Subscribe Now Subscribe Today
Abstract
Fulltext PDF
References
Research Article
 
Consumption of Vitamins and Associated Factors by Age Group



Hyo Young Lee, Hyunmi Son and Stephanie D. Short
 
ABSTRACT
Background: The intake of vitamins is generalized, however, studies of their consumption have focused on limited associated factors and has not identified the influencing factors by age. This study was designed to contribute to an understandings of factors related to the use of vitamins by age group. Methodology: This study is a nationally representative cross-sectional, population-based and the sample consisted of 5,915 Korean adults aged 20 years older. Analyses were based on a multiple linear regression model including socioeconomic factors, health status and health behaviours. Results: Many factors are linked to the increased likelihood of using vitamins, including female, high income, living with a spouse, perception of health as bad, smoking, sufficient dietary intake, exercises and attempts to control one’s weight. However, the presence of chronic diseases was not related to the use of vitamins in any age group. The associated factors differed according to age and the number of relevant factors decreased with age. Conclusion: Adults 20-44 years of age, especially who ate enough, require educations related to nutrient intake. Besides the nutrient education could be effective in adults 65 years who have regular exercises. But, people in this age group may need more vitamins than young and middle-aged adults do, they should receive relevant guidelines or information. Regarding the public health education of vitamins intake, among those aged 45-64 years, socioeconomic factors must be considered.
Services
Related Articles in ASCI
Similar Articles in this Journal
Search in Google Scholar
View Citation
Report Citation

 
  How to cite this article:

Hyo Young Lee, Hyunmi Son and Stephanie D. Short, 2017. Consumption of Vitamins and Associated Factors by Age Group. Asian Journal of Epidemiology, 10: 10-18.

DOI: 10.3923/aje.2017.10.18

URL: http://scialert.net/abstract/?doi=aje.2017.10.18
 
Received: September 16, 2016; Accepted: October 26, 2016; Published: December 15, 2016

INTRODUCTION

The use of vitamins has become widespread and is continuing to increase. Historically, people obtained vitamins and minerals from diverse foods. Today, however, nutrients are also obtained from nutrient-fortified foods, vitamin or mineral supplements or other types of vitamins. Vitamins are widely used throughout the world and contribute to the amount of nutrients consume1. Multi-complex vitamins are widely used and the percentage of the Korean population consuming these products increased2 from 25.8% in 2005 to 34.1% in 2008. This is comparable to the 35% of the US general population who report regular use of multivitamins as vitamins3.

Magazine or television advertisements for vitamins appear to promise that these supplements improve your health and well-being, prevent illness or increase your lifespan. However, not everyone needs vitamins; only people who do not obtain the necessary nutrients in their regular diet require supplements. Moreover, the amount of additional nutrients needed differs by age and symptomatology4-6.

Previous studies have reported inconsistent results7,8 or have focused on a limited number of factors associated with the use of vitamins3 have examined unrepresentative samples of adults or specific age/gender groups9,10 have not identified differences by age5,6,10 or have included few socioeconomic variables6,10. Thus, this study investigated whether and to what extent socioeconomic factors, health status and health behaviours influenced the use of vitamins by different age groups. If such factors vary by age, different guidelines or nutrient-related education programs will be needed for different age groups. Therefore, this study was designed to contribute to an understandings of factors related to the use of vitamins by age group.

MATERIALS AND METHODS

Data sources and study population: Data were obtained from the 5th Korean National Health and Nutrition Examination Survey (KNHANES-V), which was conducted in 2010. KNHANES is a nationally representative cross-sectional, population-based study conducted by the Korea Center for Disease Control (KCDC) and approved by the KCDC Institutional Ethics Review Board No. 2010-02CON-21-C. All participants provided written informed consent before data collection11. Survey participants were selected using a complex stratified, multistage probability sampling method and the response rate was 81.9%11. Of the total 8958 individuals who participated, 5,915 Koreans aged 20 years or older were included in this study. Participants were interviewed by trained interviewers. Data about the presence of diseases were based on diagnostic evaluations performed by physicians. Height and weight for the assessment of obesity (>25 kg m–2) were measured at a mobile medical centre.

Variables
Outcome variable:
The outcome variable, the regular weekly consumption of vitamins was measured with the following question: "During the most recent year, did you regularly consume vitamins on a weekly basis?" Participants could respond with "yes" or "no."

