Excessive sodium intake was a major risk factor for cardiovascular disease, stroke, and chronic kidney disease, and other diseases. The dangers of excessive sodium intake have drawn global attention. Many studies have reported that elevated sodium intake is associated with a number of non-communicable chronic diseases (including hypertension, cardiovascular disease, and stroke) and that a decrease in sodium intake may reduce blood pressure and the risk of these associated non-communicable chronic diseases [14,15,16]. Indeed, the WHO showed that reducing sodium intake could significantly reduce blood pressure in all populations and that reducing sodium intake to < 2000 mg/d was more beneficial than reducing sodium intake but still consuming > 2000 mg /d. However, the WHO also reported no association between sodium intake and all-cause mortality, including cardiovascular disease and non-fatal coronary heart disease [17]. A British study has shown that a high-salt diet can lead to obesity, independent of energy or sugar intake. The risk of childhood and adult obesity increased by 28% and 26% with an increased salt intake of 1 g/d [18]. Furthermore, a meta-analysis of 18 cross-sectional studies showed that the higher a person’s sodium intake, the larger their waistline [19].

This study evaluated sodium intake status and associated epidemiological factors for Shanghai residents enrolled in the SDHS from 2012 to 2014 across four seasons in each calendar year. The results showed that the median sodium intake for residents aged 15 years old and above was 4306 mg/d in Shanghai, and the salt intake from cooking salt was just 6.0 g/d in our study. A study of 6072 adults from 12 mainland provinces from 2009 to 2011 found that the intake of cooking salt was 9.2 g/d in China as a whole but only 6.7 g/d in Shanghai [20]. The difference may be related to the lower intake of cooking salt in Shanghai, which may be related to a series of salt-reduction health education, including the city government distributing free salt control spoons to all families [21]. Moreover, we found that the total sodium intake and main food sodium intake source did not alter significantly with the changing seasons. This phenomenon may be related to the convenience of shopping for these foods.

The average sodium intake of Shanghai residents aged 15 years and over was 4915 mg/d, representing a 13.1% decline from the national level of 5702 mg/d that that was reported in a Chinese nutrition and health monitoring study from 2010 to 2013. Furthermore, this was well below the previous average of a national nutrition survey (6268 mg/d in 2002, a 9.0% decline) [22]. This finding may be linked with the salt control program [21], increased publicity of the program in mainstream media, and the free distribution of salt control scoops. All these factors would encourage Shanghai residents to consciously focus on their daily salt consumption, guiding the formation of improved health behaviors. However, although the total sodium intake in Shanghai is far below the national average, unfortunately, it is still above the global mean sodium intake (3900 mg/d) [23]. Furthermore, it far exceeds the WHO recommended standard, which states that the adult’s daily sodium intake should be less than 2000 mg [17]. The Chinese dietary reference intake standard states that the recommended nutrient intake of sodium should be 1500 mg/d in people aged 18 years old and over because it is likely that non-communicable chronic disease can be prevented if the intake does not exceed 2000 mg/d [24]. In our study, the sodium intake was approximately 2.5 times the recommended standard. From an individual perspective, 96.4% of Shanghai residents had a higher sodium intake than the 2000 mg/d level.

The essence of controlling salt is sodium restriction; the consumption of visible salt (cooking salt) is already low in Shanghai, and we found that it approximated 55.1% of the total sodium intake. The next step in controlling salt should be targeting salt-containing condiments, such as monosodium glutamate, soy sauce, and pre-packaged foods. The Chinese National Centre for Food Safety Risk Assessment showed that sodium intake from non-cooking salt increased by 12.6% from 2009 to 2011[19]. In our study, the sodium from monosodium glutamate, soy sauce, and pre-packaged food approximated 35.4% of the total sodium intake. In the last 20 years, with the gradual development of food sales from traditional single outlets to modern food chain supermarkets, convenience stores, and online shopping systems, residents select and purchase more pre-packaged food. Consequently, global pre-packaged food consumption is increasing [25]. Some studies have shown that the pre-packaged food consumption rate of the adult population in China is 85.3%. Among these pre-packaged foods, the sodium content of convenience foods and baked goods is generally high, and the consumption rate is also high, at 52.8% and 31.7%, respectively [26]. Our study found that Shanghai residents consumed approximately 22.2% of sodium from pre-packaged foods. We believe that this has been substantially undervalued. Chinese dietary composition tables contained approximately 300 kinds of pre-packaged foods, and these were all domestic foods. As an international metropolis, people in Shanghai can easily buy all pre-packaged foods, including imported food, which can now be included as part of the Chinese dietary composition in Shanghai. When we processed the data, we used a similar pre-packaged food or the main raw material of the pre-packaged food to replace the pre-packaged food that was not in the database. When we substituted pre-packaged foods for the main raw material of the pre-packaged food, the salt added to the pre-packaged foods during processing was ignored. An Australian study, which assessed the sodium content of 15,680 pre-packaged foods in the supermarket, found that the average sodium content was 500 mg/100 g of food in Australia. Voluntary salt control in the food industry is the most economical and effective salt reduction policy [6]. People can adapt to the taste of processed food with its salt intake reduced by 10–20 percent, which does not affect consumption [27].

Nutritional labeling is one of the most recognized and effective nutrition interventions worldwide. Labeling regulations have been adopted in many countries experiencing a nutrition transition from traditional diets to contemporary patterns of food consumption [28]. Labeling regulations have been initiated in Europe, North America, Australia, New Zealand, Asia, Africa, the Middle East, and Latin America [29]. In 2011, China enacted the General Rules for Nutrition Labeling of Pre-packaged Foods (GB 7718-2011), the first mandatory legislation on nutritional labeling in China. The general rules stipulated that all pre-packaged foods must label the contents of energy, protein, fat, carbohydrate, and sodium in a prominent and easily viewed place. The labeling rate for the sodium content of pre-packaged foods was 99.8% in Shanghai [30]. From a public health perspective, it is important to teach residents to recognize and use nutrition labels to choose low-sodium foods and discourage people from consuming too much sodium in pre-packaged foods.

The limitation of the present study is that the study design may have underestimated the surplus of cooking salt and high sodium condiments. We used the proportion of condiment intake at home to estimate the intake of condiments eaten out, accordingly. However, we found that Shanghai residents have tended to dine out more often in recent years because of the development of the economy and the online takeaway industry. In 2016, China's online food takeaway turnover was about 17.0 billion USD. In general, food consumed while eating out tends to have higher levels of salt than homemade food [31]. We also recognize that caution should be taken when generalizing our results to the global population, as differences in food preferences, tastes, cooking habits, lifestyles, and health consciousness might exist between the general population and our study population.