Abstract
Background:
In New Zealand (NZ), the prevalence of dialysis is increasing with Māori and Pacific people over-represented. Cardiovascular disease (CVD) is responsible for the majority of deaths in the NZ hemodialysis (HD) population. There are few definite interventions to improve CVD mortality risk in dialysis patients except transplantation. The Sodium Lowering in Dialysate (SoLID) Trial was a multi-centre randomised controlled trial conducted in NZ from 2012 to 2016. The trial compared the use of low sodium dialysate (135 mmol/L (mM) sodium) with conventional dialysate (140 mM sodium) in HD patients over a 12-month period. The main outcomes included left ventricular (LV) mass (primary outcome), CVD mortality, blood pressure (BP), and interdialytic weight gain (IDWG).
Dietary sodium intake also has a critical role in kidney health. There are few data on sodium intake and its relationship with outcomes in dialysis populations. Therefore, this Nutritional Sub-study of the SoLID Trial represents a major opportunity to describe intake of sodium and other nutrients in a multi-ethnic sample of dialysis patients in NZ, and the response of sodium intake to manipulation of dialysate sodium (DNa). As part of the core trial design, longitudinal dietary data were therefore collected to document nutrient intake in trial participants.
Objectives:
1) Describe baseline dietary intake of sodium, and other nutrients, in the context of recommended ranges for intake from relevant clinical practice guidelines
2) Compare baseline dietary sodium intake with a range of socio-demographic and health related characteristics, as well as intake of other nutrients
3) Compare baseline dietary sodium intake with 6 months and 12-month data
4) Compare dietary sodium intake in the low DNa vs high DNa group at baseline, 6 months, and 12 months.
Methods:
Recruitment of participants
Participants were patients on HD recruited in 10 centres from 7 District Health Boards (DHBs) in NZ (Counties Manukau, Auckland, Waitemata, Waikato, Capital & Coast, Canterbury, and Southern), with an accrual period of 36 months between May 2013 to May 2016, and patient follow-up of 12 months. Participants were eligible for the study if they were incident or prevalent patients treated with maintenance HD, aged 18 years or older, suitable for both low and standard sodium dialysate with pre-dialysis serum sodium concentration of ≥135mM.
Dietary information collection and analysis
Ninety-nine participants randomised into the SoLID trial, were asked to provide baseline data and a 3-day weighed food diary (3DWFD) at baseline, 6 months, and 12 months. Diaries were analysed using Foodworks 8 professional, supplemented by other sources of nutrient information.
Univariate and logistic regression analyses were used to assess the differences in baseline characteristics of participants by sodium intake category. The significance of changes in nutrient intakes between baseline and 6 months and baseline and 12 months was assessed by paired t-tests. Univariate and multivariate regression analyses were used to model a number of factors against baseline sodium intake (mg/day). Inferential statistical analyses were undertaken and two sorts of regression models were built to analyse the treatment effect of DNa on oral sodium intake at 6 and 12 months.
Results:
Of the 99 participants recruited to the SoLID Trial, 86 completed at least one 3DWFD, 85 completed a 3DWFD at baseline, 61 completed a 3DWFD at 6 months, 60 completed a 3DWFD at 12 months, and 52 completed a 3DWFD at all three time periods.
The Nutritional Sub-study population had a mean (standard deviation (SD)) age of 52 (13) years. The largest ethnic group was NZ European and Other (NZEO) (44%) followed by Pacific (30%).
The mean (SD) sodium intake was 2502 (957) mg/day at baseline, 2738 (1251) mg/day at 6 months, 2415 (1125) mg/day at 12 months. According to NZ Renal Dietitians’ recommendation, more than half of the participants exceeded the sodium intake target. Of 59 participants who completed baseline and 12-month diaries, paired t test showed 12-month mean sodium intake (2317 mg/day) was significantly lower than baseline mean sodium intake (2636 mg/day) (P=0.0082). Multivariate regression analysis shows baseline sodium intake was positively associated with energy intake (β=211, P<0.0001).
For other nutrients, 5% (baseline), 7% (6 months) and 3% (12 months) of participants met the recommended calorie density; nine percent (baseline), 7% (6 months), and 15% (12 months) of participants ate the recommended minimum of 1.2g/kg of protein per day; about two thirds of participants were consuming inadequate fibre across all the three time periods. In contrast, saturated fat contributed around 14% of total energy. Approximately two out of five participants had excessive phosphorus intake at all the three time periods. More than 90% of participant exceeded the idea sodium/potassium ratio of 1:1 at all three time periods.
There were changes in dietary sodium intake in low DNa and high DNa groups over time. The sodium intake was similar at baseline between two groups (2531 vs 2475 mg/day); the high DNa group had higher sodium intakes at 6 months (3004 vs 2481 mg/day) and 12 months (2527 vs 2289 mg/day) compared to low DNa group, however, the mean treatment effect was not significant according to mixed-effects linear modeling (P=0.063 at 6 months, P=0.411 at 12 months) and repeated measure analysis of covariance (ANCOVA) (P=0.067).
Conclusion:
This study showed a high proportion of dialysis patients in SoLID Trial did not meet current renal-specific dietary recommendations. The data suggest excess sodium intake at baseline, 6 months and 12 months. It is also evident that there was poor intake of calorie, protein, fibre and excess intake of saturated fat, and phosphorus. Health professionals, especially renal dietitians, need to consider barriers for non-adherence and continue to promote lower sodium intake without compromising energy and protein intake. The results from the study merit further research, especially, into the effect of DNa on dietary sodium intake.