Download Reducing Dietary Sodium Intake

Canadian Council of Food and Nutrition
Phone: 905-625-5746 Email: [email protected] Web: www.ccfn.ca
Address: 2810 Matheson Boulevard East, 1st Floor, Mississauga, Ontario L4W 4X7 CANADA
Reducing Dietary Sodium Intake
A CCFN WATCHING BRIEF
NOVEMBER 2008
Prepared byi
Susan Barr, PhD, RD, FDC
Professor of Nutrition
University of British Columbia
ecently, the issue of dietary sodium has received increased attention in
Canada. In response to concern about the prevalence of hypertension,
Blood Pressure Canada and 16 health-related organizations released a
policy statement (1) calling for Canadians’ daily sodium intake to be reduced to
between 1500 mg and 2300 mg, the range recommended by the Institute of
Medicine’s Dietary Reference Intakes (DRIs). Shortly after the policy statement
was released, Health Canada announced the formation of a multisectoral
Working Group on Dietary Sodium Reduction (2), with a mandate to develop
and oversee a strategy that would lead to lowering the sodium content of
Canadians’ diets to within the recommended range. Given that 90% of
Canadian adult men and two thirds of adult women have intakes above the
upper end of the range, broadly based approaches and concerted efforts at all
levels will be required to meet this goal.
R
The purpose of this brief is to provide context for this issue. First, an overview
of sodium’s functions in the body and the basis for the current intake
recommendations are described, followed by a discussion of the adverse effects
of excessive sodium intake, with an emphasis on hypertension. Next,
information on sodium intakes of Canadians and sources of dietary sodium is
presented, followed by the functional roles of sodium as a food ingredient.
Initiatives to reduce sodium intake in other countries are highlighted to illustrate
the successes and challenges. Finally, the implications for various sectors are
discussed.
i
CCFN acknowledges the assistance of Sheryl Conrad, RD, in compiling the reviewer feedback and editing this Watching Brief.
Executive Summary
Because of concerns about the prevalence of hypertension, Canadians’ dietary sodium
intakes are receiving increased attention. Sodium is an essential nutrient for humans, and
the Adequate Intake (AI) was set at 1500 mg/d for young adults. Although many other
lifestyle variables also influence blood pressure, decreasing sodium intake has been
projected to have important effects on hypertension risk at the population level. To reduce
that risk, the Tolerable Upper Intake Level (UL; a level that people are recommended not
to exceed) was set at 2300 mg/d for adults.
Currently, most Canadians (66%–97%, depending on sex and age group) have sodium
intakes above the UL. Most dietary sodium is added during the processing of food, which
indicates that efforts to reduce intake will need to be directed to the food supply as a whole.
Sodium plays a number of roles in the food supply, including its contribution to taste and
palatability, its roles in food processing, and its action as a preservative.
According to CCFN’s Tracking Nutrition Trends (TNT) survey, Canadians appear to be
aware that salt/sodium is a health issue, although relatively few report making related
changes to their diets. The World Health Organization recommends that countries aim for
a sodium intake of no more than 2000 mg/d, and a number of countries with high intakes
have taken action to move toward this goal. To date, modest reductions have occurred,
although no country has yet attained the goal. If Canada is to successfully reduce
population sodium intakes, there are implications for consumers, government, industry,
schools, physicians, and research.
Sodium: The Basics
Sodium chloride, also termed salt, is essential for humans (3). Together, sodium and
chloride are the principal osmotic determinants of extracellular fluid volume and are thus
responsible for maintaining plasma volume. In addition, in conjunction with potassium,
sodium is a determinant of the membrane potential of cells, and plays a role in the active
transport of nutrients across cell membranes
Sodium versus Salt
(e.g. glucose and amino acids are actively
The amount of sodium in a given amount of
absorbed from the small intestine by a
table salt (sodium chloride) can be estimated
sodium-linked mechanism). Dietary sodium
by multiplying by 0.4. For example:
is almost completely absorbed in the small
♦ 1 gram of salt has about 0.4 g (400 mg) of
intestine, and is lost primarily through the
sodium (and 0.6 g (600 mg) of chloride).
urine. In non-sweating individuals, urinary
♦ A teaspoon of table salt is about 6 grams,
losses are essentially equal to sodium
and thus has 2400 mg of sodium.
intake, which means that 24-hr urine
collections provide an accurate indicator of intake. Sodium can also be lost through sweat;
in individuals who sweat heavily (e.g. endurance athletes exercising in the heat), losses can
be considerable.
