PART D: SCIENCE BASE
Section 7: Fluid and Electrolytes
This section addresses three major questions related to the intake of fluid
and the electrolytes sodium and potassium.
What amount of fluid is recommended for health? What are the effects of salt (sodium chloride) intake on health? What are the effects of potassium intake on health?
The Committee placed a strong focus on sodium and potassium because of the substantial
body of research linking these electrolytes to levels of blood pressure. Part
B, "Introduction," provides background information on the problem of elevated
blood pressure and its control. That information can help the reader appreciate
the importance of blood pressure as a modifiable risk factor for cardiovascular
and kidney diseases and of dietary factors that can lower and possibly control
blood pressure.
The conclusions in this section are largely based on evidence from an extensive,
systematic, and very recent review of the scientific literature conducted by
an expert panel for the Institute of Medicine (IOM) (IOM, 2004). For topics not
covered in the IOM report, we conducted literature searches. The search strategies
used to find the scientific evidence related to each of these questions appears
in Section C.
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QUESTION 1: WHAT AMOUNT OF FLUID IS RECOMMENDED FOR HEALTH?
Conclusion
The combination of thirst and usual drinking behavior, especially the consumption
of fluids with meals, is sufficient to maintain normal hydration. Healthy individuals
who have routine access to fluids and who are not exposed to heat stress consume
adequate water to meet their needs. Purposeful drinking is warranted for individuals
who are exposed to heat stress or who perform sustained vigorous activity.
Rationale
Recommendations for water are made to prevent the deleterious, primarily acute,
effects of dehydration. These effects include impaired cognitive function and
motor control. Although a low intake of water has been associated with some chronic
diseases, this evidence is insufficient to establish recommendations for water
consumption.
The primary indicator of hydration status is plasma or serum osmolality. Appendix
G-1 from the recent IOM report (IOM, 2004) provides the serum osmolality by decile
of total water intake in the third National Health and Nutrition Examination
Survey (NHANES III). Serum osmolality concentrations were essentially identical
(the maximum range between the lowest and highest decile was only 3 mOsmol/kg).
These data indicate that persons in the lowest and highest deciles of total water
intake were neither systematically dehydrated nor hyperhydrated. Importantly,
this pattern of findings also was evident in men and women age 71 and older.
Thirst, which is the desire to drink by both physiological and behavioral cues,
may be triggered by a decrease in blood volume or severe dehydration. Over the
course of a few hours, body water deficits can occur. However, thirst mechanisms
come into play over the ensuing 24 hours to trigger replacement of fluids lost
(Johnson, 1964). Such replacement is enhanced by consuming beverages at meals
and in other social situations (Engell, 1995; Szlyk, 1990).
Total water intake includes drinking water, water in beverages, and water contained
in food. Because normal hydration can be maintained over a wide range of water
intakes, the Adequate Intake (AI) for total water was set based on the median
total water intake from U.S. survey data (IOM, 2004). The AI for total water
intake for young men and women (age 19 to 30 years) is 3.7 L and 2.7 L per day,
respectively. In NHANES III, fluids (drinking water and beverages) provided 3.0
L (101 fluid ounces; ~13 cups) and 2.2 L (74 fluid ounces; ~9 cups) per day for
men and women age 19 to 30, representing approximately 81 percent of total water
intake. Water contained in food provided about 19 percent of total water intake.
The AI should not be interpreted as a specific requirement or recommended intake.
Individual water requirements can vary greatly, even on a day-to-day basis, primarily
because of differences in physical activity and environmental conditions but
also because of differences in diet. A total water intake above the AI often
is required by those individuals who are physically active or who are exposed
to heat stress. In individuals who are neither physically active nor exposed
to heat stress, daily consumption below the AI can be sufficient to maintain
normal hydration. Dietary factors also influence water requirements because total
water consumption must be sufficient to excrete metabolites of protein and organic
compounds, as well as excess electrolytes.
Because healthy individuals have considerable ability to excrete excess water
and thereby maintain water balance, the IOM did not set a Tolerable Upper Intake
Level (UL) for water. However, acute water toxicity can occur following the rapid
consumption of large quantities of fluids that greatly exceed the kidney's maximal
excretion rate of approximately 0.7 to 1.0 L per hour.