Socioeconomic variables: This study examined a wide array of independent variables in three categories. The first included socioeconomic factors: Age group (20-44, 45-64 and 65 years older), sex (male, female), educational level (middle school or less, high school, college or higher), income (low, middle low, middle high and high), occupation (office work, farmer or fisher, physical labourer, student/housewife, unemployed), marriage (living with spouse, not living with spouse) and private health insurance (no, yes).

Health status: The second set of independent variables dealt with health status, including subjective (perceived health and body image) and objective health status (activity limitation, hypertension, diabetes, high levels of triglycerides and obesity). The following questions were used: "In general, how would you rate your health?" and "In general, how would you rate your body image?". In terms of health status, responses were divided into "good" (excellent/good), "moderate" (fair) and "bad" (poor/very poor). With regard to body image, responses were categorised into thin, moderate and obese. In terms of objective health, the presence of the following conditions was examined and coded as "present" or "absent": activity limitations, hypertension, diabetes, high levels of triglycerides and obesity.

Health-related behaviours and other factors: The third dimension involved the following health-related behaviours: Alcohol consumption (no, yes), smoking (no, yes), exercises (no, yes), daily activity level (low, average, intense), attempts at weight control during the past year (no, yes), eating patterns at regular meals (eating enough, eating a limited variety of foods, insufficient) and having knowledge about nutrients.

Analysis: Descriptive statistics were calculated and multiple logistic regression analysis was performed to explore the factors associated with the use of vitamins, using the SPSS software (version 21.0). All analyses relied on the pooled weights of sampling clusters to yield statistics that were representative of the Korean adult population. The results are presented as weighted percentages and as Odds Ratios (ORs) with Confidence Intervals (CIs). The Х2-tests were performed to examine differences in the prevalence of using vitamins by participant characteristics according to age group.

RESULTS

Socioeconomic factors associated with using vitamins: About half of the total sample were male and 20-44 years of age. A total of 41.2% of all respondents used vitamins, 38.1% for 20-44 years, 46.5% for 45-64 years and 38.8% for 65 years and older. The use of vitamins significantly differed by all other socioeconomic factors, health behaviours and several health-status variables independent of age (Table 1).

In terms of socioeconomic factors, users of vitamins who were 20-44 years of age were more likely to be female (46.0%), not live with a spouse (42.2%), earn a high income (45.3) have college education or higher (42.7%), be employed as office workers (41.5%) and have private health insurance (38.2%). Non-users in this age group were more likely to be male (69.5%), live with a spouse (67.8%) have a low income (66.4%), be a high school graduate (70.6%), work as a physical labourer (75.1%) and have non-private health insurance (63.4%). Other age groups showed similar patterns (Table 1).

Subjective and objective health status associated with use of vitamins: With regard to health status, the users of vitamins who were 20-44 years of age were more likely to perceive their health as bad (43.9%), not be obese (33.5%) and not have high levels of triglycerides (39.6%). Users in the 45-64 year old group were more likely to view their body as obese (50.2%) have no limitations on their activities (47.5%) and not have high levels of triglycerides (49.4%). However, users who were at least 65 years of age differed significantly from non-users only with regard to body image (Table 1).

Health behaviours associated with use of vitamins: Regarding health behaviours associated with use of vitamins by age, use was most prevalent among those who did not drink alcohol, smoke or perform exercises and those who had average activity levels had attempted to control their weight, ate appropriately and had knowledge about nutrition, among those aged 20-44 and 45-64 years. Among those aged at least 65 years, users of vitamins did not differ from non-users in terms of alcohol consumption and smoking (Table 1).

Odds ratios and 95% confidence intervals of vitamin intake by age group: After adjusting for all independent variables, multiple logistic regressions showed that the factors associated with the use of vitamins differed significantly by age. Among those 20-44 years of age, the use of vitamins differed according to many socioeconomic, health-status and behavioural factors. However, use of vitamins differed primarily according to socioeconomic factors among those 45-64 years of age. Among those at least 65 years of age, the use of vitamins differed only according to participation in exercises (Table 2).

