Journal of Renal Nutrition
Volume 21, Issue 2 , Pages 160-168, March 2011

Relationship Between Beliefs Regarding a Low Salt Diet in Chronic Renal Failure Patients on Dialysis

Department of Nursing, University of Campinas, Campinas, São Paulo, Brazil

published online 31 May 2010.

Article Outline

Objective

The aim of this study was to investigate the relationship between beliefs regarding a low salt diet and the nutritional behavior of sodium consumption as well their relation with sociodemographic and clinical variables among chronic renal failure (CRF) patients on dialysis.

Design and Methods

This cross-sectional study enrolled a sample of 117 patients who answered the Brazilian version of the Beliefs about Dietary Compliance Scale (BDCS), the sodium frequency food questionnaire, and the use of discretionary salt/day.

Results

The average of total salt consumption was 10.6 g/day (±6.3) and it was positively correlated with the interdialytic weight gain (r = 0.20 P = .032) and negatively correlated with the education level (r = −0.19 P = .044). The Benefits beliefs were discriminatory of the higher and lower salt consumers, and proportionally related to monthly income (r = 0.22 P = .017). The Barriers beliefs were positively correlated only to time on dialysis (r = 0.25 P = .008). Subjects with lower schooling and those of older age tended to consume more discretionary salt (r = −0.27, P = .005; r = 0.23, P = .016, respectively); however, they reported a lower consumption of foods with high salt content (r = 0.25 P = .006; r = −0.27 P = .004).

Conclusions

Educational interventions aimed at reducing salt consumption for this group must include interventions targeted at different behaviors related to overall salt consumption, the specificities of age, and level of schooling of the patients.

 

CHRONIC RENAL FAILURE (CRF) is the final result of chronic kidney disease. The different stages of chronic kidney disease (CKD) form a continuum of time. In the initial stages, patients are generally asymptomatic and do not experience clinically evident disturbances; however, in the latter stages, the severe disturbances in water and electrolyte balance, as well as endocrine and/or metabolic abnormalities, require chronic renal replacement therapy (hemodialysis or peritoneal dialysis) and result in higher levels of morbidity and mortality.1 Rates of progression of CKD to end-stage renal disease are conflicting, varying from 0.5% up to 60%, probably because of heterogeneity of the CKD population.2 Patients on hemodialysis or peritoneal dialysis used to have, on average, two hospital admissions per year, particularly the diabetic patients.3

Lifestyle changes, primarily an improved diet, are important tools in slowing the progression of the disease. Among the nutritional guidelines, the restriction of sodium intake can be highlighted.4

It has been reported that, in both experimental and clinical conditions, a high salt intake may result in detrimental effects on glomerular hemodynamics, causing progressively higher levels of glomerular filtration rate, a rise in glomerular capillary pressure, alteration of glomerular selectivity, an increase of albuminuria and proteinuria, and an increase in the renal arteriole resistance.5 Other effects observed were elevation of blood pressure and abolishment of light or dark rhythm of blood pressure in sensitive mice.6, 7 Furthermore, salt consumption seems to influence the antiproteinuric and antihypertensive effect of several drugs, including angiotensin-converting enzyme inhibitors and calcium antagonists, besides having a potential effect on the tissue of the kidney, independent of its ability to increase blood pressure, influencing the processes of renal fibrosis and necrosis.8, 9 The kidney can be further damaged by the effects of reactive oxygen species released from the increased oxidative stress.10, 11 Reduction in salt intake has been demonstrated to optimize the effectiveness of antihypertensive drugs and their antiproteinuric effect.8

In the context of the patient on dialysis, sodium consumption has been directly related to higher water consumption and, consequently, to the expansion of extracellular volume and higher levels of blood pressure as well as interdialytic weight gain (IWG), which is considered a key factor for the increase in mortality risk from heart disease in these patients.12, 13, 14

Therefore, there is a consensus that patients with CRF, at any stage of development of renal dysfunction, should restrict the dietary intake of salt, a recommendation that is included in all CRF programs. The recommendation is that the daily intake should not exceed 2 to 3 g/d among CRF patients.15, 16, 17

In the published data, there is evidence pointing out the fact that the recommendation alone is not sufficient to guarantee the expected change in health-related behaviors. Individual and environmental factors have been highlighted as important determinants of the individual decision regarding adopting a certain behavior.

