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Public health
Research
Estimating daily salt intake based on 24 h urinary sodium excretion in adults aged 18–69 years in Shandong, China
  1. Ji-yu Zhang1,
  2. Liu-xia Yan2,
  3. Jun-li Tang1,
  4. Ji-xiang Ma2,
  5. Xiao-lei Guo1,
  6. Wen-hua Zhao3,
  7. Xiao-fei Zhang4,
  8. Jian-hong Li2,
  9. Jie Chu1,
  10. Zhen-qiang Bi1
  1. 1Shandong Center for Disease Control and Prevention, Jinan, China
  2. 2National Center for Chronic and Non-communicable Disease Control and Prevention, Chinese Center for Disease Control and Prevention, Beijing, China
  3. 3Chinese Center for Disease Control and Prevention, Beijing, China
  4. 4Department of Clinical Epidemiology and Biostatistics, Second Affiliated Hospital of Zhejiang University College of Medicine, Hangzhou, China
  1. Correspondence to Dr Zhen-qiang Bi; bizhenqiang{at}gmail.com

Abstract

Objective 24 h urinary sodium extretion was used to estimate the daily salt intake of shandong residents aged from 18 to 69 years in China.

Setting 20 selected counties/districts in Shandong stratified by geographic region (Eastern, Central Southern and North Western) and residence type (urban vs rural).

Participants Among 2184 randomly selected adults, 2061 provided usable 24 h urine samples. Urine volume <500 mL or male creatinine <3.81 (female creatinine <4.57) are not included in the analysis.

Results The mean sodium level excreted over 24 h was 237.61 mmol (95% CI 224.77 to 250.44) mmol. Overall, the estimated mean salt intake was 13.90 g/day (95% CI 13.15 to 14.65). The mean salt intake among rural residents was higher than that among urban residents (14.00 vs 13.68 g; p<0.01). Salt intake in men was higher than that in women (14.40 vs 13.37 g; p<0.01). Approximately 96% of the survey participants had a dietary salt intake of ≥6 g/day.

Conclusions The salt intake in Shandong is alarmingly higher than the current recommended amount (6 g/day). Thus, effective interventions to reduce salt intake levels to combat the increasing burden of non-communicable diseases need to be developed and implemented.

  • EPIDEMIOLOGY
  • NUTRITION & DIETETICS

This is an Open Access article distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY-NC 3.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited and the use is non-commercial. See: http://creativecommons.org/licenses/by-nc/3.0/

https://doi.org/10.1136/bmjopen-2014-005089

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    Strengths and limitations of this study

    • We used the ‘gold standard’ 24 h urine collection to estimate salt intake.

    • A limitation of the study was that only a single 24 h urine collection was obtained from each participant. A single collection will less accurately reflect 24 h sodium intake than will several collections.

    • However, the day-to-day variation in sodium excretion can vary in both directions, so it is unlikely that the findings overstated the inaccuracy of the claim of a low sodium intake.

    Background

    In China, hypertension is a major contributor to cardiovascular disease (CVD), which accounts for about 40% of all deaths1 and 23% of the healthcare costs.2 Evidence suggests that a high level of salt intake is associated with high blood pressure (BP) and a greater risk of stroke and CVD.3–6 The National China Nutrition Surveys (1982, 1992 and 2002) and the Chinese Behavioral Risk Factor Surveillance indicated that approximately 80% of Chinese individuals consume more salt than the upper limit of the recommended salt (sodium chloride) intake (6 g/day) by the Chinese Nutritional Society (2007). In 2010, the mean salt intake among the Chinese was estimated to be 9.1 g/day in urban areas and 11.5 g/day in rural areas.1

    Reducing salt intake is one of the easiest, most efficient and cost-effective ways to reduce the burden of CVD and healthcare costs, which would result in a substantial improvement in public health.1 ,7 Globally, dietary sodium reduction has been recommended as a major strategy for the prevention and control of non-communicable diseases.8 Several countries including Japan, the UK, Finland, Portugal and the USA have succeeded in reducing the population-wide sodium intake.9 10 Unlike in Western countries, a major challenge to reduce sodium intake in China is that the majority of sodium comes from the salt added to home cooked food rather than from commercially processed foods. A recent study suggested that most of the dietary sodium intake (76%) was attributed to salt added during home cooking.11 In contrast, processed foods contributed heavily to sodium intake in the UK (95%) and the USA (71%).11 Therefore, efforts to reduce sodium intake in China should focus on reducing the sodium level in home cooked foods.