Of those 20-44 years of age, women were 1.38 times (95% CI: 1.07-1.77) more likely than men and those with a high income were more than 1.38 times (95% CI: 1.03-1.74) more likely than those with a low income to use vitamins. Participants who assessed their health as "bad" were more likely to use vitamins than were those who assessed it as "good/moderate" and those who perceived themselves as "thin" were more likely to take vitamins than were those who perceived themselves as "moderate." Moreover, people who were not obese were 1.49 times (95% CI: 1.13-1.95) more likely to use vitamins compared with the obese group. Respondents who did not consume alcohol were 1.40 times (95% CI: 1.13-1.72) more likely to use vitamins than those who did and smokers were 1.34 times (95% CI: 1.04-1.74) more likely than were non-smokers to use vitamins. Additionally, most participants who regularly engaged in exercises and tried to control their weight were more likely to use vitamins compared with who did not engage in exercises or not try to control their weight. However, those who ate enough or who ate limited varieties of food were 3.49 times (95% CI: 1.34-9.10) more likely and 3.53 times (95% CI: 1.36-9.17), respectively to take vitamins compared with the insufficient eating group.

The socioeconomic factors associated with the use of vitamins among those 45-64 years old were similar to those associated with such use among those aged 20-44 years. However, among those with private health insurance were 1.37 times (95% CI: 1.10-1.85) more likely than those with non-private health insurance to take vitamins. Only one factor was related to using vitamins among those at least 65 years of age: respondents in this age group who participated in exercises on a weekly basis were 1.76 times (95% CI: 1.23-2.52) more likely to take vitamins than were those who did not.

Table 1: Characteristics of the study population by vitamin use and age groups n (Percentage among age groups)



Table 2:Vitamin use by age groups in multiple logistic regression analysis: odds ratios and 95%Cis
Odds ratios adjusted for all other variables in the model and their 95% CIs, *Significance at p<0.05

DISCUSSION

This study focused on the factors related to the use of vitamins and examined whether and to what extent the influence of such factors differed by age. The results suggested that socioeconomic factors are related to the use among those 20-64 years of age. Besides, the associated factors differed across age groups. However, the presence of chronic diseases was not related to the use of vitamins in any age group. Those aged 20-44 years who usually ate enough were more likely to take vitamins compared with the insufficient eating group. Higher educational level was associated with taking vitamins only among those 44-64 years of age and having a high income had a stronger influence on the use of vitamins among this age group than it did on that of those 20-44 years of age. Among those 65 years of age and older, the only associated factor was regular participation in exercises. Because people in this age group may need more vitamins than young and middle-aged adults do, they should receive relevant guidelines or information.

Paradoxically, those aged 20-44 year who usually ate enough were more likely to use vitamins compared with the insufficient eating group. This result is consistent with the previous study, who found that those who took vitamins had higher recorded nutrient intake from food sources alone than did those who did not take vitamins12. Besides, a study in Japan also identified that vitamin use tends to be associated with interest in aspects of Western lifestyle as an intake of energy5. This is usually known as the "Inverse supplement hypothesis9". However, this paradox was not observed among other age groups, even among those whose dietary intake was similar to or higher. This result suggests those who most needed to use vitamins were those who were least likely take to them. Thus, relevant guidelines should be emphasised and public health educational efforts should be intensified10.

Almost all the participants aged 65 or older had low levels of education, which may have affected their use of vitamins. Higher educational level was strong related to the use of vitamins, but this relationship was observed only among those 45-64 years of age in the present study3. Moreover, education was not among the many factors that influenced the use of vitamins among those 20-44 years of age. The use of multivitamin supplements was associated with the characteristics of a healthy lifestyle13 and dietary supplement intake was also associated with physical activity14. The current emphasis on well-being among young adults may influence their consumption of vitamins regardless of their educational level. The popularity of this trend among this cohort and their greater receptivity may render the influence of education weaker than it is among those 45-64 years of age. Additionally, having private health insurance may reflect the health-related concerns of those aged 45-64 years. Indeed, the mean age of applicants for private health insurance was about 50 years and that these applicants usually expressed considerable concern about their health15. Specifically, the use of vitamins is associated with healthy lifestyles and concern about one’s health and these factors may be an indicator of engagement in healthy behaviours15.