2
Recommended Sodium Intakes
It is possible to attain sodium balance on extremely low sodium intakes, as obligatory
urine, fecal, and dermal sodium losses are approximately 100–200 mg/d (3). However,
achieving this level of sodium conservation is associated with substantially increased
plasma renin activity. Renin is a proteolytic enzyme released from cells in the kidney in
response to reduced blood pressure, blood volume, or sodium concentration. It initiates the
conversion of angiotensinogen to angiotensin II, which in turn stimulates aldosterone and
antidiuretic hormone release. The results include increased renal reabsorption of sodium
and water, which contribute to restoration of blood volume, blood pressure and serum
sodium concentration. However, some research has suggested that chronically elevated
plasma renin activity may be undesirable, as it has been reported to be associated with
cardiac hypertrophy, insulin resistance, and other cardiovascular risk factors (3). There
may also be adverse effects of chronically elevated aldosterone (4).
It was challenging for the Institute of Medicine (IOM) panel to establish recommended
intakes for sodium. When possible, it is preferable to set an Estimated Average
Requirement (EAR; an intake level that meets the requirements for a specified indicator of
adequacy for half the members of an age–sex group). The EAR in turn can be used to set a
Recommended Dietary Allowance (RDA; an intake level that meets or exceeds the
requirements of nearly all healthy individuals). However, setting an EAR requires dose–
response data for an appropriate indicator of adequacy, and such data were not available
for sodium. Although it might have been possible to use the minimum intake required to
attain sodium balance as an indicator of adequacy, this would have led to the sodium RDA
being set at a very low level. The uncertainty about potential adverse effects of elevated
plasma renin (which would occur with very low sodium intakes) was a concern.
Accordingly, the IOM panel set Adequate Intakes (AI) instead. Like the RDA, the AI is an
intake level thought to meet or exceed the needs of almost all healthy individuals (3).
Dietary Reference Intakes for Sodium (3)
Age Group (years)
Adequate Intake (mg/d)
Tolerable Upper Intake
Level (mgd)
1–3
4–8
9–13
14–50
51–70
>70
1000
1200
1500
1500
1300
1200
1500
1900
2200
2300
2300
2300
The AIs for sodium are shown in the table above. The sodium AI was set at 1500 mg/d
for young adults because it was considered high enough to:
1. allow selection of an overall diet that would meet needs for other essential nutrients;
2. allow for sweat sodium losses in unacclimatized individuals exposed to heat, or in
those who are physically active in accordance with other health recommendations; and
3. minimize concerns about elevated plasma renin activity (3).
3
Tolerable Upper Intake Levels for Sodium
In addition to setting recommended sodium intakes, the IOM panel was charged with
setting a Tolerable Upper Intake Level (UL; the highest daily intake level thought to pose
no risk of adverse health effects to almost all individuals in the general population). As
intake increases above the UL, the potential risk of adverse effects may increase (3). For
sodium, adverse effects include increased blood pressure (3,5,6), as well as renal disease,
stomach cancer, renal stones and low bone mineral density (3,6). The association with high
blood pressure was identified as the adverse effect to set the UL (3).
Just as identifying recommended sodium intakes was challenging, so too was establishing
the UL. As indicated above, the definition of the UL implies that a threshold exists below
which sodium has no effect on blood pressure. However, it was not possible to identify a
clear lower threshold for sodium intake. Furthermore, identifying the UL was also
complicated by the fact that many other variables affect blood pressure (e.g. age, body
weight, physical activity, other dietary factors, genetics). Thus, the blood pressure response
to a change in sodium intake is not consistent from one person to the next. Nevertheless,
the IOM panel set the UL at an intake of 2300 mg/d for teens and adults (3). Values for
other age groups are shown in the table on the previous page.
