor of supplement use. It is likely that the ethnic differences may reflect cultural differences in attitudes towards the sufficiency of food.
The prevalence of using a supplement in the previous year was highest in the 15-24 year age group and the prevalence decreased with increasing age in both males and females. Results from other studies have shown an opposite trend to this with supplement use being lower in the younger adult groups, (2,16) although the data collected in 2000 by MacLennan and colleagues from Australia showed a similar trend with the highest use in the 15-34 year age group. (9) Thus, the higher prevalence of supplement use among younger NZ adults could be an emerging trend during the 1990s, although these cross-sectional data are unable to answer that question. Alternatively, the higher usage in younger people may be an artefact of the comparatively long time period over which usage was reported (the previous year). Younger people may be more likely to trial a supplement, or take it to alleviate an immediate problem rather than to take it every day as a preventative measure.
Consistent with other studies the odds of being a dietary supplement user were significantly higher in people who had school qualifications and post-school qualifications compared with those who did not have any qualifications. (1,9,17,18,22,23) Although supplement users have higher educational levels, the correlation between supplement use and nutrition knowledge has been shown in other studies to be zero (24) or negative. (25) A higher level of education is not necessarily associated with a sound knowledge of nutritional need.
The prevalence of supplement use in NZDep96 quartile I compared with quartile IV was nearly 20% higher in females and 10% higher in males. NZDep96 quartile, however, is a combination of a number of variables including education. Adjusting for these variables removed the association between NZDep96 quartile and supplement use. In Britain, the use of supplements was higher in both males and females among people in the two highest social classes compared with the two lowest (16% vs 11%). (3) Dorant and coworkers also showed a higher prevalence of supplement use in the highest social class in the Netherlands. (26) In so much that the variable NZDep96 can be regarded as a marker of social class, it is clear that NZ adults do not follow the patterns observed in Europe.
Consistent with the theory that supplement users are more health-conscious than the rest of the population, the odds of being a supplement user were higher for nonsmokers compared with current smokers. The majority of studies that have examined smoking status and the use of supplements have found the same results. (6,7,27,28) Certainly no studies have found supplement use to be lower among non-smokers compared with smokers. Smokers are one group in the population for whom dietary supplement use may be particularly beneficial with smokers having an increased need for anti-oxidant nutrients. (29) Smokers are also less likely than non-smokers to have adequate nutrient intakes. (6)
Non-drinkers in the present study were significantly less likely to consume supplements compared with people who drank at least two to four times per week (odds ratio: 1.44). Several studies have found that female supplement users have higher intakes of alcohol compared with non-users, (7,30,31) whereas others reported female supplement users consumed less alcohol. (6,32) This inconsistency may in part be explained by different methods used to assess alcohol intake and non-drinkers may be characteristically different from low alcohol consumers. Wallstrom and colleagues suggest that people who know that their alcohol intake is too high may choose to take supplements to reduce the perceived threat to their health from drinking. (30)
Self-reported health status was not found to be associated with supplement use. This is not surprising given the reasons for supplement use. In some cases supplements may be used as a result of ill health, or by those who perceive their health as poor, or in other cases supplement users may perceive their health as better because of the use of supplements. Supplement users were also more likely to follow an alternative diet such as vegan, vegetarian or avoidance of dairy products. Supplement use was similarly found to be higher among vegetarians and vegans in the UK Women's Cohort Study and the Dutch National Food Composition Survey. (2, 7, 27, 31) The more frequent intake of supplements by people following an alternative diet may be a compensation for dietary restriction, particularly in vegans whereby vitamin [B.sub.12] intakes may only be obtainable through the use of supplements. People who follow alternative diets may also take a greater interest in their health and diet and be more aware of nutritional issues and needs prompting them to use supplements.
The literature suggests that supplement users are more likely to follow dietary guidelines and to have better nutrient profiles. (6,7) In the present study, supplement users were more likely to report consuming more than seven servings of cereals per week. However, they were less likely to consume at least five servings of fruits and vegetables per day, an opposite finding to numerous other studies. (6,7,18,31,33) Fat intakes were the same for supplement users and non-supplement users. (6,7,18,31,34) Despite not being better followers of all dietary guidelines, supplement users had higher or equal intakes compared with non-users for all nutrients from food.