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QUESTION 2: WHAT ARE THE EFFECTS OF SALT (SODIUM CHLORIDE) INTAKE ON
HEALTH?
Conclusion
The relationship between salt (sodium chloride) intake and blood pressure is
direct and progressive without an apparent threshold. Hence, individuals should
reduce their salt intake as much as possible. In view of the currently high levels
of salt intake, a daily sodium intake of less than 2,300 mg is recommended. Many
persons will benefit from further reductions in salt intake, including hypertensive
individuals, blacks, and middle- and older-aged adults. Individuals should concurrently
increase their consumption of potassium because a diet rich in potassium blunts
the effects of salt on blood pressure.
Rationale
A recent report from the IOM (IOM, 2004) provides the basis for a recommended
daily sodium intake (an AI) of 1,500 mg and a UL of 2,300 mg for adults1,
2. These recommendations are based on an extensive examination
of the scientific literature by an expert IOM (IOM) panel. The primary basis
for setting the AI was to ensure overall nutrient adequacy, not to prevent chronic
disease. In contrast, the UL was set because of the direct relationship of salt
intake with blood pressure.
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Review of the Evidence
Studies of the Relationship of Sodium Intake and Blood Pressure.
The relationship between sodium intake and blood pressure is direct and progressive.
In addition to observational studies, supportive evidence comes from more than
50 clinical trials and meta-analyses (see IOM, 2004, Tables 6-12, 6-13, 6-15,
6-16, and Appendix I). The best available dose-response evidence comes from individual
trials that specifically examined this relationship (i.e., randomized trials
that tested the effects of three or more levels of sodium intake on blood pressure).
In these dose-response studies, the lowest level of sodium intake ranged from
approximately 230 to 1,500 mg of sodium per day, while the highest level ranged
from approximately 3,200 to over 34,000 mg of sodium per day. The largest and
most rigorous of these trials documented statistically significant, progressive
dose-response relationships (Johnson et al., 2001; MacGregor et al., 1989; Sacks
et al., 2001). Importantly, there was no evidence of a threshold; that is, the
direct relationships were evident throughout the range of salt intake.
The trial by Johnson et al. (2001) tested the effects of 5 levels of sodium
intake (lowest to highest: 920 mg per day to 7,800 mg per day) in 40 older-aged
persons (nonhypertensives, persons with isolated systolic hypertension, persons
with combined systolic-diastolic hypertension). The trial by MacGregor et al.
(1989) tested the effects of 3 levels of sodium intake (1,100; 2,300; and 4,600
mg of sodium per day) in 20 hypertensive adults. The largest of the dose-response
trials, the Dietary Approaches To Stop Hypertension (DASH) -Sodium
trial, tested the effects of three different sodium intakes separately in two
distinct diets—the DASH diet and a control diet. The DASH diet is described in
detail in Section D1 and in Table D1-18. In brief, the DASH diet is rich in fruits,
vegetables, and low-fat dairy products and reduced in total fat, saturated fat
and cholesterol. The control diet is typical of what many Americans eat, that
is, relatively high in total and saturated fats and low in fruits, vegetables,
and low-fat milk products. Mean achieved levels of sodium intake, as reflected
by 24-hour urinary sodium excretion, corresponded to approximate intakes of 1,500;
2,500; and 3,300 mg in the lower, intermediate, and higher doses, respectively.
Of the 3 dose-response trials, the DASH-Sodium trial enrolled the largest and
most diverse population; 41 percent were hypertensive, 40 percent were white,
and 57 percent were black. However, the DASH-Sodium trial had the narrowest range
of sodium intake; approximately half of the U.S. population consumes sodium in
excess of the highest level tested in this trial (Rose et al., 1988).
The main results of the DASH-Sodium trial are displayed in Figure D6-1. The
blood pressure response to sodium reduction was nonlinear. Specifically, reducing
sodium intake by approximately 920 mg per day caused a greater lowering of blood
pressure when the initial sodium intake was at the intermediate level than when
it was at the higher intake; this pattern of results was especially evident on
the control diet. Results from the INTERSALT observational study (Rose et al,
1988) and from the Johnson trial likewise suggest that the blood pressure response
to changes in sodium intake is steeper below 2,300 mg per day than above 2,300
mg per day.
