Quality in Primary Care Open Access

Keywords

Salt intake; Sodium intake; Devices; Questionnaires

Introduction

The relation of high salt-intake induced hypertension with cardiovascular diseases has received much attention. The top 10 causes of death reported from WHO (Fact sheet Nº310 (https://www.who.int/mediacentre/factsheets/fs310/en/)) shows ischemic heart disease as top (13.2%: 7.4 million people) and stroke as second (11.9%: 6.7 million people). Of the preventable risk factors, next to tobacco control and blood pressure control, salt intake control has been shown to contribute to prevent premature mortality, especially in the eastern Mediterranean region for men and western Pacific region for women [1]. Saltintake remains high in rather low-income countries [2]. There are reports of high-salt intake influencing oxidative stress rather than blood pressure [3], which target organs such as the kidney [4], with no change of the renin-angiotensin system [5]. However, blood pressure control is the endpoint of most randomized trials of salt reduction [6].

Anciently, when there were no ways in measuring blood pressure, hardening of the arteries with high salt intake was described in Chinese medicine in the 3^rd century BC [7]. In 1896, the sphygmomanometer was invented and hypertension was easily evaluated in the clinics [8]. The effect of salt restriction on hypertension has been documented shortly afterwards [9] and the earliest experimental evidence between salt intake and high blood pressure [10,11] was found in the era when hypertension was believed to be caused by toxic products. Reports of salt restriction on treating hypertension [12,13] did not change these myths, until the role of diet in various diseases was proposed in the 1930s. The low-protein, low-fat, and extremely low-salt “Kempner” rice diet showed effectiveness on severe hypertension evidenced by improvement of chest X-ray and eye examination [14-16]. The involvement of salt intake was confirmed in animals and humans, by the effect sodium chloride addition, while potassium chloride had not effect [17,18]. The famous Framingham Heart Study started at this time when half of the Americans including President Roosevelt, died of cardiovascular disease [19]. It was in the 1950s, when urinary electrolyte excretion was measured for the first time to find the relation of salt diet and hypertension [20].