Only one factor was significantly associated with the use among elderly individuals. However, it is important to note that the rate at which elderly individuals used vitamins was similar that for those aged 20-44 years despite the fact that the need for such products should be higher among the former group. Elderly individuals tended to obtain vitamins for reasons related to aging (consuming lower levels of nutrients, difficulties with eating). Moreover, vitamins are a very popular gift in Korea and children frequently give vitamins to their parents as a sign of respect or as a means of celebrating their parents’ birthdays, parents’ day, national holidays and visits to their parents’ home. Thus, the use of vitamins among elderly individuals is related to several factors. Widespread and comprehensive education should be directed at this age group because older adults may benefit more from specific supplements. Additionally, elderly individuals who engaged in regular exercises on a weekly basis were 1.76 times more likely to take supplements than were those who did not regardless of socioeconomic and health status. Thus, participation in exercises reflected concern about health among elderly individuals in all subgroups.

The relationships of smoking with vitamins use was opposite that between alcohol consumption and supplement use. Smokers were likely to use multivitamins to compensate for the loss of nutrients through smoking because smoking increases oxidants and vitamins provide antioxidants, which are known for their anti-aging properties16. Thus, smokers should take more vitamins16. A study about the use of vitamins among women found that those who did not consume alcohol and those who smoked were more likely to take vitamins9. These relationships should be explored further in future prospective studies. In contrast, some studies indicated that former smokers tended to use vitamins more than current smokers17 and dietary supplement users were less likely to smoke18,19. Patterns of dietary supplement use differed by type of drinks consumed17 and dietary supplement use has been reported to be linked to higher alcohol consumption19, obesity and young age7,20.

In females, a lower BMI, participation in physical activity and non-manual social class were associated with the use of vitamins21. A study of the use of vitamins in the US found that being female and a having higher level of education were associated with the use3. Those in the high socioeconomic class used vitamins more and certain socioeconomic factors and lifestyle factors were necessary for preventive nutrition22. Additionally, some studies have found that multivitamins may reduce the risk for chronic diseases, especially cancers23, but a study reported that vitamins did not have a beneficial effect on almost any chronic disease24. Recent studies have described inconsistencies in the benefits of vitamins for chronic diseases8,24. Besides, there was no difference in morbidity between the supplement groups in either older males or females25. Our study found no relationship between using vitamins and chronic diseases. If the main purpose of the use of multivitamin supplements by the general adult population involves the primary prevention of diseases, it should be known that these supplements may not be helpful with regard to chronic conditions. However, this study used a cross-sectional design, examined only several chronic diseases and did not analyse the data according to type of vitamin. Thus, further studies in this domain are warranted.

Before discussing the factors related to taking vitamins, we will address the effectiveness of and possibility of a harmful effect on the body resulting from the use vitamins26. The study about dietary supplement use for weight loss in obese adults, showed that both the dietary supplement group and the placebo group increased their self-efficacy, even though the mechanisms of the placebo effects were fully identified27. Indeed, dietary supplement use in lower-income adults had barriers of knowledge, income, accessibility, health and preferences28. If people used vitamins despite their having appropriate eating habits, vitamins may be useless or lead to the inefficient absorption and use of nutrients from foods. Additionally, when people consume large amounts of vitamins from two or more sources simultaneously, micronutrients may be overused and vitamins may build up. Nutrient-related education and effective guidelines targeted at specific groups of users should be provided to ameliorate these problems. When used according to such guidelines by social marketing, vitamins can be very helpful29.

This study has several limitations. First, we did not identify the kinds of nutrients used by participants. Data was not gathered regarding the kinds of vitamins used by subjects, because more than 70% of Koreans usually take vitamins and mineral supplements and they refer to these in a generic manner6. Thus, respondents generally answered with a simple "yes" or "no" to the question about the use of these products. Nevertheless, information about the characteristics of those who use vitamins should be helpful, especially our data regarding the factors related to vitamin use among different age groups in a nationally representative sample. Second, we could not discuss the effectiveness of vitamins for specific symptoms or dietary insufficiencies. Additional studies should examine the effect of vitamins on health, including the frequency of use and the prevalence of different effects.

CONCLUSION

Adults 20-44 years of age require additional and more specific interventions related to nutrient intake, because those in this age group tended to consume vitamins even though they usually ate enough, which could lead to intoxication or harmful somatic effects. Additionally, older people also require guidelines for making rational choices regarding increased needs of dietary supplements use. We must strengthen nutrition-related education and guidelines about eating appropriately directed at the public. It should be also needed to design and publicize an effective nutrition-related educational program for those who are interested in learning more about this important topic.