In protocol-specified subgroup analyses of the DASH-Sodium trial (Vollmer et
al., 2001), a reduced-sodium intake significantly lowered blood pressure in each
of the major subgroups on the control diet. On the control diet, reduced-sodium
intake led to greater systolic blood pressure reduction among hypertensive individuals,
blacks, and persons age 45 years and older compared to their counterparts. Net
systolic/diastolic blood pressure reductions associated with reducing salt from
the higher to the lower level in hypertensives and nonhypertensives were 8.3/4.4
and 5.6/2.8 mmHg, respectively. On the DASH diet, a qualitatively similar pattern
was evident, but the extent of blood pressure reduction was less; net systolic/diastolic
blood pressure reductions associated with reducing salt from the higher to the
lower level in hypertensives and nonhypertensives were 4.9/2.5 and 1.7/1.1 mmHg,
respectively. In each subgroup, the lowest blood pressure was observed on the
DASH diet with the lower sodium level.
In subsequent pos-hoc analyses, Bray et al. (2004) presented results in joint
subgroups (age and hypertension status, race/ethnicity and hypertension status,
and sex and race/ethnicity). In the control diet, sodium reduction significantly
lowered systolic blood pressure in each subgroup. In the DASH diet, many but
not all blood pressure changes associated with sodium reduction were statistically
significant in the subgroups. In all subgroups, the DASH diet significantly lowered
blood pressure at the higher sodium level; however, at the lower sodium level,
several blood pressure reductions did not achieve statistical significance. Overall,
the general pattern of results in subgroup analyses was similar to that of the
main results. Deviations between main results and subgroup analyses, especially
post-hoc analyses, should be interpreted cautiously because of reduced sample
size and hence the potential for false negative associations.
The control diet, in which the blood pressure effect of sodium reduction was
the largest, is closer to what most Americans currently eat than is the DASH
diet. For instance, in the United States less than 10 percent of adult men and
1 percent of women consume 4.7 g per day of potassium (the potassium goal of
the DASH diet), and less than 25 percent of adult men and less than 5 percent
of adult women have a daily calcium intake from foods of 1,240 mg per day (the
calcium goal of the DASH diet) (IOM, 2004). The low-saturated fat and total-fat
contents of the DASH diet (goals of 6 percent and 27 percent kcal, respectively)
are likewise uncommon in the U.S. population. These observations, coupled with
the consistency of the findings across subgroups, support recommendations to
concurrently reduce sodium intake and consume the DASH diet. Although the duration
of each feeding period lasted only one month, it is reasonable to speculate that
adherence to the combination of the DASH diet and reduced sodium intake might
help blunt the well-documented rise in blood pressure that occurs with age, especially
because reductions in systolic blood pressure were greater in the older than
younger participants.
As documented above (also, see IOM, 2004 Tables 6-13 and 6-15) the effects of
sodium on blood pressure are large and clinically relevant in hypertensive individuals
not on medication. Sodium reduction also lowers blood pressure in the presence
of antihypertensive drug therapy (Appel et al., 2001). Although the effects of
sodium intake on blood pressure are smaller in nonhypertensive individuals, the
potential benefits of sodium reduction on blood pressure have substantial public
health relevance. Stamler et al. (1989) estimated that a 3 mmHg reduction in
systolic BP could lead to an 8 percent reduction in stroke mortality and a 5
percent reduction in mortality from coronary heart disease. In observational
studies, a reduced salt intake (as manifest by 24-hour urinary sodium excretion)
is also associated with a blunted ate-related rise in blood pressure (Rose et
al., 1988).