It has been reported that most users of dietary supplements appear to believe that they prevent sickness, or may cure their medical problems and are influenced to buy these products by a variety of agencies, many of whom have a vested interest in the sale of the product. (9) It is also of concern that many people assume dietary supplements are harmless, when there is evidence of possible side-effects. (35) There are also wider issues with the use of dietary supplements such as waste of health resources, drug interactions and delay in effective treatment. (36)
In summary, supplement users within NZ are similar to users in other countries in that they are more likely to be female, of NZEO more highly educated and are more likely to display health-conscious behaviour such as not smoking. However, unlike other studies the prevalence of supplement use was highest in the younger age groups compared with the older age groups.
These results indicate that, in general, the diets of supplement users are similar to those of non-users; certainly supplement users do not have lower intakes from food for any of the nutrients examined. Supplement users in NZ are not a group more likely to need supplements compared with non-users. The results also indicate that, in general, in NZ adults, the use of supplements is not displacing food, as a source of essential nutrients.
The 1997 National Nutrition Survey (NNS97), was funded by the Ministry of Health (NZ). Principal investigators of NNS97 were from University of Otago (Emeritus Professor David Russell, Dr Winsome Parnell, Dr Noela Wilson, Dr Jim Faed, Dr Elaine Ferguson, Mr Peter Herbison, Dr Caroline Horwath, Dr Ted Nye, Dr Papaarangi Reid, Dr Rob Walker, Dr Barry Wilson) and Auckland University (Dr Colin Tukuitonga). Statistical advice was obtained from M. Wohlers (LINZ), and programming services from C. Blakey (LINZ).
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University of Otago, Dunedin, New Zealand
C. Smith, MSc, Life in New Zealand Activity and Health Research Unit N.C. Wilson, PhD, Life in New Zealand Activity and Health Research Unit
W.R. Parnell, PhD, Department of Human Nutrition
Correspondence: W.R. Parnell, Department of Human Nutrition, University of Otago, PO Box 56, Dunedin, New Zealand. Email: firstname.lastname@example.org
W.R. Parnell and N.C. Wilson were involved in the study design, data collection and primary data analyses of the survey. C. Smith completed these secondary data analyses. The consultant statistician was J. McKenzie.
Table 1. Categorical variables over which supplement users and non-users were compared Variable Categories Derived from Socio-demographics Gender Male; female NZHS-Adult Ethnic group NZ European and others; NZ NZHS Maori; Pacific people NZDep96 I; II; III; IV NZHS quartile Age (years) 15-24; 25-34; 35-44; 45-54; NZHS 55-64; 65-74; 75+ Education No qualifications; school or NZHS post-school qualifications only; school and post- school qualifications Family income Loss/zero-$20 000; $20 001- NZHS 30 000; $30 001- 50 000; $50 000+ Place of Metropolitan; provincial NZHS residence Marital status Single; married/living as NZHS married Dietary habits Following an Normal; alternative (a) NNS-FFQ (QI) alternative diet Fruit and <5 servings of fruit and NNS97-FFQ (Q2 and 3) vegetable intake vegetables per day; [less than or equal to]5 servings of fruit and vegetables per day Cereal intake <7 servings of cereals NNS97-FFQ (Q6) (excludes bread) per week; [less than or equal to] servings of cereals per week Use of fat or Yes; no NNS97-FFQ (Q6) oil to cook chicken or meat Lifestyle/health characteristics Body mass index Underweight; normal; NNS97-anthropometry overweight; obese Smoking status Current smoker; ex-smoker; NZHS-adult never smoked questionnaire (Q59-62) Level of physical Sedentary; relatively NZHS-adult activity inactive; relatively questionnaire active; highly active (Q59-62) Alcohol intake Non-drinker; less than NZHS-general health monthly; two to four questionnaire times per month; two (Q14-15) to three times per week; four times per week Self-reported Very good/excellent; good; NZHS-general health health fair; poor questionnaire (Q1) (a) All diets that did not include 'a variety of all foods, including animal products'. NNS = National Nutrition Survey; NZ = New Zealand; NZHS = New