SIGNIFICANT STATEMENTS

This study focused on the factors related to the use of vitamins and examined whether and to what extent the influence of socioeconomic status, health status and/or health behaviours differed by age
Socioeconomic factors are related to the use among those 20-64 years of age and the associated factors differed across age groups. Higher educational level was associated with taking vitamins only among those 44-64 years of age and having a high income had a stronger influence on the use of vitamins among this age group
The presence of chronic diseases was not related to the use of vitamins in any age group
Those aged 20-44 years who usually ate enough were more likely to take vitamins compared with the insufficient eating group

ACKNOWLEDGMENTS

This study was supported by Dongseo University Professor Research Grant, 2014 and the National Research Foundation of Korea Grant funded by the Korean Government (NRF-2014S1A2A1A01025886).

REFERENCES
Bailey, R.L., J.J. Gahche, C.V. Lentino, J.T. Dwyer and J.S. Engel et al., 2010. Dietary supplement use in the United States, 2003-2006. J. Nutr. 10.3945/​jn.110.133025

Brownie, S., 2005. Characteristics of older dietary supplement users: Review of the literature. Aust. J. Ageing, 24: 77-87.
CrossRef  |  Direct Link  |  

Denison, H.J., K.A. Jameson, H.E. Syddall, E.M. Dennison, C. Cooper, A.A. Sayer and S.M. Robinson, 2012. Patterns of dietary supplement use among older men and women in the UK: Findings from the hertfordshire cohort study. J. Nutr. Health Aging, 16: 307-311.
CrossRef  |  Direct Link  |  

Dorant, E., P.A. Van den Brandt, A.M. Hamstra, M.H. Feenstra, R.A. Goldbohm, R.J. Hermus and F. Sturmans, 1993. The use of vitamins, minerals and other dietary supplements in The Netherlands. Int. J. Vitamin Nutr. Res., 63: 4-10.
PubMed  |  Direct Link  |  

Earnest, C., K.H. Cooper, A. Marks and T.L. Mitchell, 2002. Efficacy of a complex multivitamin supplement. Nutrition, 18: 738-742.
CrossRef  |  Direct Link  |  

George, G.C., D.M. Hoelscher, T.A. Nicklas and S.H. Kelder, 2009. Diet-and body size-related attitudes and behaviors associated with vitamin supplement use in a representative sample of fourth-grade students in Texas. J. Nutr. Educ. Behav., 41: 95-102.
CrossRef  |  Direct Link  |  

Gregory, J., K. Foster, H. Tyler and M. Wiseman, 1990. The Dietary and Nutritional Survey of British Adults. HMSO Publications Centre, USA.

Hoggatt, K.J., 2003. Commentary: Vitamin supplement use and confounding by lifestyle. Int. J. Epidemiol., 32: 553-555.
CrossRef  |  Direct Link  |  

Huang, H.Y., B. Caballero, S. Chang, A.J. Alberg and R.D. Semba et al., 2006. The efficacy and safety of multivitamin and mineral supplement use to prevent cancer and chronic disease in adults: A systematic review for a National Institutes of Health state-of-the-science conference. Ann. Internal Med., 145: 372-385.
CrossRef  |  PubMed  |  Direct Link  |  

Hulshof, K.F., M.R. Lowik, F.J. Kok, M. Wedel, H.A. Brants, R.J. Hermus and F. Ten Hoor, 1991. Diet and other life-style factors in high and low socio-economic groups (Dutch nutrition surveillance system). Eur. J. Clin. Nutr., 45: 441-450.
PubMed  |  Direct Link  |  

Ishihara, J., T. Sobue, S. Yamamoto, S. Sasaki and S. Tsugane, 2003. Demographics, lifestyles, health characteristics and dietary intake among dietary supplement users in Japan. Int. J. Epidemiol., 32: 546-553.
CrossRef  |  Direct Link  |  

KMHW and KCDCP, 2012. Guidelines for using raw data of the Korean National Health and Nutrition Examination Survey, the first and second survey of the fifth phase. Korean Ministry of Health and Welfare (KMHW), Korea Center for Disease Control and Prevention (KCDCP).

KMHW., 2012. The 2nd wave plan for the development of Korean oriental medicine. Korean Ministry of Health and Welfare (KMHW), February 3, 2012.