Blood Pressure, Hypertension, and the Role of Sodium
Blood pressure is the force exerted on the arterial walls as blood circulates throughout the
body. Normal blood pressure is generally defined as systolic pressure <120 mm mercury
(Hg) and diastolic pressure <80 mm Hg. Hypertension is diagnosed when systolic pressure
is ≥140 mm Hg and/or diastolic pressure is ≥90 mm Hg, or when these levels exceed
130/80 mm Hg in those with diabetes or chronic kidney disease. In Canada, two recent
population-based studies conducted in Ontario suggest that 21%–24% of the adult
population has hypertension, and that this increases steadily with age to reach over 50%
among older adults (7,8). Many more people have “high normal” blood pressure, and in
the United States, systolic blood pressure between 120 and 139 mm Hg or diastolic
pressure between 80 and 89 mm Hg is now identified as prehypertension (9).
Elevated blood pressure is a leading risk factor for stroke, ischemic heart disease, renal
disease and heart failure. Globally, the World Health Organization (WHO) estimates that
62% of strokes and almost half of ischemic heart disease can be attributed to systolic blood
pressure above 115 mm Hg (10). The relationship between blood pressure and disease risk
is graded and continuous, and begins to increase in the prehypertension range. Because so
many people have blood pressure in this range, more cardiovascular events and deaths
occur in this group than in those with hypertension. This argues in favour of a populationbased approach, in which shifting the entire distribution of blood pressure downwards,
even by a relatively small amount, would lead to a large decrease in cardiovascular
morbidity and mortality.
4
Blood pressure is influenced by many lifestyle variables, including obesity, physical
activity, and diet (11). Modifying these variables has been estimated to reduce systolic
blood pressure as follows (9):
ƒ 5–20 mm Hg for a 10-kg weight loss in an overweight or obese individual;
ƒ 8–14 mm Hg for consuming a diet rich in vegetables, fruit and low-fat dairy products;
ƒ 4–9 mm Hg for including regular daily aerobic activity;
ƒ 2–8 mm Hg for a reduction in dietary sodium to less than the UL; and
ƒ 2–4 mm Hg for limiting alcohol consumption to <2 drinks/d in men or 1 drink/d in
women.
Clearly, excessive dietary sodium intake is not the only or even the most important
determinant of blood pressure. The impact of other dietary factors is clearly illustrated by
the DASH (Dietary Approaches to Stop Hypertension) diet, which emphasizes vegetables,
fruit, and low-fat dairy products (and accordingly, high intakes of potassium, calcium and
magnesium) (12). This diet, which is similar to Canada’s Food Guide (13), results in
substantial reductions in blood pressure even when sodium intake is not decreased (12).
Nevertheless sodium has an effect, and reduced intake at the population level has been
projected to have considerable impact (14).
Although numerous trials have demonstrated that reducing sodium intake leads to modest
reductions in blood pressure (5,6), there is less evidence of an impact on related clinical
outcomes. To date, what many consider to be the strongest data are from the observational
follow-up of the Trials of Hypertension Prevention (TOHP) (15). In these trials,
participants aged 30–54 years with high normal blood pressure (now referred to as
prehypertension) were randomized to a sodium reduction intervention or to usual care. The
interventions lasted either 18 or 36 months, and resulted in modest decreases in urinary
sodium (about 1000 mg/d) and in systolic and diastolic blood pressure. Five or 10 years
after completing the trials, observational follow-up for cardiovascular disease was initiated
and continued for almost 5 years. Cardiovascular disease outcomes were detected in 7.5%
of those originally assigned to the sodium intervention and in 9.0% of those assigned to the
control group. With adjustment for variables such as age, sex, and race, this 1.5% decrease
in absolute risk represented a significant 25% decrease in the relative risk of a negative
cardiac outcome. Questionnaire responses showed that those in the sodium intervention
group were more likely to report disliking salty foods and liking low-sodium or unsalted
foods. They were also more likely to report using low-sodium products, reading food
labels for sodium content, and keeping track of their sodium intake at least sometimes.