    Shandong Province is one of the most populous provinces in China. Sodium intake among most Shandong residents considerably exceeds the recommended limit. In 2002, the mean daily salt intake was 12.6 g and the prevalence of hypertension was 25.1%. High salt intake has caused huge economic loss.2 In 2011, the Chinese Ministry of Health (MOH) selected Shandong province as a national pilot area and the Shandong-MOH Action on Salt and Hypertension (SMASH) Project was set up to reduce the sodium level in the Chinese diet. To help develop effective interventions for the SMASH Project, a baseline survey was conducted in 2011 to assess salt consumption and the prevalence of hypertension among residents in Shandong. The measurement of 24 h urinary sodium excretion is considered the gold standard method for obtaining data on the sodium intake in population surveys.1 However, in China, few studies have been conducted on the salt intake of the general population based on the 24 h urinary sodium excretion method. This study analyses the baseline survey to estimate the daily intake of dietary sodium among the general adult Shandong population (aged 18–69 years) based on 24 h urinary sodium excretion.

    Methods

    A cross-sectional survey was conducted in Shandong province from June 2011 to July 2011. Multistage (four-stage) cluster sampling was used to select a provincially representative sample of the adult population (aged 18–69 years). Briefly, a total of 140 counties/districts in Shandong province were stratified by geographic region (Eastern, Central Southern and North Western) and residence type (urban vs rural). Twenty counties/districts were selected (12 rural counties and 8 urban districts) for this study. Using a proportional probability sampling (PPS) method, three townships (in rural areas) or two suburbs (in urban areas) were chosen from each selected county or district. The PPS method was also used to select one village/community from each sampled township/suburb. From each chosen village/community, 42 participants were randomly chosen, resulting in a total sample size of 2184 participants.

    A face-to-face interview using a close-ended questionnaire was conducted by trained public health professionals with experience in conducting interviews for similar health surveys to collect information on demographics (gender, age, education and income).

    All participants were given written and verbal instructions to collect a 24 h urine sample. The first urine of the day was discarded and all urine over the following 24 h was collected in provided bottles. All bottles from the same participant were carefully mixed and the volume of urine was measured. Then a 20 mL aliquot of mixed urine was taken and immediately frozen at −20°C and sent to the laboratory for analysis.

    The samples were first assayed for creatinine concentration using the picric acid method (ADICON Clinical Laboratory; CAP accredited). A sample was excluded from the analysis if the 24 h urine volume was below 500 ml or its creatinine content referred to body weight was found to be lower than the mean minus 2 S.D. from the population mean (male creatinine <3.81 mmol; female creatinine <4.57 mmol). The sodium concentration was determined using the direction selective electrode method. Daily salt intake was estimated by assessing 24 h urinary sodium excretion based on the assumption that all sodium was ingested in the form of sodium chloride.

    Height, weight and waist circumference were measured in all participants. Of 2184 participants, 2061 participants with complete data were included in this analysis (response rate, 94.4%). Educational level and annual household income were used as indicators for social economic status. We divided participants into five groups based on education level: no education, primary school, junior high school, senior high school, college and higher education. Annual household income was classified into high, middle and low, according to tertiles. Body mass index (BMI=weight (kg)/height (m2)) was calculated. individuals with a BMI ≥24 but <28 kg/m2 were defined as being overweight and those with a BMI ≥28 kg/m2 were considered obese.