Kirk, S.F., J.E. Cade, J.H. Barrett and M. Conner, 1999. Diet and lifestyle characteristics associated with dietary supplement use in women. Public Health Nutr., 2: 69-73.
CrossRef  |  Direct Link  |  

Lee, H.B. and K.R. Hyun, 2011. A study of the characteristics and factors of private health insurers. Korean Soc. Secur. Stud., 27: 217-240.

McNaughton, S.A., G.D. Mishra, A.A. Paul, C.J. Prynne and M.E. Wadsworth, 2005. Supplement use is associated with health status and health-related behaviors in the 1946 British birth cohort. J. Nutr., 135: 1782-1789.
PubMed  |  Direct Link  |  

Mullie, P., P. Clarys, M. Hulens and G. Vansant, 2011. Socioeconomic, health and dietary determinants of multivitamin supplements use in Belgium. Int. J. Public Health, 56: 289-294.
CrossRef  |  Direct Link  |  

Murphy, S.P., K.K. White, S.Y. Park and S. Sharma, 2007. Multivitamin-multimineral supplements' effect on total nutrient intake. Am. J. Clin. Nutr., 85: 280S-284S.
Direct Link  |  

Ock, S.M., S.S. Hwang, J.S. Lee, C.H. Song and C.M. Ock, 2010. Dietary supplement use by South Korean adults: Data from the national complementary and alternative medicine use survey (NCAMUS) in 2006. Nutr. Res. Practice, 4: 69-74.
CrossRef  |  Direct Link  |  

Radimer, K., B. Bindewald, J. Hughes, B. Ervin, C. Swanson and M.F. Picciano, 2004. Dietary supplement use by US adults: Data from the national health and nutrition examination survey, 1999-2000. Am. J. Epidemiol., 160: 339-349.
CrossRef  |  Direct Link  |  

Radimer, K.L., A.F. Subar and F.E. Thompson, 2000. Nonvitamin, nonmineral dietary supplements: Issues and findings from NHANES III. J. Am. Dietetic Assoc., 100: 447-454.
CrossRef  |  Direct Link  |  

Rautiainen, S., L. Wang, J.M. Gaziano and H.D. Sesso, 2014. Who uses multivitamins? A cross-sectional study in the physicians' health study. Eur. J. Nutr., 53: 1065-1072.
CrossRef  |  Direct Link  |  

Rock, C.L., 2007. Multivitamin-multimineral supplements: Who uses them? Am. J. Clin. Nutr., 85: 277S-279S.
Direct Link  |  

Sien, Y.P., N. Sahril, M.H.A. Mutalip, N.A.M. Zaki and S.A. Ghaffar, 2014. Determinants of dietary supplements use among adolescents in Malaysia. Asia-Pacific J. Public Health, 26: 36S-43S.
CrossRef  |  PubMed  |  Direct Link  |  

Smith, C., N.C. Wilson and W.R. Parnell, 2005. Dietary supplements: Characteristics of supplement users in New Zealand. Nutr. Dietetics, 62: 123-129.
CrossRef  |  Direct Link  |  

Tippens, K.M., J.Q. Purnell, W.L. Gregory, E. Connelly, D. Hanes, B. Oken and C. Calabrese, 2014. Expectancy, self-efficacy and placebo effect of a sham supplement for weight loss in obese adults. J. Evidence-Based Complementary Alternat. Med., 19: 181-188.
CrossRef  |  Direct Link  |  

Van der Horst, K. and M. Siegrist, 2011. Vitamin and mineral supplement users. Do they have healthy or unhealthy dietary behaviours? Appetite, 57: 758-764.
CrossRef  |  Direct Link  |  

Warnick, E., K.A. Dearden, S. Slater, B. Butron, C.F. Lanata and S.L. Huffman, 2004. Social marketing improved the use of multivitamin and mineral supplements among resource-poor women in Bolivia. J. Nutr. Educ. Behav., 36: 290-297.
CrossRef  |  Direct Link  |  

Whiting, S.J., H. Vatanparast, J.G. Taylor and J.L. Adolphe, 2010. Barriers to healthful eating and supplement use in lower-income adults. Can. J. Dietetic Practice Res., 71: 70-76.
CrossRef  |  Direct Link  |  

©  2018 Science Alert. All Rights Reserved
Fulltext PDF References Abstract