Metabolic syndrome (MetS), as a clustering of metabolic abnormalities, is a well-known risk factor for CVD, type 2 diabetes and early mortality. The growing trend in the prevalence of MetS is a global health problem in both developed and developing nations( Reference O’Neill and O’Driscoll 1 ). Several dietary and non-dietary factors have been involved in the pathogenesis of MetS. Among others, obesity, unhealthy food intake, lack of physical activity and smoking contribute extensively to these conditions( Reference Al Thani, Al Thani and Al-Chetachi 2 ). Elevated concentrations of inflammatory biomarkers have also been suggested as a possible mediating mechanism through which dietary intakes and obesity might affect the risk of chronic disorders.
Dietary energy density (DED) is defined as the amount of energy per unit weight of food( Reference Kant and Graubard 3 ). High-energy-dense diets are associated with higher energy intake and weight gain( Reference Vernarelli, Mitchell and Rolls 4 – Reference Rouhani, Haghighatdoost and Surkan 9 ). Excess body weight and fat mass are linked to greater inflammatory markers and chronic diseases. High-energy-dense diets are characterized mainly by low consumption of fresh fruits and vegetables but high consumption of fats and refined carbohydrates( Reference Esmaillzadeh and Azadbakht 10 , Reference Wang, Luben and Khaw 11 ). Although the association of dietary intakes with metabolic abnormalities has frequently been examined, data on the association of DED with chronic diseases are scarce. Examining the association of DED with chronic diseases could be more reliable than single dietary components, since it accounts for the enormous interactions between nutrients and foods.
Available evidence assessing the association between DED and risk of MetS is limited and inconsistent. While some epidemiological investigations indicated a positive link between DED and the odds of diabetes, insulin resistance, MetS and abdominal adiposity( Reference Esmaillzadeh and Azadbakht 10 , Reference Esmaillzadeh, Boroujeni and Azadbakht 12 , Reference Wang, Zhang and Sun 13 ), others failed to find such significant relationships( Reference Vernarelli, Mitchell and Rolls 4 , Reference van den Berg, van der and Spijkerman 14 ). Besides cardiometabolic abnormalities, few studies have examined the association of DED with inflammation( Reference Vernarelli, Mitchell and Rolls 4 , Reference Melanson, Summers and Nguyen 7 , Reference Tabesh, Hosseinzadeh and Tabesh 15 ). Despite a slight decrement in serum concentrations of C-reactive protein (CRP) following the consumption of a low-energy-dense diet in a short-term clinical trial( Reference Melanson, Summers and Nguyen 7 ), findings from a large population-based study and an 8-week clinical trial revealed no significant correlation between DED and serum high-sensitivity (hs)-CRP or other inflammatory adipokines( Reference Vernarelli, Mitchell and Rolls 4 , Reference Tabesh, Hosseinzadeh and Tabesh 15 ).
It must be kept in mind that previous studies on diet–inflammation–CVD associations were mostly conducted in Western countries and limited data are available in this regard in the understudied region of the Middle East. Different dietary patterns and lifestyle in this part of the world might reveal different associations between energy density and these conditions. Due to low intakes of fruits and vegetables and high intake of refined grains, Middle Eastern populations consume more energy-dense diets than Europeans( Reference Esmaillzadeh and Azadbakht 10 , Reference Wang, Luben and Khaw 11 ). Therefore, assessing the association between DED and these cardiometabolic abnormalities is particularly relevant in these populations. We hypothesized that higher DED is associated with increased risk of MetS as well as higher levels of inflammatory biomarkers, and conducted the present study to identify the association of DED with inflammation and MetS in a large group of Iranian female nurses.