Questionnaires to Evaluate Sodium-Intake

Questionnaires or dietary assessment methods have been used for evaluating habitual diets where intervention and diet modification are key factors in preventing cardiovascular disease. The first questionnaires on salt intake were targeted on people with poor income [21] and on people who survived severe heat waves [22]. The issue of salt intake influence on high blood pressure came from studies of hypertension in the Bahamas, where nearly 40% of the natives had high systolic blood pressure [23]. In order to find out the etiology, a questionnaire was distributed to physicians and a result of about 1,000 cases showing the incidence of hypertension very high even in young people. The death rate due to hypertensive complications was also very high. Urine examinations were not so reliable but the interesting finding was the high sodium concentration in well water, and the commonly used salt pork oil. It was estimated that 12 g or more salt was excreted in the urine [24]. The questionnaire in this study did not include questions on salt intake, but the importance of salt intake became aware in this period of the late 1950s. There are various regions in the world where salted fish is heavily consumed, as the Pacific islands including Japan, but not only coasted people have high salt-intake. Inland people, where salt was used as a preservative can be a source of high salt-intake. One of the first questionnaires including questions of salt intake was reported in a study on coronary heart disease asking the timing of salting [25]. Twenty years thereafter, the reliability and validity of these questionnaires were variable and difficult to state anything from the results. Two issues were a problem at this time. First is the difficulty to develop a method that targets both group and individual, and the second is accurate quantification or importance of categorization based on less accurate quantification. Several methods have been developed and evaluated since then. The first questionnaire method, “dietary history approach” or “dietary records”, was actually developed far way back [26], and was concerned the gold standard where quantitative accurate information is provided. This method requires extensive interviews, where long-term history of the patterned intake is taken into account. Detailed preparation methods, recipes and portion size of food mixture are recorded. At the end of the recording, a trained interviewer reviews the record with the subject. This method is based on motivated interviewers and subjects to complete daily records which may limit the results to a proportion of the general population. The recording per se may even affect the amount and variety of food the subjects select, and have the tendency to be underreported. The second questionnaire method is the easier “24 h dietary recall” where the interview is often structured with specific probing questions to increase reliability of the questionnaire. However, diets vary day to day, individually but maybe not as a mean of a group [27] and a seven-day recall method (“history method”) was attempted and modified to record the actual intake (“record method”) since it is difficult to recall a meal a week before. Still, the method needs intense cooperation of the participants and is difficult for use in large epidemiologic studies. Short cut studies trying to circumvent these difficulties were developed between the 1950s and 1970s, but always could not confirm these results against the “truth”, which they were trying to show. Comparing the same assessment at different times also had difficulties since it is difficult to seek correlation on one item when all other items are not always consistent. Validation of different methods were studied at this period and found that the “history method” had higher values than the “record method” [28-31]. This difference can be overcome by various regression techniques. Indeed, studies have been conducted for both methods to see whether they can categorize individuals in a similar manner [32-34]. Repeated dietary history was evaluated in the Framingham study, showing that a two-year interval showed a similar result, but not four [35]. Some studies on the 24 h dietary recall method have shown that serum and urine values reflect the reported intake [36], but other do not [37]. Thus, it has been reported that the history method (24 h recall method) may not be appropriate for the individual, but informative in evaluating groups [27,38-40]. A study comparing the 24 h dietary recall and seven-day intake record showed the results were comparable if the number of subjects was greater than 50 [41]. Methods tried to solve this problem by seeking a 24 h method with comparable validity as sevenday histories and records. Accordingly, the third questionnaire method is the “food frequency approach (or prediction equation approach)”, where respondents report the consumed frequency of specific food for a specific period, was developed as one of the short-cut methods. In order to evaluate the accuracy of this approach, 33 specific foods were recorded for 50 subjects and the subjects were asked to recall the frequency of intake for the specific food. The approach was quite accurate, although the preference of food must be taken into concern. Short quantitative methods attempted to develop equations from simple questions of consumption to calculate precise level in the diet, but these did not always show satisfactory results [27,40]. In fact, this method is difficult to generalize due to different diet preferences in various regions throughout the world. In the late 1980s, coding of the foods and usage of automated software, has made data processing easier and accurate with standardized interviews (the third National Health and Nutrition Examination Survey) [42]. Nonetheless, concerning nutrition and disease, it is important to place the individual in broad categories with rough consumption levels, rather than focusing on the precise amount of the nutrient. It is also important to choose the appropriate item, which differs among population and research purpose. The number of questions is also an important factor, since long food lists overestimate and short food list underestimate the actual intake.

A special salt-check sheet (questionnaire) based upon special Japanese ingredients was developed [43], and applied to local residents [44] living in Fukuoka and Saga prefectures (Figure 1). Fukuoka and Saga prefectures are located in the western part of Japan where the salt intake ranks low (but still much higher than the WHO recommended 5 g/day) (Fukuoka: 36th out of 47 (11.4 g/day) (Men), 39^th out of 47 (9.6 g/day) (women), Saga: 46^th out of 47 (10.9 g/day) (men), 44^th out of 47 (9.3 g/ day) (women)) compared to other prefectures in Japan (mean: Men (11.8 g/day), Women (10 g/day)) according to the five year-report (2006-2010) from the Japanese Ministry of Health, Labour and Welfare. The scores of the special salt-check sheet correlated well with 24 h urinary sodium excretion and imply that the top seven salty ingredients are enough to validate the salt-intake. Of course, the top seven salty ingredients will differ between regions or countries. The relatively low salt intake in this region compared to other parts of Japan correlates well with the relatively low death rate for heart disease (Fukuoka ranking 47th out of 47 prefectures in Japan, Saga ranking 40th out 47 for both men and women) and cerebrovascular disease (Fukuoka: 40th (men) 37th (women), Saga: 36th (men) 39th (women)) found in the 2010 report from the Japanese Ministry of Health, Labour and Welfare. Among the list, soy sauce is well known for its high concentration of salt (sodium calculated) (1.6 g/10 mL), but it should be also taken into account that the attractive “umami (amino acid)” taste is mainly evoked by monosodium glutamate [45], also found in soy sauce and is usually a sodium salt form. Since this is misleading, all-consuming products in Japan having been labeled by sodium content, have changed labeling to salt “equivalent” content which is easier to understand, starting on April 1^st, 2015. Interestingly, in spite of the very high salt-intake in Japan, the Japanese people rank top class for longevity of life span in the world. This means further life span, or a further healthy life span can be expected with appropriate salt restriction. The salt-check sheet above may, in a sense, be categorized as a “brief dietary assessment method”, the fourth questionnaire method, which is used for intervention or education for groups in clinical settings. For studying the whole diet of an individual, more than 100 questions are required, but for a particular nutrient only 15-30 items of food are said to be enough [46,47]. The selection of items is difficult and must be evaluated. For instance, a brief self-administered diet-history questionnaire for salt intake was validated by 24 h urinary salt excretion [48]. The fifth questionnaire method is the “diet history” which is not a simple frequency of food intake but also includes the preparation method, since processed food has increased during these decades [49]. An electronic questionnaire, 24 h dietary recall method has also been developed for assessing salt intake. It was compared with 24 h urinary sodium excretion with a correlation coefficient of 0.72 [50].