There is growing recognition that educational interventions need to be grounded in the beliefs of the population of interest to be effective. According to social-cognition theories, motivation (or intention) is the proximal determinant of the behavior and is based on expected consequences of the behavior. Furthermore, intention can be changed when beliefs about the expected consequences change.18, 19

The Health Belief Model, a social-cognitive theory, is based on the understanding that a person will pursue a health-related action if that person feels that a negative health condition can be avoided (i.e., there are benefits in adopting a certain behavior); the person also has a positive expectation that by taking a recommended action, he or she will avoid a negative health condition, and believes that he or she can successfully take a recommended health action (i.e., there is a perception of few or no barriers to engaging in the behavior).20, 21 This theoretical model was the framework applied to the development of the Beliefs about Dietary Compliance Scale (BDCS), aimed at identifying the Benefits and Barriers perceived by patients with heart failure in following a low salt diet.22, 23 The beliefs investigated by this instrument may be present in other populations who are chronically ill and who have been given a formal recommendation of dietary salt restriction, such as CRF subjects undergoing dialysis.

The purpose of this study was to investigate the relationship between beliefs regarding a low salt diet and the nutritional behavior of sodium consumption as well their relation with sociodemographic and clinical variables among CRF patients on dialysis.

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Methodology 

This cross-sectional, correlational study enrolled 121 CRF patients aged >18 years who received tri-weekly, in-centre hemodialysis for at least 3 months at 2 clinics in Southeast Brazil, one clinic being a public clinic at a large urban teaching hospital and the second a private clinic. In both clinics, patients received regular information regarding dietary salt restriction in individual appointments with the local dietitian. Patients with cognitive impairment and who had limited understanding of the study purpose and of the instruments used for data collection (n = 2), those with a diagnosed psychiatric disorder (n = 1), and patients following other dietary restrictions (n = 1) were excluded.

Subjects were enrolled consecutively in such a way so as to include all patients who met the inclusion criteria until the estimated pre-set sample size was met. Sample size was estimated considering correlation coefficients of 0.40 (of moderate magnitude, according to psychosocial variables, according to Ajzen and Fishbein24), values of α = 0.05 and β = 0.20, which pointed to a minimum of 75 subjects. The sample was composed of 117 subjects to avoid too small a size during the process of statistical analysis.

All patients took part in structured interviews to obtain sociodemographic data: age, monthly income, schooling, gender, and race; clinical data: residual renal function, length of dialysis treatment, length of CRF diagnoses, main co-morbidities, dry weight, average of IWG/last 10 dialysis sessions (data presented in Table 1); and to evaluate beliefs regarding a low salt diet and the behavior of salt consumption. Patients were recruited on one of their regular days of dialysis. One of the researchers explained the purpose of the study and invited the patients to participate. All patients involved in the study signed the informed consent form approved by the local ethics committees, in accordance with ethical principles, including the World Medical Association Declaration of Helsinki and resolution 196/96 of Brazil. Questionnaire items were read to each participant, who responded verbally to each item, and the interviewer recorded the responses. Reading the items to participants standardized the method of data collection, minimized missing data, and decreased respondent burden.

Table 1. Sociodemographic and Clinical Data (n = 117)
N (%)Mean (SD)Median (IQR)Variance
Gender
Male61 (52.1)
Race
Caucasian41 (35.0)
Non-Caucasian76 (64.9)
Residual renal function
Yes66 (57.4)
CRF etiology
Diabetes Mellitus27 (23.3)
CGN20 (17.1)
Hypertension17 (14.7)
Indetermined26 (22.4)
Other41(35.5)
Comorbidities
Diabetes29 (25.0)
Hypertension76 (65.5)
Age 51.5 (15.8)53.0 (25.0)20-80
Schooling (years) 5.8 (4.1)4.0 (7.0)0-16
Family income (US$) 437 (463.9)291.3 (437)0-2141.8
Length of CFR diagnosis (years) 6.1 (5.0)4.0 (6.0)1-25
Length of dialysis (months) 34.8 (25.4)35 (24.0)4-148
Dry weight (kg) 65.3 (14.6)63 (16.2)40-124
IWG (kg) 2.6 (1.2)2.8 (1.5)0.1-6.6

CGN, chronic glomerulonephritis; Indet, indetermined; SD, standard deviation; IQR, inter quartile range; IWG, interdialytic weight gain.