Zealand Health Survey. Table 2. Reported frequency of supplement use in the previous year Vitamin and Frequency Total % minerals % Other supplements % Less than once per month 3 3 3 One to three times per month 3 3 2 Once per week 2 2 1 More than once per week 9 10 9 Daily 43 41 47 (a) Episodically 38 39 36 Don't know 1 1 2 (a) Significantly different from vitamins and minerals (P < 0.05). Table 3. Mean difference (a) in macronutrient intakes from food (derived from 24-hour recall) of supplement users (b) and non-supplement users Males Nutrient Difference 95% CI for the difference Energy (kJ) 208 -318, 734 Fat (g) 0.40 -6.16, 6.97 Carbohydrate (g) 9.74 -5.69, 25.2 Protein (g) -2.34 -8.42, 3.73 Dietary fibre (g) 1.04 -0.48, 2.56 % energy from fat -0.15 -0.91, 0.61 % energy from carbohydrate 0.15 -1.13, 1.44 % energy from protein -0.43 -0.98, 0.11 Females Nutrient Difference 95% CI for the difference Energy (kJ) 122 -235, 481 Fat (g) 0.30 -3.89, 4.50 Carbohydrate (g) 6.22 -3.09, 15.5 Protein (g) 2.20 -1.09, 5.48 Dietary fibre (g) 1.28 (c) 0.45, 2.11 % energy from fat -0.59 -1.30, 0.12 % energy from carbohydrate 0.80 -0.36, 1.96 % energy from protein 0.06 -0.48, 0.60 (a) Difference between the mean intake of supplement users and non-users after adjustments have been made for NZDep96 quartile, ethnic group, age, smoking status and family income. (b) A supplement user is defined as someone who used at least one supplement over the previous year. (c) Mean difference is significantly different between supplement users and non-users (P<0.05). Table 4. Ratio for the geometric means (a) of vitamin and mineral intakes from food of supplement users and non-supplement users Males Females Nutrient Ratio (b) CI for the ratio Ratio CI for the ratio Cholesterol 0.98 0.90, 1.07 1.00 0.92, 1.09 Vitamin C 1.05 0.93, 1.18 1.12 (d) 1.02, 1.23 Folate 1.12 (c) 1.04, 1.21 1.07 (d) 1.02, 1.13 Vitamin 0.98 0.90, 1.07 1.01 0.96, 1.07 [B.sub.12] Vitamin A 1.07 0.95, 1.21 1.00 0.93, 1.09 equivalents Vitamin E 1.05 0.99, 1.12 1.05 0.99, 1.10 [beta]- 1.08 0.90, 1.29 1.09 0.96, 1.24 Carotene Niacin 1.01 0.95, 1.07 1.05 (d) 1.01, 1.10 Riboflavin 1.07 (c) 1.03, 1.12 1.03 1.00, 1.06 Iron 1.0 0.94, 1.06 1.02 0.97, 1.07 Zinc 1.0 0.93, 1.07 1.04 0.98, 1.10 Calcium 1.21 (c) 1.13, 1.30 1.06 0.99, 1.12 (a) The geometric mean is approximately equal to the median and has been used because of the skewed distribution of micronutrient intakes. (b) The ratio equals the geometric mean of supplement users divided by the geometric mean of non-supplement users, hence the closer the ratio is to one the lower the degree of difference. (c) The geometric mean in male supplement users is significantly higher than the geometric mean of non-supplement users (P<0.05). (d) The geometric mean in female supplement users is significantly higher than the geometric mean of non-supplement users (P<0.05). Table 5. Multivariate analysis using logistic regression for predictors of dietary supplement use in the previous year Variable Odds ratio 95% CI Gender Male 1.00 Female 2.31 1.95, 2.74 Age (years) 15-24 1.00 25-34 0.80 0.58, 1.11 35-44 0.58 0.54, 1.02 45-54 0.74 0.41, 0.77 55-64 0.56 0.40, 0.83 65-74 0.50 0.35, 0.70 75+ 0.25 0.19, 0.44 Education School and post-school 1.00 qualifications School qualifications or post-school 0.92 0.75, 1.12 qualifications No qualifications 0.71 0.56, 0.88 Ethnic group NZ European and others 1.00 Maori 0.55 0.43, 0.71 Pacific 0.16 0.11, 0.22 Smoking status Never smoked 1.00 Current smoker 0.68 0.55, 0.85 Ex-smoker 1.15 0.92, 1.42 Alcohol intake Non-drinker 1.00 Less than monthly 1.02 0.79, 1.33 Two to four times per month 1.44 1.09, 1.88 Two to three times per week 1.50 1.11, 2.01 [greater than or equal to]Four times per week 1.45 1.06, 1.98 Dietary habits Follows a normal diet 1.00 Follows an alternative diet 1.70 1.14, 2.51 Does not eat meat or chicken fried or 1.00 roasted in fat/oil Eat meat or chicken fried or roasted 0.83 0.68, 1.01 in fat/oil [greater than or equal to]Seven servings of 1.00 cereal per week <Seven servings of cereal per week 0.83 0.69, 0.98
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|Author:||Parnell, Winsome R.|
|Publication:||Nutrition & Dietetics: The Journal of the Dietitians Association of Australia|
|Date:||Dec 1, 2005|
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