Introduction

Hypertension is a public health problem in both economically developing and developed countries that is associated with cardiovascular disease, morbidity and mortality [1, 2]. It accounts for at least 45% of death from heart disease and 51% of death from stroke [3]. In US, 33% of adults, aged ≥ 20 years, have hypertension [4]. In addition, 25% of Iranian adults are affected that seems to be high [5]. Therefore, detecting the contributing factors in the occurrence of hypertension is of great importance.

Although the etiology of hypertension is unknown, there are some established risk factors for hypertension including obesity, sedentary life style, high fat diet, alcohol consumption, and high intake of salt or salty foods [6,7,8,9,10,11,12,13,14]. In contrast, adherence to healthy dietary patterns such as the dietary approaches to stop hypertension (DASH) dietary pattern decreases the blood pressure, particularly in patients with hypertension [15]. Recently, it has been shown that carbohydrate intake contributes to hypertension [16, 17]. Sugar-sweetened beverages (SSBs) that are rich in refined carbohydrates may have a role. Consumption of SSBs has greatly increased in western nations [18, 19], and is increasing even in developing countries [20, 21] Soft drinks consumption with added sugar has been positively associated with risk of obesity and diabetes in children and adolescents and also with the development of metabolic syndrome in adults [22,23,24]. Furthermore, SSBs consumption has also been related to elevated levels of serum triglycerides and insulin resistance [25]. Consumption of SSBs leads to greater caloric intake, less satiety and overconsumption, and then obesity that is known contributing factor for hypertension [26,27,28]. Therefore, SSBs consumption may affect blood pressure.

Conflicting data are available linking SSBs consumption and hypertension. Chen et al. [29]reported that consumption of SSBs was positively associated with increased risk of hypertension. In addition, such positive association was reported in two prospective and one cross-sectional studies [3, 30, 31]. Additionally, in a survey on Iranian children and adolescents, consumption of junk foods such as SSBs, salty snacks, sweets, and fast foods was positively associated with odds of hypertension [32]. Such evaluation on school students in urban areas of Nanjing, China, revealed similar findings [33]. In contrast, Dhingra et al. [34] reported that SSBs consumption of ≥ 1 serv/day was not associated with increased odds of hypertension. Furthermore, previous studies on the association between SSBs consumption and hypertension were mostly done on western population, and few evidences are available from the understudied region of the Middle East, where the prevalence of hypertension is estimated to be high [5]. Besides, little attention has been laid down on university students who may had different dietary behaviors or dietary pattern compared with general adults. Food choices in university students might be affected by foods’ price, busy daily life and preferences [35]. It has been shown that university students consume lower fruits and vegetables and higher SSBs than general population [36, 37]. Therefore, current study purposed to assess the association between SSBs consumption and hypertension among a large population of Iranian university students.

Materials and methods

Participants

This cross-sectional study was performed in the framework of Mental and Physical Health Assessment of University Student (MEPHASOUS) study which was designed by the Counseling and Health Organization of the Ministry of Science and Technology (CHOMST), Tehran, Iran, in 2012–2013. The purpose of this project was to assess the health issues and behaviors of Iranian university students. Detailed information about the study design, participants, and data collection method has been published elsewhere [28, 38]. Briefly, all students who admitted in 74 governmental eligible universities [from 28 provinces of Iran], affiliated to Ministry of Science and Technology (MST), were invited to participate in this project. Students were included if newly admitted at university and aged 18 years and more. To gather data, students were invited to health center of universities. Data on demographic characteristics, anthropometric measures, medical history and dietary habits were collected from each student. From 657,845 university students who were invited, 84,332 individuals participated in the mentioned project. After excluding those with missing information, 66,634 university students were included in the current analysis. All participants provided signed informed written consent forms. The whole project was ethically approved by the Ministry of Science and Technology, Tehran, Iran.

Dietary habits

A self-administered questionnaire was used to collect data on dietary habits including dietary intake of some food groups as well as breakfast eating pattern. Reliability and validity of this questionnaire had been shown previously [38]. Participants were asked to report frequency of consumption for fruits (rarely, ≤ 1 serv/day, 2–3 serv/day, ≥ 4 serv/day), vegetables (not weekly, 1 serv/wk, 2–3 serv/wk, ≥ 4 serv/wk), dairy (not weekly, 1 serv/wk, 2–3 serv/wk, ≥ 4 serv/wk), fast foods (rarely, 1 time/wk, 2–3 times/wk, ≥ 4 times/wk) and SSBs (< 1 time/wk, 1–2 times/wk, ≥ 3 times/wk) during the last year. The response categories for intake of fruits were different compared with other food groups consumption, because Iranian consume fruits frequently than vegetables, dairy, fast foods and carbonated beverages which are infrequently consumed. Breakfast eating pattern was assessed by a question as follow: “how many days do you eat breakfast in a week”. Participants were asked to answer the question by these response categories: < 1 day/wk, 1–2 days/wk, 3–4 days/wk, ≥ 5 days/wk. Those who consumed breakfast 4 days/wk or less were defined as breakfast skipper. Previous studies revealed that our questionnaire presented validated and reliable data on dietary intakes [28, 39].