Materials and methods
Participants
The current cross-sectional study was conducted in a representative sample of Isfahani female nurses aged >30 years. To perform random sampling, we provided a list of all hospitals (both public and private) in different parts of Isfahan. Then, we randomly selected some hospitals considering the number of private and public hospitals in each area (n 12). Nurses in the selected hospitals were listed and then a random sample of nurses in each hospital was chosen based on the number of nurses working in that hospital. Totally, 1102 nurses of 1196 invited nurses agreed to participate in our study. We excluded individuals who had a prior history of CVD, diabetes, cancer and stroke (n 11), or infection (n 3). In addition, those who had left at least seventy items blank on the FFQ (n 12), those who reported a total daily energy intake outside the range of 3347–17 573 kJ (800–4200 kcal; n 29) and those who were taking medications that affect glucose homeostasis (n 11) were excluded. After these exclusions, the current analysis was done on 1036 nurses. All participants declared their willingness to participate in the current study by providing a written informed consent. The study was approved by the research council and ethics committee of the School of Nutrition and Food Science, Isfahan University of Medical Sciences, Isfahan, Iran.
Assessment of dietary intake
Dietary data were collected using a Willett-format( Reference Willett 16 ), dish-based, 106-item semi-quantitative FFQ that was designed and validated specifically for Iranian adults. Detailed information about the design, the foods included( Reference Keshteli, Esmaillzadeh and Rajaie 17 ) as well as the validity of this questionnaire to assess the association between food consumption and MetS( Reference Zaribaf, Falahi and Barak 18 ) has been reported elsewhere. Briefly, the questionnaire contained five categories of foods and dishes: (i) mixed dishes (cooked or canned, twenty-nine items); (ii) starchy foods (different types of bread, cakes, biscuits and potato, ten items); (iii) dairy products (dairies, butter and cream, nine items); (iv) fruits and vegetables (twenty-two items); and (v) miscellaneous food items and beverages (including sweets, fast foods, nuts, desserts and beverages, thirty-six items). To develop the questionnaire, a comprehensive list of foods and dishes commonly consumed by Iranian adults was constructed. Then, we chose those foods that were nutrient-rich, consumed reasonably often or contributed to between-person variations. This process led to the remaining of the 106 food items in the questionnaire. The portion size for food items and mixed dishes was defined based on the most commonly consumed portion size for each item in the general population. To increase precision and accuracy of estimates, we attempted to give the portion size of foods and mixed dishes as a unit with the same perception for all people. Participants were asked to report their dietary intakes of foods and mixed dishes based on nine multiple-choice frequency response categories varying from ‘never or less than once a month’ to ‘12 or more times per day’. The number of frequency response categories was not constant for all foods. For foods consumed infrequently, we omitted the high frequency categories, while for common foods with a high consumption, the number of multiple-choice categories increased. The frequency response categories for the food list varied from six to nine choices. For instance, the frequency response for tuna consumption included six categories, as follows: never or less than once per month, 1–3 times/month, once per week, 2–4 times/week, 5–6 times/week, 1–2 times/d; and for tea consumption the frequency response included nine categories, as follows: never or less than 1 cup/month, 1–3 cups/month, 1–3 cups/week, 4–6 cups/week, 1 cup/d, 2–4 cups/d, 5–7 cups/d, 8–11 cups/d, ≥12 cups/d. Finally, we computed daily intakes of all food items and then converted to grams per day using household measures( Reference Ghaffarpour, Houshiar-Rad and Kianfar 19 ). Daily intakes of nutrients for each participant were calculated using the US Department of Agriculture’s national nutrient databank. Total energy intake was calculated by summing up energy intakes from all foods. Total weight of foods consumed by participants was calculated by summing up the weights of foods only. We did not consider weight of drinks consumed because findings from available publications supporting the effect of DED on body weight are based on changes in weight of food intake, not drinks( Reference Ledikwe, Blanck and Khan 20 ). In a systematic review Johnson et al. also concluded that focusing on weight of food consumed reduces the variability in the results and facilitates data interpretation( Reference Johnson, Wilks and Lindroos 21 ). To calculate DED, we divided each individual’s reported daily energy intake (kcal/d) by the total weight of foods consumed (g/d).