Beliefs About Dietary Compliance Scale 

The Brazilian version of the BDCS,22, 23, 25 was used to measure perceived benefits and barriers related to dietary sodium consumption. The BDCS contains 12 items, 7 items assessing perceived benefits and 5 items assessing perceived barriers related to adherence to dietary sodium restrictions. Participants were asked to respond to each item on a scale from 1 (strongly disagree) to 5 (strongly agree). Responses for each of the BDCS subscales are summarized, with higher numbers reflecting greater agreement. In this study, the Benefits subscale presented Cronbach's α = 0.82, and the Barriers subscale, α = 0.55.

Salt Consumption 

For the salt consumption assessment the sodium Food Frequency Questionnaire (FFQ) was used, as well as the estimation of discretionary salt. Sodium consumption was evaluated considering the 2 methods separately (FFQ; discretionary salt) and summed (FFQ+discretionary salt) to obtain the value of total salt intake.

Sodium FFQ 

The FFQ is composed of a 15-item list of foods known to be high sodium sources frequently consumed by the local population.26 Participants were asked how frequently each food was consumed during the last year, with responses ranging from 1 = “never” up to 7 = “twice or more per day”. For each food on the list, there is a sodium conversion factor, a number between 0.01 and 1.00, derived from food composition tables that represent the amount of sodium contained in 1 g of the food. The sodium content of the average portion consumed for each participant is calculated by multiplying the weight of the portion (in grams) by the conversion factor. Finally, the number of milligrams of sodium in the usual serving size is multiplied by the frequency of intake and corrected for the frequency of monthly consumption (0 for the score of 1, representing never; 0.5 for the score of 2, representing less than once a month; 2 for a score of 3, representing a frequency of 1 to 3 times per month; 4 for a score of 4 representing once a week; 12 for a score of 5, representing a frequency of 2 to 4 times a week; 30 for a score of 6, representing a frequency of once a day; and 60 for a score of 7, representing a frequency of 2 or more times per day). The result is the monthly sodium consumption for each item.

Discretionary Salt 

Patients were asked to rate the usual monthly salt consumption (considering as reference, 1 kg package of salt) as well as the number of persons in the household who ate at least 5 meals per week at home, to correct the salt consumption per person, adapted from the methodology described.27 The United States Department of Agriculture reference (1 g of salt = 400 mg of sodium) was used to obtain the monthly and daily average of number of milligrams of sodium per person.28, 29

Data were analyzed with the Statistical Package for Social Sciences SPSS-PC, Version 10.0 (Chicago, IL). Descriptive statistics (mean scores, standard deviations, ranges, and frequencies) were computed for each variable. The Spearman correlation coefficient was used to verify correlations between beliefs, salt consumption, and sociodemographic and clinical variables. Salt consumption was further split into quartiles (Q25 and Q75), forming 2 groups of low and high salt consumers, respectively. The Mann–Whitney test was used to assess differences between the high and lower salt consumers regarding the Benefits and Barriers beliefs.

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Results 

Salt Intake 

The sodium intake assessed by the FFQ and by the reference measures was higher for men than for women (P < .05). No difference in discretionary salt use was observed between genders.

The sodium intake assessed by the FFQ pointed to a mean consumption of 2.4 g (±2.6) of salt per day sourced from high-content salt foods. Among the 15-item list of foods, seasonings and instant noodles were the items with greater contributions to salt consumption. The less consumed items were snacks, sardines, and canned beef burger.

Discretionary salt was the main contributor to salt intake among the studied group, pointing to a high daily addition of salt (8.9 ± 5.7 g/d). Total salt intake, estimated by summing the discretionary salt and the FFQ values, was 10.6 g/day (±6.3) (Table 2).