Sodium reduction can also prevent incident hypertension. To date, three trials
have explored the effects of a reduced sodium intake as a means to prevent hypertension
(Hypertension Prevention Trial [HPT], Trial of Hypertension Prevention
Phase I [TOHP1], and Phase II [TOHP2 Collaborative Research Group,
1997]). HPT and TOHP1 were pilot studies that were conducted to inform the design
of TOHP2. Each study was a controlled trial in which a behavioral intervention
focused exclusively on reducing sodium intake. HPT and TOHP2, also included groups
that simultaneously implemented other interventions: increased potassium intake
in HPT and weight loss in TOHP2 (1997). Net reductions in urinary sodium excretion
on the sodium reduction arm were modest in the three studies, ranging from the
equivalent of 300 mg to ~1,300 mg of sodium per day, at the end of followup.
In this setting, a reduced sodium intervention that did not include any other
lifestyle change led to a decreased relative risk of incident hypertension (range
0.69 to 0.82).
Results from TOHP2 are especially relevant because this trial was designed and
adequately powered to test the effects of a reduced dietary sodium intervention
as a means to prevent hypertension. TOHP2 was a randomized, controlled 2 x 2
factorial trial that tested the effects of 3 lifestyle interventions (sodium
reduction, weight loss, or combined weight loss and sodium reduction) on blood
pressure and incident hypertension over 3 to 4 years of followup in overweight
individuals aged 30 to 54 years with an initial diastolic blood pressure of 83
to 89 mm Hg and a systolic blood pressure < 140 mm Hg. At 6 months, the height
of intervention adherence, the incidence of hypertension was lowest in the combined
group (2.7 percent), intermediate in the weight loss only (4.2 percent) and sodium
reduction only (4.5 percent) groups, and highest in the control group (7.3 percent).
At 18 months, the pattern persisted. By the end of follow-up, the incidence of
hypertension was 18 to 22 percent less in each lifestyle group (p< 0.05 compared
to control) but not different from each other. Results of this trial indicate
that lifestyle interventions can prevent hypertension over the long-term. Also,
the pattern of incident hypertension at 6 and 18 months suggests that the effects
of weight loss and reduced sodium intake, under optimal conditions of adherence,
may be additive.
Relying on behavioral interventions to reduce dietary intake of sodium presents
a major barrier to the achievement of greater reductions in blood pressure and
to a reduction in the associated CVD complications. In contrast to the short-term
(3-day) feeding trials that could achieve contrasts in sodium intake of nearly
34,300 mg per day (Luft et al., 1979), the maximum contrast in the primary prevention
trials was 1,300 mg per day in TOHP1. The average contrast in long-term trials
lasting 6 months was only 800 mg per day (Hooper et al., 2002). The limited contrast
in sodium intake in these trials reflects the difficulties of sustaining behavior
change when the most common source of sodium, namely processed foods, accounts
for > 75 percent of total sodium intake and when discretionary salt intake
accounts for only 11 percent (5 percent added during cooking and 6 percent added
at the table) (Mattes, 1997). The sodium that occurs naturally in foods accounts
for the remainder (approximately 10 percent).
Salt Sensitivity. Evidence from a variety of studies,
including observational studies and clinical trials, has demonstrated heterogeneity
in the blood pressure responses to sodium intake. Those individuals with the
greatest reductions in blood pressure in response to decreased sodium intake
are termed salt sensitive. Despite the use of the terms
salt sensitive and salt resistant
to classify individuals in research studies, the change in blood pressure in
response to a change in salt intake is not binary. Rather, the reduction in blood
pressure from a reduced sodium intake has a continuous distribution across individuals.
Also, there are no standardized diagnostic criteria and tests. Despite these
limitations, it is possible to make some general observations.
Salt sensitivity is modifiable. The rise in blood pressure
from increased salt intake is blunted in the setting of
a high potassium intake (4.7 g of supplemental
potassium per day in one trial, (Morris et al., 1999); 6.7
g per day in another trial, (Schmidlin et al., 1999)). The
rise in blood pressure from increased salt
intake was also blunted in the setting of the DASH diet,
which is rich in potassium (4.6 g of potassium per day)
as well as other minerals (Table 6-1) (Bray et al.,
2004; Karanja et al., 1999; Sacks et al., 2001; Vollmer et
al., 2001); nonetheless, a dose-response relationship between
sodium intake and blood pressure persisted.