Assessment of biochemical indicators
To quantify biomarkers of inflammation and metabolic profile, we collected blood samples after a 12 h overnight fast. Fasting plasma glucose was assessed on the day of blood sampling via an enzymatic colorimetric method. Serum TAG levels were measured using enzymatic colorimetric tests with glycerol phosphate. HDL cholesterol (HDL-C) concentrations were determined after precipitation of the apo B-containing lipoproteins with phosphotungstic acid. All measurements were done using commercially available enzymatic reagents (Pars Azmoon, Tehran, Iran), adopted to an auto-analyser system (Selectra E; Vitalab, Holliston, the Netherlands). Serum concentrations of IL-6, IL-2, TNF-α and amyloid A were determined with the use of ELISA by means of commercially available kits (Bender MedSystems and Biosource International, Vienna, Austria). hs-CRP was assayed by ultra-sensitive latex-enhanced immunoturbidimetric assay (Randox Laboratories, Crumlin, UK). Intra- and inter-assay CV for all biochemical measurements were less than 10 %.
Assessment of blood pressure and other variables
To evaluate blood pressure, we first asked participants to rest for 10 min. Blood pressure was then measured using a standard mercury sphygmomanometer, twice with a 5 min interval, while participants were sitting. The mean of the two measurements was considered as the participant’s blood pressure. Physical activity was assessed by use of the short-form International Physical Activity Questionnaire and expressed as metabolic equivalent-hours/week (MET-h/week). Information about age, smoking habits, socio-economic status, medical history, menopause status and current use of medications was obtained using pre-tested questionnaires. Height, weight and waist circumference were measured based on standard protocols( Reference Esmaillzadeh, Mirmiran and Azizi 22 ). BMI was calculated as weight (in kilograms) divided by the square of height (in metres).
Definition of terms
Obesity was defined as BMI ≥30·0 kg/m2. In the current study, MetS was defined as the presence of at least three components of the following abnormalities, according to the National Cholesterol Education Program Adult Treatment Panel III (ATP III definition)( 23 ): (i) abdominal obesity (waist circumference>88 cm); (ii) low HDL-C concentration (<50 mg/dl); (iii) high serum TAG (≥150 mg/dl); (iv) abnormal glucose homeostasis (fasting plasma glucose >100 mg/dl); and (v) elevated blood pressure (systolic blood pressure ≥130 mmHg or diastolic blood pressure ≥85 mmHg).
Statistical analysis
Participants were categorized based on quartile cut-off points of DED. Higher quartiles of DED indicate higher energy density than lower quartiles. General characteristics of study participants across quartiles of DED were examined using ANOVA for continuous variables or the χ 2 test for categorical variables. Age- and energy-adjusted intakes and food and nutrient consumption across quartiles of DED were calculated using ANCOVA. To determine the association between DED and MetS and its components, we used multivariable logistic regression analysis in different models. First, we controlled for the confounding effect of age. In the second model, we further adjusted for cigarette smoking (yes or no), physical activity (MET-h/week), socio-economic status (categorical), current oestrogen use (yes or no), menopausal status (yes or no) and family history of diabetes and stroke (yes or no). Further statistical control was performed for BMI (continuous) in the last model. The overall trend of odds ratios across quartiles of DED was calculated by considering the median of DED in each quartile as a continuous variable.
Due to the skewness in the distribution of inflammatory markers, all biomarkers of inflammation were first logarithmically transformed. Then, geometric means of inflammatory markers across quartiles of DED were estimated by ANCOVA in several models (covariates were same as above). All statistical analyses were performed by using the statistical software package PASW Statistics Version 18.0. P<0·05 was considered significant.
Results
General characteristics of the study participants across quartiles of DED are shown in Table 1. Participants with higher DED were more likely to be older, obese and physically active (P<0·001 for all). Prevalence of MetS and its components including central obesity, elevated blood pressure, hypertriacylglycerolaemia and low HDL-C were significantly higher in the top quartile of DED compared with the bottom quartile (P<0·001 for all except high serum TAG, where P=0·01). No other significant differences were seen in the general characteristics when comparing the quartiles of DED.
Table 1 General characteristics by quartile of dietary energy density among female nurses (n 1036) aged >30 years, Isfahan, Iran
WHR, waist-to-hip ratio; LDL-C, LDL cholesterol; HDL-C, HDL cholesterol; MET, metabolic equivalent of task.
Data are presented as means and standard deviations unless indicated otherwise.
* By using linear regression.
† High socio-economic status was defined based on educational level, income, family size, being owner of the house or renting the house, and house area.
‡ Obesity: BMI ≥30·0 kg/m2.