Figure 1:Salt check sheet for Japanese local residents. The typical salt concentration of each Japanese ingredient is as follows. Miso soup (3 g NaCL/250 mL), Tsukemono (0.46 g NaCL/ 20 g), Chikuwa/Kamaboko (0.6 g NaCL/ 30 g), Ramen (6 g NaCL/ 100 mL) The amount is calculated as estimated per meal.

Urinary Sodium Excretion and Sodium Intake

In 1951, a report of the effect of various diets with different amounts of sodium chloride on urinary electrolyte and blood pressure was documented [51]. The well tolerable anti-hypertensive drug, chlorothiazide, which increases salt excretion, was available in 1958 [52]. The randomized controlled trial for anti-hypertensive drugs including chlorothiazide at this time was so extremely beneficial that the control group receiving no medication was decided to dropout the study and start medication by ethical reasons. The effect was also found in patients with mild hypertension [53]. Thus, the importance of salt intake was widely recognized and the effect of salt restriction on blood pressure was evaluated in a randomized controlled trial [54]. Large group studies of the average population on urine analyses (salt intake evaluation) and blood pressure have been reported since then [55-64]. Though there are some conflicting meta-analysis short term trial reports questioning benefits from salt restriction [65,66], the main picture on salt restriction in preventing cardiovascular diseases has obtained unshakable status and a guideline on sodium intake for adults and children has been established by the World Health Organization (WHO) [67]. The guideline shows <2 g sodium/day (5 g salt/day) intake is related to prevention of high blood pressure, but with no direct effect on all-cause mortality, incident cardiovascular disease or non-fatal coronary heart disease. Since blood pressure is known to affect these outcomes, WHO strongly recommend sodium intake reduction. These is also for children, where the amount of <2 g sodium/day (5 g salt/day) should be adjusted by reduced energy requirements. The recommendations are applied to all individuals with or without hypertension, except for people with illness where sodium restriction will result in adverse effects.

Devices to Measure Sodium-Intake

Sodium research began from Michael Faraday, the namer of “ions”. And from Svante Arrhenius who discovered that dissolved solid crystalline salts disassociate into paired charged particles, and won the 1903 Nobel Prize in Chemistry. Total sodium level of a 60 kg adult is 2,400-4,200 mmol. Extracellular fluid contains 55%, bones 43% and intracellular fluid 2%. Sodium is measured by photometry (wave length: 589 nm) excited by gas flame. The strength of light correlates with concentration and can also be measured by ion selective electrodes; directly by change in electric membrane level and indirectly by measuring the concentration of sodium in blood samples. Blood sodium level does not correlate with total sodium level, but with the proportion of total sodium level and extracellular fluid volume. Sodium is the main electrolyte found in extracellular fluid and potassium is the main intracellular electrolyte. Since 95% of blood sodium is excreted in urine, evaluation of blood sodium concentration can be measured by urine sodium level and extracellular volume. Blood sodium level can be influenced by various diseases including disruption of vasopressin (ADH: antidiuretic hormone) a hormone secreted from the posterior pituitary and regulating sodium homeostasis. Diabetes insipidus, where vasopressin secretion is impaired or sensitivity decreased can cause elevated blood sodium level. SIADH, frequently found in small cell lung cancer, can cause decreased blood sodium level by excess secretion of vasopressin. Thus, evaluation of sodium intake by urine sodium level needs precautions such as ruling out the above diseases and other related factors. Measurement of 24 h urine sodium excretion has been used as a golden standard for evaluating sodium intake. However, 24 h urine samples are inconvenient, costly for a large population. Alternative methods, such as spot, overnight, daily, timed urine samples have been taken into concern whether it can reflect sodium intake of individuals as well as large populations. There were studies showing correlation between spot and 24 h urine collection [58]. However, sodium excretion is not constant throughout the day, and intra-individual variation has to be concerned. A study suggested that at least a week of overnight samples is necessary to evaluate urinary sodium excretion level [55]. Fixing the amount of sodium intake of the subjects also showed the accuracy of 24 h urinary sodium measurements greater than overnight ones [57], while other studies showed a significant correlation [60,61,68]. A study using longer 12 h urinary sodium measurement was more accurate than an 8 h (overnight) measurement, and warned the use of partial samples due to inaccuracy [64]. Repeated measurements can increase the reliability of spot urine analyses [69]. Spot-urine evaluation is not always useless, because there are studies where evidence can be discovered by this simple, easier method, especially in clinical practice [70]. B-type natriuretic peptide levels correlated to salt intake in patients with permanent atrial fibrillation and heart failure [71,72].