Assessment of SSBs consumption

Participants were asked to report frequency of consumption for SSBs by these response categories: < 1 time/wk, 1–2 times/wk, ≥ 3 times/wk. In addition, they were instructed to consider SSBs as soft drinks [cola, sugar-sweetened soda, sweetened cola] and artificially sweetened fruit juices [including natural or industrial fruit juices with added sugars or any types of fruit juices which are available in the market] that commonly used in Iran. Other types of SSBs and soft drinks such as those made with powdered flavoring, non-juice fruit-flavored drinks, sports drinks or sweetened coffee drinks are not of usual intake among Iranian population and therefore, were not assessed in the current study. In fact, some of these drinks are not available in Iran market, and consequently are consumed rarely by Iranian. Students with 3 or more times/wk consumption of SSBs were considered as high consumers. However, this cut-offs for high consumption is different from those reported from Western countries. This discrepancy can be explained by lower intake of SSBs in Iranian compared with Western populations.

Assessment of hypertension

Blood pressure was measured twice in a seated position after a 5-min rest. The average of two measurements was considered as the final systolic and diastolic blood pressure. Hypertension was defined as systolic blood pressure (SBP) of ≥ 140 and diastolic blood pressure of ≥ 90 according to the Seventh Report of the Joint National Committee on Prevention, Detection, Evaluation and Treatment of High Blood Pressure (JNC7) [40].

Assessment of other variables

A self-reported questionnaire was used to gather data on age, gender (male/female), education (advanced diploma/bachelor of science (BSc)/master of science (MSc)/medical science (MD)/philosophy of doctor (Ph.D)), marital status (single/married) and history of diabetes mellitus (yes/no) (31). We considered students who were in advanced diploma, BSc and MD courses as under-graduated and those in MSc and Ph.D courses as graduated students. In addition, students were asked to answer two questions about sleep pattern: “how is your pattern of sleeping and awaking?” and “how many hours do you sleep in a day?”. The response categories for the first question were “regular”, “irregular” and for the second question were as follow: “< 6 h/day”, “6–8 h/day”, “8–10 h/day”, “>10 h/day. To assess physical activity, we asked a question as follow: “how often do you exercise lasting 30 min?” The response categories of this question was “rarely”, “1–2 times/wk”, “3–4 times/wk”, “>5 times/wk”. Students who did exercise 3 times/wk or more were considered as physically active. In addition, we considered those who did exercise 1–2 times/wk as somewhat active and those who did rarely as inactive. In terms of anthropometric measures, weight was measured with minimal clothing and without shoes by analogue scale with a precision of 100 g and height was determined in a standing position without shoes by a tape measure with the nearest 0.5 cm. Body mass index (BMI) was calculated as weight in kilograms divided by height in square meters. We categorized students into four categories based on BMI status: underweight (BMI < 18.5 kg/m2), normal-weight (18.5 ≤ BMI < 25 kg/m2), overweight (25 ≤ BMI < 30 kg/m2) and obesity (BMI ≥ 30 kg/m2).

Statistical analysis

Participants were categorized based on three levels of SSBs consumption as follow: < 1 time/wk, 1–2 times/wk, ≥ 3 times/wk. One-way analysis of variance (ANOVA) was used to compare categories of SSBs consumption in terms of continuous variables. To assess distribution of students in terms of categorical variables including demographic characteristics and dietary habits across categories of SSBs consumption, Chi-square test was applied. Binary logistic regression in different models was used to assess the association between SSBs consumption and hypertension among the whole population and separately by gender (men/women) and BMI status (underweight/normal-weight/overweight/obesity). In the first model, age (continuous) and gender (male/female) were adjusted. Further adjustment was made for marital status (single/married), education (under-graduated/graduated), physical activity (inactive/somewhat active/active), sleep pattern (regular/irregular), diabetes mellitus (yes/no) and breakfast skipping (yes/no) in the second model. In the third model, we additionally adjusted for dietary intake of fruits (rarely, ≤ 1 serv/day, 2–3 serv/day, ≥ 4 serv/day), vegetables (not weekly, 1 serv/wk, 2–3 serv/wk, ≥ 4 serv/wk), dairy (not weekly, 1 serv/wk, 2–3 serv/wk, ≥ 4 serv/wk) and fast foods (rarely, 1 time/wk, 2–3 times/wk, ≥ 4 times/wk). Further control was made for BMI in the last model to see if the association between SSBs consumption and hypertension is independent of obesity. In this analysis, students in the lowest category of SSBs consumption were considered as the reference group. To compute the overall trend of odds ratios across increasing categories of SSBs consumption, we considered these categories as an ordinal variable in the logistic regression models. All statistical analyses were done using SPSS software (version 19.0; SPSS Inc, Chicago IL). P values were considered significant at < 0.05.