This study is important, as it is the first to document effects of sodium reduction on
prevention of cardiovascular disease outcomes (rather than on the intermediate marker of
blood pressure). However, the effect on overall mortality was not significant, although the
low number of deaths in both groups likely limited the power to detect a difference.
Additional evidence is needed to confirm effects on cardiovascular outcomes and to
provide definitive conclusions on mortality.
5
Sodium Intakes of Canadians
Data on Canadians’ sodium intakes are available from the 2004 Canadian Community
Health Survey (CCHS), Cycle 2.2 (Nutrition) (16). Mean intakes and the proportions
exceeding the UL are shown in the table below. These estimates do not include salt added
to foods at the table or during food preparation (e.g. addition of salt to cooking water used
for vegetables), so they likely underestimate actual intakes to some extent. A substantial
majority in all age–sex groups exceeded the UL, while the proportions with intakes below
the AI were negligible (16).
Mean Sodium Intakes of Canadians and
Proportions with Intakes above the Upper Level
Age Group (years)
Mean Intake (mg/d)
% Intakes >UL
1–3
4–8
1903
2677
77
93
9–13
14–18
≥19
3555
4142
3587
97
97
90
2962
2936
2658
83
82
66
Males
Females
9–13
14–18
≥19
Sodium is widely distributed throughout the food supply, and it is estimated that ≥75% of
dietary sodium is added during the processing of foods. Analysis of the CCHS data (17)
indicated that the top contributor was a food grouping including pizza, sandwiches,
submarines, hamburgers, and hotdogs, which provided 19% of total sodium. This was
followed by soups (7.4%) and pasta (5.7%). The British Columbia Nutrition Survey
examined food intake data relative to Canada’s Food Guide, and found that grain products
contributed about 25% of total sodium, meat and alternatives contributed about 18%, dairy
products and vegetables/fruits each contributed about 12.5%, and “other foods” contributed
32% (18). This indicates that efforts to reduce sodium intake will need to be directed to the
food supply as a whole (including efforts from the food industry, foodservice
establishments, catering services, restaurants and others), rather than to a few selected
categories of food.
Although most Canadians have high intakes, sodium now appears to be on Canadians’
radar screens. The Tracking Nutrition Trends (TNT) survey conducted in 2008 (19) asked
“When you are selecting food to eat, to what degree, if any, does each of the following
influence your food choice?” Of the 12 attributes listed, “The food is low in salt/sodium”
was a very or somewhat influential consideration for 71% of respondents, falling below
“The food is low in trans fat” and “The food is made from whole grains” (rated as
influential for 80% and 78%, respectively), but similar to “The food is low in sugar”
(influential for 72%). Individuals who did not consider an attribute to be influential on
their food choices were asked why it was not, and could respond that they were “not
aware” of the health benefits, that they “do not care” about them, or that they “don’t
know”. For sodium, 80% reported that they did not care.
6
Despite the indication that sodium content influenced food choices, relatively few
Canadians are making concerted efforts to reduce sodium intake. Similar to previous TNT
surveys, in 2008 about 6 of 10 Canadians reported that they had made an effort to change
their eating habits over the past year (19). The most frequently reported changes were to
increase vegetables (27%), consume less fat (17%), eat more whole grains/fibre (17%),
reduce sugar intake (15%), and reduce calorie intake (14%). In comparison, 12% reported
that they had tried to reduce their salt intake.
Functions of Sodium in Food
Sodium and salt play a number of roles in the food supply. These are important to
understand when considering initiatives to reduce the sodium content of foods. Although a
comprehensive review is beyond the scope of this article and can be obtained elsewhere
(20,21), some of the key roles are highlighted below.
Taste and palatability—As described earlier, sodium is an essential element. Two
mechanisms ensure that animals (and humans) are able to maintain sodium levels: 1) the
kidney’s ability to conserve sodium when intake is limited, and 2) the ability to identify a
source of sodium when it is encountered, through the sense of taste (20). Although we now
have access to sodium in amounts that greatly exceed our needs, this was not always the
case and from an evolutionary perspective, the ability to taste—and have a preference for—
sodium was critical. Sodium (but not other ions with the exception of lithium) interacts
with specific taste receptors in the oral cavity, and leads to propagation of an electrical
signal that is received in the brain (20). In addition to activating the salt taste sensation,
sodium may also enhance other taste sensations and can block the bitterness taste sensation.