Table 2. Discretionary Salt Use (According to Salt Packages and Grams, and Daily Salt Intake), Daily Salt Intake According the FFQ, Total Salt Intake (Discretionary Salt+FFQ), and Beliefs Scores According to Benefits and Barriers Subscales
Mean (SD)MedianRange
Discretionary salt
Salt packages∗ (per month)0.8 (0.5)0.50-3
Grams of salt (per month)830.0 (548.7)660.084-3000
Salt per meal (g)5.2 (3.2)4.41.4-18.5
Daily salt intake (g)8.9 (5.7)7.71.5-35.0
FFQ
Daily salt intake from usual consumption of foods with high sodium content (g)2.4 (2.6)1.50-12.5
Total salt intake
Discretionary salt+FFQ10.6 (6.3)9.81.5-35.7
Mean (SD)Min.Max.α∗
BDCS—benefits
1. Eating a low salt diet will keep me healthy4.1 (1.0)15
2. Salty food is not good for me4.3 (0.6)25
3. Eating a low salt diet will keep my heart healthy4.2 (0.7)25
4. Eating a low salt diet will control my swelling4.1 (0.7)15
5. Eating a low salt diet will prevent the retention of fluid (swelling) in my body4.0 (0.8)15
11. When I follow my low salt diet I feel better4.0 (0.9)15
12. Eating a low salt diet will help me breathe more easily3.6 (1.0)15
Total score4.0 (0.6)1.35.00.82
BDCS—barriers
6. Eating a low salt diet makes it difficult to eat out3.7 (1.0)15
7. The food does not taste good in a low salt diet3.8 (1.0)15
8. It is expensive to follow a low salt diet2.2 (0.8)14
9. Following a low salt diet takes too long2.2 (0.8)14
10. It is very difficult to understand how to follow a low salt diet2.7 (1.1)15
Total score3.0 (0.5)1.44.00.55

SD, standard deviation; Min, minimum; Max, maximum; α*, Cronbach's α.

∗Salt packages contain 1 kg of salt.

Beliefs About the Salt Consumption 

A mean score of 3.0 (±0.5) was observed for the Barriers subscale and of 4.0 (±0.6) for the Benefits subscale, indicating that the subjects perceived less barriers and more benefits to following a low-sodium diet. Beliefs with the highest score in the subscale of Benefits were “salty food is not good for me” and “eating a low salt diet will keep my heart healthy.” Moreover, the lowest scores were found in the beliefs “when I follow my low salt diet, I feel better” and “eating a low-salt diet will help me breathe easier.”

In the Barriers subscale, beliefs with the highest score were “eating a low-salt diet makes it hard to go to restaurants” and “food does not taste good on the low-salt diet” (Table 2). Correlation between items and between item-total score of the subscales was also examined. It was found that within the Benefits as well as the Barriers subscales, correlations between items and between item-total scores was positive; however, correlations were stronger within the Benefits subscale than within the Barriers subscales, ratifying the observation of greater internal consistency for the Benefits subscale (Table 3).

Table 3. Inter-Item and Item-Total Score Correlations of the Benefits and Barriers Subscales
BenefitsItem 1Item 2Item 3Item 4Item 5Item 11Item 12Total Score
Item 11.00.516†0.409†0.331†0.401†0.400†0.228†0.633†
Item 20.516†1.00.589†0.596†0.433†0.575†0.325†0.685†
Item 30.409†0.589†1.00.542†0.438†0.457†0.393†0.717†
Item 40.331†0.596†0.542†1.00.722†0.485†0.349†0.726†
Item 50.401†0.433†0.438†0.722†1.00.476†0.323†0.717†
Item 110.400†0.575†0.457†0.485†0.476†1.00.514†0.724†
Item 120.228†0.325†0.393†0.349†0.323†0.514†1.00.624†
BarriersItem 6Item 7Item 8Item 9Item 10Total Score
Item 61.00.217∗−0.104−0.1300.0670.425†
Item 70.217∗1.00.1180.0820.185∗0.583†
Item 8−0.1040.1181.00.492†0.275†0.532†
Item 9−0.1300.0820.492†1.00.312†0.546†
Item100.0670.185∗0.275†0.312†1.00.652†

∗P ≤ .005.

†P ≤ .001.