    Statistical analyses were performed using the Statistical Analysis System (SAS) V.9.3 (SAS Institute Inc, Cary, North Carolina, USA). p<0.05 was considered significant. All the results shown were weighted by the total demography in Shandong province. Survey weight (total weight=design weight×post-stratification weight) was used to calculate weighted proportions. Design weight was calculated to account for different factors including cluster design, strata and individual factors. The population data from Shandong province was used to generate the post-stratification weight. Means and 95% CIs were calculated for all variables and normality of the data was assessed. Analysis of variance and the χ2 test were used to test the significance of differences between groups.

    This study received ethics approval from the Ethics Committee of the Shandong Centre for Disease Control and Prevention. All participants provided written informed consent to participate in this study.

    Results

    Study subjects

    A total of 2061 participants provided urine specimens out of which 123 samples were further excluded from the analysis because of incomplete or inaccurate urine collection. There were no significant differences between included and excluded participants in terms of age, sex and residence type (t=1.52, p>0.05; χ2=0.02, p>0.05; χ2=0.32, p>0.05; table 1).

    View this table:
    Table 1

    Distribution of included and excluded participants

    The characteristics of each subpopulation are summarised in table 2. Of 2061 eligible samples, 1076 were male (52.21%) and 985 were women (47.79%). The mean age, height and BMI were 41.23 years, 163.00 cm and 24.22 kg/m2, respectively. There were no significant differences in age and BMI between men and women (t=0.34, p>0.05; t=0.29, p>0.05). However, men were significantly taller than women (t=37.65, p<0.05). The prevalence of overweight and obese individuals was 33.72% and 18.00%, respectively, with no significant differences between men and women (χ2=0.90, p>0.05;χ2=0.001, p>0.05). Of the tested participants, 82.90% had an income <10 000 RMB and 42.60% had an education level of junior high school.

    View this table:
    Table 2

    Personal characteristics of the study sample by sex and age

    Salt intake

    The 24 h urinary sodium level ranged from 22.05 to 750.79 mmol, with a mean of 231.10 mmol. Tables 3 and 4 show the 24 h urinary sodium excretion and estimated daily salt intake for each subpopulation. The mean sodium level excreted over 24 h was 237.61 mmol (95% CI 224.77 to 250.44 mmol), which corresponds to 13.90 g NaCl (95% CI 13.15 to 14.65 g). The mean salt intake by rural residents was more than that of urban residents (14.00 vs 13.68 g; p<0.01). Salt intake in men was higher than that in women (14.40 vs 13.37 g; p<0.01). There were no significant differences in the salt intake between age groups.

    View this table:
    Table 3

    Urinary sodium excretion of Shandong adults aged 18–69 years in 2011 (Weighted mean, SD, 95% CI mmol/24 h)

    View this table:
    Table 4

    Salt intake of Shandong adults aged 18–69 years in 2011 (Weighted mean, SD, 95% CI g/24 h)

    Discussion

    It is challenging to accurately estimate sodium intake because it is very difficult to know the exact amount of salt added during cooking (even in restaurants) and at the table. It is also difficult to estimate the amount of salt that has been left on the plate, and to determine the salt content in food and drinking water.1 Therefore, as suggested by a number of reports,1 ,3 ,12 the best method to determine salt intake is to measure the amount excreted in 24 h urine. In China, very few studies have measured the salt intake using 24 h urinary excretion and none have used a representative sample. Liu et al13 reported an intake of 188.4 mmol/day (corresponding to 11.02 g NaCl) in a population of 48–56-year-old individuals in China, a value much smaller than that obtained in this study (224.77 mmol/24 h, 13.9 g). The current study indicates that the sodium intake is very high in a representative population of Shandong adults, higher than that in a UK population aged 19–64 years (148 mmol).14 Salt intake by men and women was found to be 246.23 and 228.52 mmol/day, respectively, which is higher than that in the population in Toyama (men, 224 mmol/day; women, 201 mmol/day),10 ,15 the USA (men, 180–190 mmol/day; women, 130–150 mmol/day)16 and the UK (men, 161 mmol/day; women, 127 mmol/day).16