The preference for sodium, also referred to as the “sodium appetite”, has recently been
reviewed (22). There are clear animal data to indicate that an innate preference for the salt
taste exists, and that this is enhanced during periods of sodium depletion. Suggestive data
are also available in humans: for example, during experimental sodium depletion, subjects
preferred higher concentrations of sodium in soup and crackers, and also rated saltier foods
as more desirable. However, data also exist to indicate that under normal conditions, the
preferred sodium concentration in a given food product can be gradually reduced (20).
Food processing roles—Sodium, in the form of salt, has many roles in food formulation
(21). For example, it controls cheese ripening, moisture content, and physical changes that
influence texture. In meat, it has tenderizing actions and in meat mixtures, it leads to
increased binding of protein components. Salt controls the rate of yeast fermentation in
bread; inadequate amounts of salt lead to excessive fermentation. Salt also affects the
properties of gluten resulting in a less sticky, easier-to-handle dough. It is also important
for the texture of some foods, such as canned vegetables.
Preservative—Salt acts as a food preservative by reducing the water activity of food, which
in turn decreases the amount of water available for bacterial growth (21). In cured meats,
salt together with nitrite are critical preservatives for inhibition of Clostridium botulinum.
In pickled vegetables, salt allows desirable fermentation to occur while inhibiting the
growth of undesirable organisms. Other sodium compounds (e.g. sodium benzoate, sodium
erythorbate, sodium sulfite) also function as food preservatives.
7
International Initiatives to Reduce Sodium
Initiatives around the world to reduce sodium intake have been encouraged by the WHO.
As part of the Global Strategy on Diet, Physical Activity and Health, the WHO organized a
forum and technical meeting on “reducing salt intake in populations” and published a
report of the proceedings (23). This comprehensive report reviews the evidence linking
excessive sodium consumption and adverse health effects, provides examples of strategies
directed at decreasing sodium intake, and discusses the cost effectiveness of population
interventions. It also describes methods of monitoring salt intake and highlights the role of
salt as a vehicle for prevention of iodine deficiency. Recommendations to various
stakeholders are presented, along with guiding principles and specific issues to be
considered by ministries of health when developing sodium reduction policies. A key
recommendation was that countries should aim for a salt intake of no more than 5 g/d
(equivalent to about 2000 mg sodium).
Many countries throughout the world have begun to take action to reduce sodium intakes
of their populations. One of the most comprehensive programs to reduce population
sodium intake was initiated in 2003 by the Food Standards Agency in the United Kingdom
(24). The program set an ambitious initial target of reducing the average population salt
intake to 6 g/d (2400 mg sodium) by 2010 (from an average intake of about 9.5 g/d, or
3800 mg sodium). Extensive consultations were held with stakeholder groups and food
industry to establish voluntary targets for the sodium content of various types of processed
foods, and a major public education campaign is occurring. Progress is being monitored
using 24-hr urinary measurements, and the survey conducted shortly after program
initiation reported that average salt excretion had decreased by 0.5 g to 9 g/d (equivalent to
about 3600 mg sodium). Results of the latest survey released in 2008 indicate that this has
further decreased to 8.6 g/d (9.7 g/d for men and 7.7 g/d for women).
Efforts to reduce sodium intake in Finland are also informative, as they illustrate the
challenges of achieving large reductions in sodium intake at the population level (25).
Initiatives to reduce sodium intake began in the 1970s, and included both collaboration
with food industry to decrease the sodium content of foods and education programs to
increase public awareness of the link between salt and health. Urinary sodium excretion
studies were conducted between 1979 and 2002, and over that time period, average sodium
excretion decreased from over 5000 mg/d to about 3700 mg/d in men, and from over
4000 mg/d to under 3000 mg/d in women (25). While these decreases are substantial, they
occurred over a period of 23 years, and Finnish intakes are still very high in comparison to
current recommendations.
