The Critical Examination of Sodium Intake and Its Impact on Human Health
Sodium is an essential mineral in the body, playing an important role in the maintenance of physiological homeostasis. It is the principal cation of the extracellular fluid, controlling its volume as well as aiding in transmission of neural impulses and muscular contractions. Hyponatremia is a rare condition in the typical Australian diet as sodium is readily available in a wide range of foods and beverages (2). In instances when this may occur however the kidneys respond by reducing sodium losses in urine and sweat, resulting in the further prevention of deficiencies. The National Health and Medical Research Council (NHMRC) has provided nutrient reference values (NRVs), for sodium intake, low enough to prevent this deficiency yet high enough to ensure absorption of other nutrients within a balanced diet.
The upper level (UL) for sodium is an important marker as high intakes have been associated with high blood pressure (5). Research from the National Health Survey (NHS) 2011 – 2013 has found that children and adolescents were more likely than adults to exceed the UL and so varied levels have been set specific to each age group (Fig 1.1) (3).
The main method of dietary data collection employed within the Australian Health Survey, 2011-2013 (AHS) was a 24-hour recall method. This interview method was broken up into five phases which included: quick list, forgotten foods, time and occasion, detail cycle and a final probe (3). This was an improvement on the original 3 phase 24-hour recall method used in the 1995 National Nutrition Survey as this addition better stimulated respondents to provide a more complete and accurate picture of foods intake within the 24-hour period prior to the interview (3). The simplistic nature of conducting the 24-hour recall method made it cost effective and relatively low in burden to both the 12, 153 participants and the non-nutrition trained ABS interview panel tasked with implementing it. It is also important to note however, that respondents within the 2011-2013 AHS originated from the 2011-2012 NNPAS. As such, the risk of respondent bias must be considered as their involuntary involvement in the study may have affected their motivation to provide a complete recollection of their dietary intake throughout the NHS (3).
The 24-hr recall method may not have been as successful as other study methods in obtaining adequate data to realistically construct a picture of the relationship between the national diet and health outcomes. It has been shown that one major issue within this study was the difficulty of accurately recording portion size, for example, when respondents may have shared a family meal (3). A study method which may have eliminated this issue would have been a food journal asking respondents to weigh and record foods and beverages at the time of consumption. Though this would have been considerably more expensive, arduous for the respondents and time consuming for the data processors to implement on a national scale. A way in which the NHS attempted to compensate for the floors of this data collection method was by conducting a second 24-hr recall 8 days after the completion of the AMPM via phone interview and thus were provided with data depicting respondent’s dietary patterns on more than just a singular 24-hr period.
Sodium Intake Compared to Sodium NRV’s
The nutrient reference values (NRV) are established to avoid deficiency diseases whereas the suggested dietary targets (SDT) aim to prevent or reduce the risk of chronic diseases (2). In the National Health Survey (NHS) it was found that 91% of males and 74% of females within the child and adolescent population group exceeded the upper level of sodium intake specific to their age group (3).
In 2017 the National Health and Medical Research Council (NHMRC) approved a revision of SDT for sodium intake in adults. The rationale for the reduction from 3600 mg/day to 2000 mg/day is that it would not only meet all nutritional requirements within a healthy diet but it would also reduce the average blood pressure of the Australian population (2). Data analysis failed to determine a point when the relationship between increased sodium intake and increased blood pressure did not occur therefor suggesting that it transpires at all measurable levels of intake (1). Therefor health benefits associated with reducing sodium intake, such as a reduction in blood pressure, would also be seen by sodium intake reductions at all levels.
Findings from the Australian Health Survey recorded a mean intake of 3116.8 mg/day of sodium (Figure 1.3), being 35.15% above that of the recommended upper level for children between 14 to 18 years of age, 2300 mg/day. The study also showed that children and adolescents were the most likely of all other population groups within the study to exceed the SDT for sodium, and that males were more likely to consume sodium in excess of the UL than females (Figure 1.3). Due to the fact that health concerns associated with sodium are prevalent at all measurable levels, children and adolescents should be encouraged to reduce their intake to meet the adequate intake (AI) level of 460-920 mg/day.
Summary of Evidence Linking Sodium to Elevated Blood Pressure
A critical health outcome associated with elevated levels of sodium intake in children and adolescents aged between 5 to 18 years is elevated blood pressure. The major concern for this population group is twofold, as not only is it the period where preferences for high sodium foods are formed but the antecedents of high blood pressure begin in early life continuing into adult hood and is strongly associated with further health risks such as hypertension and blood pressure related cardiovascular disease (1).
The NHS has indicated that 91% of males and 74% of females aged between 2-18 years exceed the upper level recommendations for sodium (figure 1.1). This therefor highlights the importance of implementing healthy eating strategies at a young age. One such trial conducted in the Netherlands suggested that infants fed on a low-sodium formula were found to have significantly lower BP levels than those fed on regular-sodium formula within the first 6 months of life (1). These findings were then further supported when the population group was restudied at 15 years of age and continued to display significantly lower blood pressure levels than that of the control group, despite following a normal dietary intake post infancy.
An elevated level of sodium intake, exceeding the upper level recommendation, is positively associated with an increase in systolic blood pressure amongst all age groups (1). Therefor given the prevalence of blood pressure related diseases such as CVD and hypertension in the adult Australian population; the notion of this issue tracking from childhood strongly supports the need for implementing interventions focused on reducing sodium intake within the child and adolescent population to better reduce its prevalence in later years (4).
Critical Evaluation of Evidence Linking Sodium to Elevated Blood Pressure
Whilst the clinical manifestations of sodium intake in excess of the upper limit occurs predominantly in the adult population, many studies have indicated that the antecedents to this begin in childhood. These studies have undertaken various methodologies in their data collection including 24-hr recalls, controlled feeding studies and observational studies. The importance of these studies being conducted in the child and adolescent population is due to evidence indicating that prevention strategies introduced at a younger age may be the most appropriate approach to reducing the incidence of high blood pressure in later life (4).
Though the positive relationship between sodium intake and hypertension has been frequently examined and supported across a variety of study methods, knowledge into its true relationship is handicapped by the preferential use of short term studies due to their low cost, low burden approach (4). One example by which this method may inhibit true results is in the initial measuring of sodium intake. Studies such as the Sodium Intake and Blood Pressure Among US Children and Adolescents obtained their sodium intake data by means of estimation from 24hr food recalls and thus would obtain far inferior quality data compared to urinary electrolyte excretion methods used in the longitudinal Sodium intake and blood pressure study. Controlled feeding methods such as in the Reducing sodium intake in children study provide powerful data into food intake patterns when considering the sodium intake and hypertension relationship, however they are inherently expensive and logistically challenging to perform (4).
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