Devices have been developed to measure urine sodium level. A urine-sampling pipe was used to trap overnight urine and estimate 24 h urine sodium excretion [73]. Electric devices have also been developed to monitor salt intake at home [74]. The KME-03 self-monitoring device (Kohno ME Laboratory, Kanagawa, Japan) contains a volume sensor and conductivity sensor (Figure 2) [75]. The volume sensor contains small resistant chips and conductivity sensor has two gold-plated nickel metal plates. The device measures urine volume, urinary chloride concentration and temperature. Estimate 24 h salt excretion is measured and calculated by collecting overnight urine of eight hours. NaCL concentration was modified because conductivity is affected by other electrolytes [75]. Overnight sodium content (X) can be converted to 24 h-salt excretion (Y) by Y=5.76(X)^0.53 g/day [74]. These self-monitoring devices have been considered inaccurate [76], but careful comparison of overnight with 24 h urine collection methods have developed the above converting formula making these devices more reliable. The sodium ion is not always paired with chloride ion in processed food. Nonetheless, this device will be an effective motivational, educational [75,77-79] tool for personal salt restriction, when standardized by a normotensive population, especially for people living in regions of high salt-intake [80]. Salt restriction is a difficult task especially for young, obese males, and patients complicated with diabetes mellitus and hyperuricemia in a Japanese study [81]. Dietary habits start from younger ages and evaluation and intervention of salt-intake at young ages may have a better effect [82]. Salt-intake is Indeed, There is a tendency for Japanese females to be more aware of salt restriction (salt conscious) than males who are hypertensive. Importantly, these devices must be cheap, easy-to-handle, and easy-to-understand. Commercially available salt measuring devices (around 15 dollars) are not so accurate but have a simple indication of low, medium and high salt concentration, which can alert high-salt concentrated liquid intake. Once the cost is lowered, we believe it can be distributed to regions of the world with high-salt intake [1,2].

Figure 2:Estimated urine salt self-monitoring devise, KME-03 (Kohno ME Laboratory, Kanagawa, Japan). Eight-hour urine samples collected in the morning are measured of volume and NaCL concentration and converted to estimate sodium intake.

Conclusion and Perspectives

Historical improvement of questionnaires and devices on salt-intake are still on their way to prevent devastating cardiovascular, cerebrovascular diseases. Recent development of devices that measure urinary salt excretion correlate well with sodium intake making them promising tools for preventing these diseases. First, they will evoke awareness of high-salt intake in regions where high-salt intake prevails. Second, these tools will be helpful in evaluating salt intake on various upcoming trials on high salt intake-induced diseases. The key essence of this review is that “elaborate but simple” questionnaires and devices must be developed to evoke awareness of deadly consequences of high-salt intake in regions where high-salt intake prevails. Cardiovascular diseases are not only deadly, but once diagnosed; also have undesirable impact on the individual as well as health economics. Once a way to prevent high-salt intake is established, we will be able to live a happier healthier longer life.

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