§ Metabolic syndrome was defined as the presence of three or more of the following components: (i) abdominal adiposity (waist circumference >88 cm); (ii) low serum HDL-C (<50 mg/dl); (iii) high serum TAG (≥150 mg/dl); (iv) elevated blood pressure (≥130/85 mmHg); (v) abnormal glucose homeostasis (fasting plasma glucose ≥110 mg/dl).
Age- and energy-adjusted dietary intakes of participants across quartiles of DED are presented in Table 2. As expected, individuals in the highest quartile of DED had significantly higher intakes of energy (P=0·005), fat (P=0·001), refined grains, and meat and fish (P<0·001). In contrast, they had lower intakes of carbohydrate (P=0·01), fibre, whole grains, fruits, vegetables, and nuts and legumes (P<0·001). Dietary intakes of other foods and nutrients were not significantly different across quartiles of DED.
Table 2 Dietary intakes by quartile of dietary energy density among female nurses (n 1036) aged >30 years, Isfahan, Iran
Data are presented as means with their standard errors adjusted for age and energy intake. Data for energy intake have just been adjusted for age.
Crude and multivariable-adjusted odds ratios for MetS across quartiles of DED are provided in Table 3. After accounting for potential confounders, we found that participants in the top quartile of DED had greater odds for MetS (OR=1·78; 95 % CI 1·36, 2·98) compared with those in the bottom quartile. Further control for BMI attenuated this association slightly (OR=1·59; 95 % CI 1·19, 2·81).
Table 3 Multivariable-adjusted odds ratios and 95 % confidence intervals for metabolic syndrome across quartile of dietary energy density among female nurses (n 1036) aged >30 years, Isfahan, Iran
* By the use of Mantel–Haenszel extension χ 2 test.
† Metabolic syndrome was defined as the presence of three or more of the following components: (i) abdominal adiposity (waist circumference >88 cm); (ii) low serum HDL cholesterol (<50 mg/dl); (iii) high serum TAG (≥150 mg/dl); (iv) elevated blood pressure (≥130/85 mmHg); (v) abnormal glucose homeostasis (fasting plasma glucose ≥110 mg/dl).
‡ Adjusted for age.
§ Further adjusted for cigarette smoking, physical activity, socio-economic status, current oestrogen use, menopausal status and family history of diabetes and stroke.
|| Additionally adjusted for BMI.
Multivariable-adjusted odds ratios for components of the MetS across quartiles of DED are indicated in Table 4. After adjustment for potential confounders, individuals in the highest quartile of DED were more likely to be abdominally obese (OR=1·51; 95 % CI 1·00, 2·63), have hypertriacylglycerolaemia (OR=2·95; 95 % CI 1·58, 3·91) and low HDL-C levels (OR=1·36; 95 % CI 1·17, 2·54) compared with those in the lowest quartile. Consumption of energy-dense diets was not significantly associated with the odds of abnormal glucose homeostasis or elevated blood pressure either before or after controlling for confounding variables.
Table 4 Multivariable-adjusted odds ratios and 95 % confidence intervals for components of the metabolic syndromeFootnote * across quartile of dietary energy density among female nurses (n 1036) aged >30 years, Isfahan, Iran
* Components of the metabolic syndrome were defined as follows: abdominal adiposity (waist circumference >88 cm); low serum HDL cholesterol (<50 mg/dl); high serum TAG (≥150 mg/dl); elevated blood pressure (≥130/85 mmHg); abnormal glucose homeostasis (fasting plasma glucose ≥ 110 mg/dl).
† By the use of Mantel–Haenszel extension χ 2 test.
‡ Adjusted for age.
§ Further adjusted for cigarette smoking, physical activity, socio-economic status, current oestrogen use, menopausal status and family history of diabetes and stroke.
|| Additionally adjusted for BMI.