Results

In total, 66,634 university students were included in the statistical analysis. In terms of general characteristics, participants who were included in the current study were not significantly different from those who were excluded. Mean age of students was 21.4 ± 3.9 years and 55.5% were female. Overall, 6.3% of total participants were high consumer for SSBs. In addition, hypertension was prevalent among 6.9% of students.

General characteristics and dietary habits of participants across categories of SSBs consumption are shown in Table 1. Participants in the highest category of SSBs consumption were more likely to be younger, physically active, obese and have hypertension, and less likely to be female, married, graduated, diabetic, have regular sleep pattern, sleep 6–8 h/day compared with those in the lowest category. In addition, weight, SBP and DBP were higher in participants in the top category of SSBs consumption than those in the bottom category. In terms of dietary intakes, students in the highest category of SSBs consumption had higher intake of fruits, dairy and fast foods, and lower intake of vegetables compared with those in the lowest category. In addition, greater intake of SSBs was positively associated with breakfast skipping (Table 2).

Table 1 General/anthropometric characteristics of the study subjects
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Table 2 Dietary habits of participants across categories of sugar-sweetened beverages consumption
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Multivariable-adjusted odds ratios and 95% CIs for hypertension across categories of SSBs are indicated in Table 3. Compared with those in the bottom category, university students in the top category of SSBs consumption were more likely to have hypertension (OR 1.71, 95% CI 1.54–1.90). This association remained significant even after adjusting for confounding variables; such that participants in the highest category of SSBs consumption had 2.17 times greater risk of hypertension compared with those in the lowest category (OR 2.17, 95% CI 1.91–2.47). Gender-stratified analysis revealed a significant positive association between SSBs consumption and hypertension in men (OR 2.12, 95% CI 1.79–2.50) and women (OR 2.03, 95% CI 1.67–2.48) (Table 4). Furthermore, stratified analysis based on BMI status indicated such positive association in all categories of BMI including students with underweight (OR 2.53, 95% CI 1.81–3.52), normal-weight (OR 2.13, 95% CI 1.81–2.51), overweight (OR 2.14, 95% CI 1.58–2.89) and obesity (OR 1.59, 95% CI 1.00–2.53).

Table 3 Multivariable-adjusted odds ratios and 95% CIs for hypertension across categories of sugar-sweetened beverages consumption
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Table 4 Stratified analyses for the association between sugar-sweetened beverages consumption and hypertension
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Discussion

In the current study, we found a positive association between SSBs consumption and hypertension. This association was significant even after taking potential confounders into account. Stratified analysis based on gender and BMI status revealed such significant association in both genders and either in participants with normal weight (BMI < 25 kg/m2) or those with overweight or obesity (BMI ≥ 25 kg/m2). To the best of our knowledge, current study was the first in the Middle East to examine the association between SSBs consumption and hypertension.

SSBs are carbonated or noncarbonated beverages containing high amounts of sugar and flavored with natural or artificial additives [41]. Because of the provision of just calorie to the diet, SSBs are usually considered as caloric beverages with adverse effects to human health [42, 43]. These kinds of beverages are rich in calories and generally are consumed as additional food intake [44]. Recently, SSB consumption has been associated with increased risk of hypertension. However, findings about this association are conflicting. Based on our findings, SSBs consumption was positively associated with risk of hypertension. Some studies reported similar results. In a study on high school students, Qin et al. [45] reported that SSBs was a contributing factor in developing childhood hypertension. In a cross-sectional study, SSBs consumption increased the odds of hypertension in children and adolescences [32]. The positive association between SSBs consumption and odds of hypertension was reported among school students of Nanjing, China [33]. Feig et al. [46] showed that an average reduction in SSBs consumption by one serving per day was associated with an average of 1.8 mmHg reduction in systolic blood pressure and an average of 1.1 mmHg reduction in diastolic blood pressure. In a prospective study on Korean adults, those who consumed a high amount of sweetened beverages had greater risk of hypertension [47]. Findings from a cross-sectional study on US adults revealed that SSBs consumption at ideal level was associated with 17% lower odds of hypertension [31]. In contrast, Gui et al. reported no significant association between SSBs consumption and hypertension among adolescences [48]. Lack of significant association in Gui et al. study might be explained by low BMI of individuals participated in this study. In addition, Gui et al. study was done on adolescences, while other studies as well as current study were conducted on adults. Other factors like different study design, sample size, or lack of control for confounding variables might provide some additional reasons for inconsistent results.

