In this retrospective infodemiological study, we ranked the global popularity of searches on topics related to dietary supplements among Google users. The analysis revealed significant differences in interest in topics across different countries, as well as over time.
4.1. Main Findings
NHANES is currently the largest investigation into the prevalence of supplement use. Supplement use among US citizens has remained stable from 1999 to 2012 [4
], but global sales (including the US) of dietary supplements increased in the years 2011–2016, which may be due to a further increase in the prevalence of supplement use or to an increase in the number of products being taken by each consumer [28
]. In the NHANES study, the most commonly used supplements were multivitamins and multiminerals [4
]. In the years 2005–2012, the ten most popular of these, in order of decreasing prevalence of use, were vitamin D, vitamin C, calcium, cobalamin, vitamin E, folic acid, pyridoxine, niacin, vitamin A, and riboflavin. In the consumer survey performed by the Council for Responsible Nutrition in 2019, the ten most popular dietary supplements among US adults were multivitamins, vitamin D, vitamin C, protein, calcium, vitamin B or vitamin B complex, omega-3 fatty acids, green tea, magnesium, probiotics, iron, vitamin E, and turmeric [29
]. The ranking of the popularity of dietary supplements among Google users partially reflects the data from the NHANES study and the consumer survey. The difference may be associated with the number of ingredients considered in real-world studies. Moreover, our study concerns global interest in dietary supplements, while the NHANES study is limited to the US population. Nevertheless, it seems that vitamins and minerals are both the most commonly searched for among Google users and the most commonly consumed supplements.
The popularity of the topics examined here was related to the number of PubMed publications on them. This association may be bilateral: the online discourse may be stimulated by new studies that are reported by news media; inversely, high consumption of certain supplements, and high interest in them, may motivate researchers to investigate their properties and efficacy.
The between-country differences show a high degree of diversity in interest among Google users. It is hard to clearly explain the observed popularity rank. We speculate that the greatest interest in certain dietary supplements might be associated with the geographical distribution of herbal supplements (such as aloe vera, mangosteen) or with high latitude (as with the great popularity of vitamin D in Norway). However, the great popularity of iron in France and Japan cannot be explained by the frequency of iron deficiency in those countries, both of which have a low-to-moderate prevalence of iron deficiency anemia [30
]. We determined the most popular dietary supplement topic for each country and listed the ten most common of these. A valid interpretation of these rankings would require a profound knowledge of local markets, cultural circumstances, and internet discourse. We here aim to present the utility of GT and to encourage readers to interpret the rankings and make use of the results in their own investigations.
The RSV of most topics and categories increased over the study period. This suggests that general interest in dietary supplements is growing among Google users. This is not surprising, as the market for dietary supplements, as well as the number of investigations into dietary supplements, continues to increase [1
Search trends are vulnerable to media clamor. We identified news items that could explain the peaks in Figure 4
. The publication of the World Health Organization’s recommendations on salt, sodium, and potassium consumption on 1 February 2011 may explain the peak of interest in potassium in February 2011 [31
]. There is a distinct peak in the interest in dietary fiber in March 2007, which could be explained by the publication on 12 March 2007 of King et al. on the effects of high fiber intake on C-reactive protein level; the article has been cited 220 times to date [32
]. Similarly, a review on the positive effects of garlic on cardiovascular disease was published in April 2013 [33
]. The review of Qidwai and Ashfag, which has been cited 89 times to date, may have stimulated news portals to write about garlic. The peak in search queries about iodine may be associated with the Fukushima Daiichi nuclear disaster of March 2011. The need for iodine intake to prevent thyroid cancers in individuals exposed to nuclear radiation was widely reported on [34
]. The peak in searches in vitamin E in November 2004 may be related to the online publication on 10 November 2004 of a meta-analysis showing that high doses of this vitamin may increase all-cause mortality [35
]. This research was widely discussed, and the media clamor may have led to an increase in interest in vitamin E.
We found that interest in all of the dietary supplement categories showed seasonality. Most Google users live in the northern hemisphere, so seasonal variation might be driven by users from Northern America, Northern Africa, Europe, and Asia [14
]. The greatest interest was observed mainly in February and March. We hypothesize that this may be associated with the onset of seasonal outdoor activities as well as with a personal decision to lose weight or live more healthily. Search volume was lowest during December, which may be related to Christmas and winter holidays. A similar observation has been reported elsewhere [21
]. However, many of the topics related to dietary supplements had the lowest interest during June, July, and August, which are the warm months in the northern hemisphere. This might be partly associated with the absence of indications (e.g., for vitamin D), the lower prevalence of common cold (for vitamin C), and the holiday period, when some users may not take time to seek information on dietary supplements. Also, a substantial number of searches over the year may be generated by scholars who are on holiday during these months, leading to a lower interest in certain topics.