Geometric means of inflammatory markers across quartiles of DED are shown in Table 5. Compared with those in the lowest quartile of DED, participants in the highest quartile had greater concentrations of hs-CRP, TNF-α and IL-6, even after adjustment for potential confounders including BMI. Mean plasma hs-CRP concentrations across increasing quartiles of DED were 1·7, 1·7, 2·0, 2·4 mg/l (P for trend=0·04). Such increasing concentrations across increasing quartiles of DED were also seen for TNF-α (4·1, 4·5, 4·5, 4·8 ng/l; P for trend=0·03) and IL-6 (1·6, 1·6, 1·5, 2·5 ng/l; P for trend <0·01). We found no significant association between consumption of energy-dense diets and serum concentrations of amyloid A and IL-2 either in crude or adjusted models.
Table 5 Geometric means of inflammatory markers across quartile of dietary energy density among female nurses (n 1036) aged >30 years, Isfahan, Iran
hs-CRP, high-sensitivity C-reactive protein; SAA, serum amyloid A.
Data are presented as geometric means with their standard errors,
* By the use of ANOVA in crude models and ANCOVA in the adjusted models.
† Adjusted for age.
‡ Further adjusted for cigarette smoking, physical activity, socio-economic status, current oestrogen use, menopausal status and family history of diabetes and stroke.
§ Additionally adjusted for BMI.
Discussion
In the current cross-sectional study, DED was positively associated with MetS and features of MetS including abdominal adiposity, low HDL-C levels and hypertriacylglycerolaemia in Iranian female nurses. Additionally, individuals in the top quartile of DED had higher plasma concentrations of hs-CRP, TNF-α and IL-6. All associations were independent of BMI.
Despite a growing interest in evaluating health outcomes related to DED in nutritional epidemiology, there is still no standardized calculation method. The major controversy is related to including or excluding beverages in the calculations. The most common method in epidemiological studies is using foods only, excluding beverages, since it has been suggested this method could better demonstrate the meaning of DED( Reference Ledikwe, Blanck and Kettel Khan 8 , Reference Johnson, Wilks and Lindroos 21 , Reference Mendoza, Drewnowski and Christakis 24 ). Therefore, in the present study, we included only foods, not beverages, in DED calculation.
A growing body of evidence has demonstrated adverse effects of high-energy-dense diets on insulin resistance, metabolic profile and anthropometric measures. Therefore, as expected, high DED was associated with increased risk of MetS. This was in line with previous epidemiological studies( Reference Esmaillzadeh and Azadbakht 10 , Reference Esmaillzadeh, Boroujeni and Azadbakht 12 , Reference Mendoza, Drewnowski and Christakis 24 ). This finding concurred with the results of a cross-sectional study among American adults, where Mendoza et al.( Reference Mendoza, Drewnowski and Christakis 24 ) demonstrated that higher DED is significantly associated with higher risk of MetS. In the current study, dietary data from a validated FFQ were used; however, the findings were similar to those of Mendoza et al.( Reference Mendoza, Drewnowski and Christakis 24 ) in spite of the use of data from one-day 24 h dietary recall in their study. DED provides an overall picture of diet since it considers all foods together. Nevertheless, it has its own characteristics in each population and might lead to different and specific outcomes. For example, in the Mediterranean dietary pattern, despite the higher content of fat, an inverse correlation has been reported between olive oil consumption and DED. This might be attributed to consuming olive oil along with vegetables in salad. Further studies are required to clarify the associations between DED and metabolic abnormalities in different populations with different dietary patterns. Adverse effects of energy-dense diets on metabolic profile and inflammatory markers might be explained by their unhealthy components such as refined grains, trans- and saturated fatty acids, and processed and red meats. Furthermore, higher-energy-dense diets are associated with greater energy intake( Reference Ledikwe, Blanck and Kettel Khan 8 ), but lower dietary diversity score, which leads to lower intakes of fruit, vegetables and whole grains( Reference Esmaillzadeh and Azadbakht 10 , Reference Azadbakht and Esmaillzadeh 25 ). Altogether, these dietary factors may lead to increased adiposity, particularly abdominal adiposity, and insulin resistance, as components of MetS( Reference Radhika, Van Dam and Sudha 26 – Reference Azadbakht, Mirmiran and Azizi 29 ). On the other hand, regarding the poor diet quality of Iranians( Reference Mirmiran, Azadbakht and Azizi 30 , Reference Azadbakht, Mirmiran and Hosseini 31 ), lower-energy-dense diets may ameliorate metabolic abnormalities by improving diet quality.