Correlation Between Sociodemographic Variables, Beliefs, and Methodologies for Evaluation of the Sodium Intake 

The correlation between sociodemographic variables, the Beliefs scores, and salt consumption was also investigated. It was found that the Barriers subscale was positively correlated with length of dialysis treatment (r = 0.25, P = .008), and the Benefits subscale was positively correlated with monthly income (r = 0.22, P = .017). Discretionary salt use was positively correlated with age (r = 0.232, P = .016) and negatively correlated with schooling (r = –0.27 P = .005). The FFQ was positively correlated with length of dialysis treatment (r = 0.249 P = .008) and with schooling (r = 0.25, P = .006) and negatively with age (r = −0.27 P = .004). The total salt consumption was positively correlated with IWG (r = 0.20, P = .032) and negatively correlated with schooling (r = −0.186 P = .044). Thus, data pointed out that subjects with a longer length of dialysis treatment perceived more barriers to following a low-salt diet, and moreover, had a higher consumption of foods with high salt content. Surprisingly, age and level of schooling presented divergent correlations with discretionary salt use and consumption of foods with high salt content. Those with lower schooling and of older age tend to consume more discretionary salt; however, they report a lower consumption of foods with high salt content. Subjects with lower monthly income recognize less benefits of consuming a low-salt diet. Importantly, subjects reporting higher total salt consumption presented higher IWG.

Benefits and Barriers Beliefs Discriminating Salt Intake 

To evaluate the potential of the Benefits and Barrier beliefs in discriminating low and high salt consumers, the consumption, given by each one of the methods used in this study to quantify the salt intake, was divided in quartiles (Q25 and Q75, lower and higher consumption, respectively) (Table 4). Comparing the medians of the beliefs scores, it was observed that, in general, higher scores on the items of the Benefit subscale were obtained for the low-salt consumers, considering all the methods used in this study to evaluate salt consumption: discretionary salt, FFQ and total salt. The opposite was observed for the items of the Barriers scale: lower scores among the low-salt consumers. However, these differences were significant only for some beliefs of the Benefit subscale: “eating a low-salt diet will keep my heart healthy” and “eating a low-salt diet will keep my swelling down”, discriminating higher and lower salt consumers according to the discretionary salt method (P ≤ .05) and the beliefs “eating a low salt diet will keep me healthy” and “salty food is not good for me” according to the FFQ method (P ≤ .05).

Table 4. Comparison of the Scores of the Benefits and Barriers Beliefs Between Low and High Salt Consumers, According the Methods: Discretionary Salt, FFQ, and Total Salt Intake
Discretionary SaltFFQTotal Salt Intake
Beliefs
BenefitsLow-ConsumersHigh-ConsumersLow-ConsumersHigh-ConsumersLow-ConsumersHigh-Consumers
Item 1
Mean3.93.44.43.94.23.9
Median4.04.04.0∗4.0∗4.04.0
Item 2
Mean4.34.24.54.24.44.2
Median4.04.05.0∗4.0∗4.04.0
Item 3
Mean4.44.04.24.24.24.2
Median4.0∗4.0∗4.04.04.04.0
Item 4
Mean4.33.94.33.94.24.0
Median4.0∗4.0∗4.04.04.04.0
Item 5
Mean4.04.14.03.93.94.1
Median4.04.04.04.04.04.0
Item 11
Mean3.83.94.03.83.94.0
Median4.04.04.04.04.04.0
Item 12
Mean3.73.83.73.53.54.0
Median4.04.04.04.04.04.0
Total score
Mean4.14.04.23.94.04.0
Median4.04.04.04.04.04.0

Barriers

Item 6
Mean3.63.63.83.83.53.8
Median4.04.04.04.04.04.0
Item 7
Mean3.83.94.03.73.93.7
Median4.04.04.04.04.04.0
Item 8
Mean2.32.42.12.02.102.34
Median2.04.02.02.02.02.0
Item 9
Mean2.32.42.12.12.12.2
Median2.02.02.02.02.02.0
Item 10
Mean2.62.12.72.42.62.6
Median2.02.02.02.02.02.0
Total score
Mean2.93.02.92.82.82.9
Median2.82.93.02.82.82.8

∗P < 0.05.

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Discussion 

Data obtained in this study provide important knowledge for the diagnosis of the problem of salt consumption and the individual factors associated with this complex behavior among patients with renal failure on dialysis.

We observed a high salt consumption in the studied population, primarily in the use of discretionary salt. Similar results were found in other studies that applied similar and additional methodology to assess the behavior of sodium consumption among hypertensive subjects.26, 30 In these studies, salt consumption was also evaluated by the 24-hour recall and the gold standard method for evaluation of salt consumption: 24-hour urine sodium levels (which obviously, would not be feasible for the population of this study).