    The salt intake in China is double the upper limit of the recommended intake of 80–110 mmol/day (5–6 g salt/day) by the World Health Organization in 198217 and in the China Nutrition Recommendations.18 In China, the high salt intake is mainly due to the high consumption of soya sauce and the tradition of adding a large amount of salt to the food both during cooking and at the table.3 ,19 ,20 Consistent with other studies,3 ,5 ,19 salt intake in men (14.4 g) was higher than that in women (13.9 g) in Shandong, possibly because of the higher overall food intake by men and the differences in food habits between men and women. In most populations, salt intake is high and well above the daily recommended values. Now the resident health records are being established in Shandong province, it is necessary to evaluate and record the salt intake for individuals by the FFQ or other methods, especially among patients with chronic disease and high-risk individuals; it is very important to focus on the salt intake of individuals and to persuade them to change the habit of high salt intake.

    Strong and consistent evidence from animal studies, clinical trials and epidemiological data from within and across populations suggests that high salt intake is an important risk factor for high BP among hypertensive and normotensive individuals.21 In addition, high salt intake is also associated with increased risk of coronary heart disease and stroke.22 Therefore, public health initiatives are needed to reduce salt consumption3 ,4 and, in turn, lower the burden of CVDs and increase life expectancy. The government needs to focus and resolve this severe situation. Salt reduction must be undertaken. Health education and promotion based on media campaigns, as well as timely community-based initiatives to limit the amount of salt added to food by individuals, would increase public awareness. Meanwhile, it is necessary to collaborate with restaurants and food industries to reduce excessive salt. Such public health approaches can be simple, at low cost and effective. Indeed, campaigns such as the SMASH and the China Rural Health Initiative Sodium Reduction Study are already underway.23 As reported by Yan et al,24 in China, measures for salt reduction will include government initiatives, health education and promotion, promotion of low sodium salt in food processing companies and restaurants. In Shandong, education through radio, television, internet and movies is underway to improve the knowledge of residents about low-salt diets. To bring about behavioural changes, salt spoons have been provided for every family in the entire province. Key groups comprising governors, medical personnel and stakeholders from the food processing enterprise and restaurants are trained. Local standards for the amount of salt in soya sauce pickles and other dishes in the cuisine of Shandong have been formulated. The results from this study could be useful in these endeavours.

    A limitation of the study was that only a single 24 h urine collection was obtained from each participant. A single collection will less accurately reflect 24 h sodium intake than will several collections. However, the day-to-day variation in sodium excretion can vary in both directions, so it is unlikely that the findings overstated the inaccuracy of the claim of a low sodium intake.

    Acknowledgments

    The authors thank all the investigators from the national, provincial and county level CDCs, and all the participants of the survey.

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    View Abstract

    Footnotes

    • Contributors J-yZ, L-xY, J-lT and Z-qB conceived and designed the experiments. J-yZ, J-lT and Z-qB performed the experiments. J-yZ, J-lT and Z-qB analysed the data. J-yZ, J-xM, X-lG, W-hZ, X-fZ, J-hL and JC contributed reagents/materials/analysis tools. J-yZ, L-xY, J-lT and Z-qB wrote the manuscript.

    • Funding The survey was supported by funds from the Chinese Centre for Disease Control and Prevention (China CDC), National Centre for Non-Communicable and Chronic Disease Control and Prevention (NCNCD, China CDC)and the Technical Development Plan in Shandong (implemented by Shandong CDC, Grant number: 2012GSF11828).

    • Competing interests None.

    • Patient consent Obtained.

    • Ethics approval The survey was approved by the Ethics Committee of the Shandong Centre for Disease Control and Prevention, China.

    • Provenance and peer review Not commissioned; externally peer reviewed.

    • Data sharing statement No additional data are available.