The possible mechanisms underlying the association of DED and inflammation are not well understood. Earlier studies have indicated that consuming a more energy-dense diet is associated with weight gain( Reference Savage, Marini and Birch 6 , Reference Vergnaud, Estaquio and Czernichow 32 ) and greater adiposity tissue( Reference Johnson, Mander and Jones 33 ). Adipose tissue is the main source of inflammatory biomarkers, and therefore this might be the link between DED and higher inflammatory biomarkers. However, even after adjustment for BMI, this association remained significant. It should be kept in mind that there is not a simple linear relationship between BMI and abdominal adiposity, as a component of MetS, and the main source of inflammatory biomarkers. It is possible that the saturated fat, cholesterol and refined carbohydrate contents of high-energy-dense diets, which are positively correlated with dietary inflammatory index( Reference Cavicchia, Steck and Hurley 34 ), contribute to its relationship with chronic inflammation status.
The lack of association between DED and abnormal glucose homeostasis in the current study is in line with earlier investigations( Reference Vernarelli, Mitchell and Rolls 4 , Reference Esmaillzadeh, Boroujeni and Azadbakht 12 , Reference van den Berg, van der and Spijkerman 14 ). The possible explanation for this finding might be the lower prevalence of abnormal glucose homeostasis in our study population. Another explanation might be the quality of fibre and fat, besides their quantity, which might differently affect glucose homeostasis( Reference Sartorelli, Freire and Ferreira 35 – Reference Bjermo, Iggman and Kullberg 37 ). Energy-dense diets contain greater amounts of trans and saturated fats( Reference Bes-Rastrollo, van Dam and Martinez-Gonzalez 5 ). Additionally, we have used only fasting plasma glucose to define abnormal glucose homeostasis, which might lead to the misclassification of study participants in terms of this condition. To further explore the relationship between DED and glycaemic control, one would need to consider fasting insulin levels as well.
Our study has several limitations that should be taken into account when interpreting our findings. The inherent limitation of its cross-sectional design does not allow us to draw conclusions on causality. Although having a metabolic disorder does not necessarily lead to unhealthy eating, longitudinal studies are required to investigate causal relationships. The study was conducted among female nurses, which may further limit the external validity of our findings. On the other hand, shift work and sleep deprivation in this population may affect their dietary intakes( Reference Haghighatdoost, Karimi and Esmaillzadeh 38 ) as well as their metabolic profile and weight( Reference Kim, Son and Park 39 – Reference Ohkuma, Fujii and Iwase 41 ). Using the US Department of Agriculture’s food composition table to estimate nutrient and energy intakes in this population is another limitation of the present study, although efforts have been made to adapt this software for Iranians. Our observational study was conducted among free-living individuals, which could better reflect the real associations because of considering habitual dietary intakes rather than short-term intervention. Although we controlled for several potential confounders, unknown or unmeasured confounding variables like sleep deprivation cannot be excluded. Misclassification of participants due to using a self-administered, dish-based semi-quantitative FFQ is another limitation of our study, as with all other epidemiological studies. Substantially large sample size and considering the confounding effects of various variables are the strengths of the present study.
Conclusion
In summary, we found that consumption of high-energy-dense diets was associated with increased chance of MetS and most of its features in Iranian female nurses. Additionally, DED was positively associated with inflammatory markers including hs-CRP, TNF-α and IL-6 concentrations. Longitudinal studies are needed to explore the relationship of DED with MetS and inflammation.
Acknowledgements
Financial support: The study was supported by the Food Security Research Center, Isfahan University of Medical Sciences, Isfahan, Iran in conjunction with Lorestan University of Medical Sciences, Khorramabad, Iran. Conflict of interest: The authors declared no conflict of interest. Authorship: L.A. and A.E. were involved in conception, design, data collection and statistical analysis. F.H. was involved in data interpretation and paper drafting. B.L. was involved in paper drafting and editing. L.A. and A.E. supervised the study. Ethics of human subject participation: This study was approved by the research council and ethics committee of the School of Nutrition and Food Science, Isfahan University of Medical Sciences, Isfahan, Iran. All participants declared their willingness to participate in the study by providing written informed consent.