In the current study, we adjusted for marital status on the association between SSBs consumption and hypertension. In a cross-sectional study, married individuals had a greater likelihood of nocturnal dipping and lower night-time blood pressure compared with single ones [49]. Another study revealed that being married was associated with higher probability of medication adherence and a lower probability of being a current smoker which both were associated with hypertension [50]. With regard to the SSBs, its consumption might be different between married and single individuals. Education was another variable that was controlled. Individuals with high education level usually have higher health literacy than those with lower education [51]. Health literacy is inversely associated with hypertension and also SSBs consumption [52]. Overall, we can consider marital status and education as important confounders on the association between SSBs consumption and hypertension.

University students have different dietary behaviors and lifestyle compared with general population [36, 37]. Several factors may affect food choices of this population. The most important food choice priorities of them are foods’ price, busy daily life and preferences [35]. Therefore, students may choose cheap or ready-to-eat foods such as fast foods which are considered as unhealthy foods [37, 53]. These foods are mostly consumed with soft drinks such as sweetened cola. Hence, SSBs consumption is estimated to be high among university students. However, the association between SSBs consumption and chronic disease such as hypertension among this population has less been paid attention. It must be kept in mind that consumption of SSBs in Iranian students is lower than those reported from western nations. In the current study, the highest category of SSBs consumption was ≥ 3 times/wk and only 6.3% of participants were in this category, while in prior studies which were done in western nations, intake of ≥ 1 serving/day SSBs was considered as the highest category and 9.6% of participants were in it [3]. Despite the low consumption, we found a positive association between SSBs consumption and hypertension among Iranian students.

Several mechanisms have been presented through which SSBs consumption might contribute to hypertension. Less satiety-inducing effects of SSBs than those of solid foods as well as high energy content of these beverages result in overconsumption and then obesity that is known contributing factor for hypertension [26, 27]. According to this context, we conducted stratified analysis based on BMI status (< 25/≥ 25 kg/m2) to exclude the mediator effect of obesity in the association between SSBs consumption and hypertension. However, we found no attenuation of the risk estimate and the positive association between SSBs consumption and hypertension remained significant in both normal weight participants and those with overweight or obesity. Therefore, obesity does not provide a full explanation of the observed association. It is possible that fructose content of SSBs is involved in increasing blood pressure [54]. Fructose consumption is related to elevation of serum uric acid, thereby inducing reduction of NO and insulin sensitivity [55]. Decreased insulin sensitivity, particularly in obese individuals, can lead to renal sodium retention, activate the renin–angiotensin system, and promote endothelial dysfunction, leading to hypertension [56, 57].

This study has several strengths. As far as we know, this is the first study in the Middle East with a large sample size examining the association between SSBs consumption and hypertension among university students. In addition, we controlled the analyses for a wide range of confounding variables including dietary habits to obtain an independent association. A large sample size enabled us to do stratified analysis by gender and BMI categories. Several limitations should be paid attention when interpreting of our findings. Based on the cross-sectional design of our study, we cannot confer a causal link between SSBs consumption and hypertension. Therefore, our findings should be confirmed by prospective studies. The questionnaire used for dietary data gathering had no information on portion size, and we had collected data about consumption frequency. Therefore, lack of accuracy on the assessment of SSBs intake might increase the rate of misclassification. However, it should be noticed that putting information on portion size in the questionnaire does not add so much, particularly in the case of foods that come in natural units (such as SSBs). In addition most of the variations in dietary intakes have been shown to be obtained by frequency of consumption rather than their portion size. Therefore, information on portion size is relatively unimportant. Furthermore, portion sizes are positively associated with frequency of consumption meaning that some of the information on portion size is already accounted for by frequency of use. Although several confounders were controlled for assessing the association between SSBs consumption and hypertension, further controlling for other variables such as energy intake, psychological disorders and residual lifestyle factors might be needed. In addition, we did not adjust for dietary intake of sodium, salt as well as salty and fatty snacks which may affect our findings. However, intake of some food groups including fast foods which are rich in sodium or salt was controlled in obtained associations. Fast foods are the most commonly used snacks among Iranian adults. However, we cannot exclude the effects of other salty and fatty snacks such as nuts in our findings.

In conclusion, we found that university students in the highest category of SSBs consumption had greater odds for having hypertension compared with those in the lowest category. Such association was seen even after adjusting for potential confounders. Stratified analysis based on gender and BMI status revealed such significant association in all categories of gender and BMI. Further studies, particularly of a prospective nature, are needed to confirm our findings. Overall, it seems that recommendation to reduce the intake of SSBs might be associated with lower chance of high blood pressure. Reducing the consumption of fast foods, which are usually consumed with SSBs, help us to decrease the intake of SSBs.