8
Issues Related to Recommendations for Sodium
Implications for consumers
If a voluntary system of reducing sodium in processed and prepared foods is implemented,
consumers will have the choice of selecting, or not selecting, foods with lower levels of
sodium. Further, if a population-based strategy is to be successful, the sodium intake of
virtually all Canadians—not just those with hypertension—needs to decrease.
Accordingly, the public will need to be aware of the health benefits of a lower sodium
intake, able to identify food products with lower sodium content, and motivated to choose
those products.
Implications for government
As noted previously, Health Canada has established a Working Group on Dietary Sodium
Reduction. The group’s mandate is to develop, implement and oversee a population-based
strategy to reduce Canadians’ dietary sodium intake to within the range recommended by
the IOM. The strategy is to include education, research, and voluntary reduction of sodium
in processed foods and foods sold in foodservice establishments. It is essential that the
education strategy be developed in such a way that sodium reduction is promoted as part of
an overall approach to healthy eating and physical activity, rather than as the only
consideration for blood pressure management and cardiovascular risk reduction. It will be
important to monitor sodium intakes and the sodium content of the food supply to assess
whether the voluntary strategy is effective. It will also be critical to evaluate the impact of
changing patterns of iodized salt use on the population’s iodine status, and if necessary
establish regulations to ensure adequate intakes of this nutrient.
Implications for industry
Many Canadian food manufacturers have already started to lower the sodium content of
their products. Industry will need to be committed to this process over the long term, as
anecdotal evidence indicates that consumers may tolerate repeated modest reductions in
sodium content, but not single large decreases. Furthermore, unless sodium reduction
occurs throughout a company’s product lines (rather than being restricted to products
marketed to consumers with an interest in sodium reduction), it is unlikely to be effective
at the population level. Finally, a key consideration will be maintaining the
microbiological safety of the food supply.
Implications for schools
Dramatic increases in childhood overweight and obesity have prompted many provincial
governments to develop or review policies for the types of foods and beverages available in
schools. In addition to developing criteria for energy and macronutrient content, criteria
for sodium have been identified. For example, in Ontario recommendations have been
made to limit the sodium content of meat products and “other” foods to no more than
480 mg per portion (26). However, no recommendations have been established for foods
in other food groups. British Columbia’s guidelines address sodium in almost all food
categories, but levels are relatively high (27). As the food supply evolves and lowersodium products become more widely available, the guidelines should be revised
downwards.
9
Implications for physicians
The Canadian Hypertension Education Program provides recommendations for the
diagnosis, management, and treatment of hypertension (28). There is evidence that
Canadian physicians are adopting these recommendations, as substantial gains have
occurred in the proportion of Canadians who are aware they have hypertension, and who
are being successfully treated for it (7). The guidelines also include recommendations for
management of lifestyle factors, including dietary sodium. Currently, the focus is on
hypertension per se. Based on the continuous association between blood pressure and
disease risk, it may be appropriate for physicians to provide lifestyle recommendations
(including those related to sodium) to their patients with prehypertension.
Implications for research
Although there is consistent evidence linking high intakes of sodium to increased blood
pressure, some data suggest that the relationship may be J-shaped, with a slight increase in
blood pressure at intakes below the AI (29). Additional research is needed to confirm
whether this observation is consistent; if so, monitoring would be warranted to ensure that
intakes did not decrease excessively. However, even if these data are confirmed, they do
not outweigh the potential benefits of reducing Canadians’ sodium intakes. In this regard,
to the extent that sodium reduction is not universally adopted by food industry and
foodservice establishments (and therefore still requires active choices by consumers), there
will be a need to better understand what would motivate consumers to reduce their sodium
intake. This is particularly true for those without diagnosed hypertension. Research
directed towards a greater understanding of the salt taste mechanism may lead to
identification of more effective salt substitutes, other flavour enhancers, or methods to
trigger the salt taste sensation using smaller amounts of sodium chloride.
CCFN’s is a multi-sectoral trusted voice for science and evidence-based food and
nutrition policy and information in Canada.
CCFN Watching Briefs provide CCFN Members with the current state of the science
on key priority areas. CCFN Watching Briefs are written by Canadian experts and
reviewed by at least three members from among CCFN’s Policy and Communication
Committees and Board of Trustees.
10
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