The present and previous data point out that the daily consumption of salt is high among the general population31 and even in the critical populations of CRF and hypertensive patients, despite the recommendation of sodium restriction for both groups (recommendation for daily salt intake for hypertensive patients is approximately 4 g,29, 30, 31, 32 and 2.4 g for kidney patients.16)

The report of salt consumption was also correlated positively with the IWG between dialysis treatments, as previously reported.14 The IWG is considered to be an important risk factor for mortality from heart disease among patients in renal replacement therapy,12 highlighting again the importance of a low-salt diet for this group of patients.

Another interesting finding was the observation of a distinct relationship between the sociodemographic variables: age, schooling, and the consumption of sodium estimated by the complementary methods, FFQ, and discretionary salt. Older patients with lower levels of schooling reported adding more salt during or after food preparation; however, those who were younger and who had higher levels of schooling consumed greater amounts of foods with high sodium content. The total salt consumption (estimated from the sum of discretionary salt and FFQ methods) exhibited a similar pattern of correlation as that observed for the discretionary salt method (positive correlation with age and negative correlation with level of schooling), probably due to the major contribution of the discretionary salt to the total sodium intake. The major consumption of foods with high salt content among the younger individuals can be explained by their easier access to processed salty foods, probably more frequent than for older patients. These results point to the need for specifically designed educational interventions aimed at reducing salt addition and the consumption of foods with high salt content.

Regarding the Benefits and Barriers Beliefs, the subjects tended to agree with the Benefits of a low-salt diet and to disagree with the Barriers beliefs. A previous study,33 which evaluated the reliability and validity of an adaptation of the BDCS to CRF patients on dialysis, observed similar findings—more agreement with benefits than barriers.

Four different beliefs from the Benefits subscale were able to discriminate high and low salt consumers: “eating a low-salt diet will keep me healthy”; “eating a low-salt diet will keep my heart healthy”; and “salty food is not good for me”. Indeed, the beliefs in the benefits were positively related to monthly income. Thus, educational activity must encompass specific strategies to strength benefit beliefs regarding salt intake, especially for those with a lower monthly income.

Although none of the Barriers beliefs had been able to discriminate higher and lower salt consumers, the total score of this subscale was related to the length of dialysis treatment, suggesting that over time, patients exhibit an increasing perception of more barriers to following a low salt diet. Therefore, it would be interesting to conduct future studies to explore in depth the barriers identified by the patients to reducing salt consumption and its variability over time after beginning renal replacement therapy.

In the study involving hypertensive patients,26 it was observed that the perceived barriers were positively correlated to salt consumption. It is possible, however, that the beliefs outlined in the Barrier subscale (originally developed for heart failure patients) might not be truly representative of the barriers beliefs in the studied group. This assumption may be reinforced by the low internal consistency observed for the subscale in this study, and by the weak correlation observed between the items of the subscale and its total score. Future studies must consider evaluating the content validity of the Barriers subscale of the BDCS—Brazilian version.

As for the implications of this study, it can be highlighted that it is necessary to propose educational interventions aimed at reducing salt consumption in this group. Planning the interventions must consider the needs of specific populations: for older people and for those with lower levels of schooling, interventions must be aimed at reducing the use of additional salt to foods during or after preparation. Another set of interventions should be directed at the younger people with higher levels of schooling, aiming at clarifying the salt content in processed foods, identification of items with high salt content, and how to verify the salt content on labels of processed food, as well as the importance of replacement of these foods with other healthier options. In this sense, interdisciplinary work involving the presence of a dietitian may result in diversification of strategies, greater targeting of interventions, and greater chances of achieving the goals set for this nutritional behavior. Further studies using longitudinal designs with the follow-up of patients receiving intensive diet counseling based on the present findings could provide significant contributions to the management of CRF patients.

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Acknowledgments 

The authors acknowledge The State of São Paulo Research Foundation for the financial support (process n. 2007/04341-6).

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PII: S1051-2276(10)00078-6

doi:10.1053/j.jrn.2010.03.007

Journal of Renal Nutrition
Volume 21, Issue 2 , Pages 160-168, March 2011

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