Individuals with hypertension, diabetes, and chronic kidney disease, as well
as middle- and older-aged persons and blacks tend to be more salt sensitive than
their counterparts. Genetic factors also influence the blood pressure response
to salt. Each of the 14 identified genes that affect blood pressure affects renal
salt handing. Such evidence provides indirect support of an etiologic role of
sodium in blood pressure homeostasis (Lifton, 2002).
Relationships Between Salt Intake and Health Outcomes Other Than
Blood Pressure. As documented by the IOM (IOM, 2004), an increased
sodium intake might have adverse effects on additional health outcomes. These
include clinical cardiovascular outcomes (i.e., stroke and coronary heart disease),
subclinical cardiovascular outcomes (i.e., left ventricular mass), and noncardiovascular
outcomes (e.g., urinary calcium excretion, osteoporosis, and gastric cancer).
Cross-sectional studies consistently document an association between urinary
sodium excretion and left ventricular mass, but only one small controlled trial
assessed the effects of sodium reduction on this endpoint. Numerous trials document
that a reduced sodium intake lowers urinary calcium excretion (Table 6-19; IOM,
2004), but urinary calcium excretion, by itself, is not a well-accepted surrogate
marker for bone mineral density or dietary induced osteoporosis. Evidence that
links sodium intake with gastric cancer is reasonably strong but still insufficient
to establish a UL for sodium. No trial has tested the effects of sodium reduction
as a means to prevent cardiovascular disease (CVD). However, the most rigorous
observational studies (He and MacGregor, 1999; Tuomilehto et al., 2001; see Table
6-17 IOM, 2004) have documented a direct relationship of sodium intake with CVD.
Salt Taste Preferences. At birth, there is no indication
that salty substances are distinguishable or preferred (Beauchamp et al., 1986).
Preference for the salty taste appears at about four months postnatal (Beauchamp
et al., 1994; Beauchamp et al., 1986; Harris and Booth, 1987). Limited evidence
suggest that infants' and children's salt preference is shaped by their experience
with salt in foods (Beauchamp, 1990; Stein et al., 1996)
Adult salt preferences can be influenced by dietary exposure. Studies have demonstrated
that reducing one's dietary sodium intake can decrease one's preference for salty
foods and increase acceptance of foods with reduced sodium content (Bertino et
al., 1982). Several studies document a temporary increased preference for salt
over the initial few weeks when sodium intake is reduced (Bertino et al., 1981;
McCance, 1936, Teow et al., 1985–1986; Yensen, 1959). Subsequently, a shift in
preference occurs such that by 8 to 12 weeks individuals prefer less salty foods
(Bertino et al., 1982; Mattes et al., 1991; Mattes, 1997). This phenomenon also
has been demonstrated in long-term studies lasting one year or more (Blais et
al., 1986).
On average, the natural salt content of food accounts for only 10 percent of
total intake, while discretionary salt use (i.e., table and cooking salt) provides
another 5 to 10 percent of total intake. The remaining 75 percent is derived
from salt added by manufacturers (James, 2000; Mattes 1991, 1997). When total
intake of salt is decreased, discretionary salt use is fairly stable, even when
available ad libitum (Mattes, 1997). Therefore, any program for reducing
the salt consumption of a population should concentrate primarily on a reduction
in the salt used during food processing (James, 2000) and on changes in food
selection (e.g., more fresh, less-processed items, less sodium-dense foods) and
preparation (Mattes, 1997). Previous guidelines have focused on decreasing the
intake of foods and beverages high in salt (HHS 1985, 1990, 1995, 2000) because
of their large contribution of salt intake in the diet.
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Recommendations for Salt (Sodium Choride) Intake
The IOM set the AI for sodium for adults at 1,500 mg per day to ensure that
the overall diet provides sufficient amounts of other nutrients and to cover
sodium sweat losses in unacclimatized individuals who are exposed to high temperatures
or who are moderately physically active (IOM, 2004). This amount of sodium does
not apply to highly active individuals, such as endurance athletes and certain
workers (e.g., foundry workers) who lose large amounts of sweat on a daily basis
and thus require a higher sodium intake.
The IOM set the UL at 2,300 mg of sodium per day (IOM, 2004). In dose-response
trials, this level of sodium intake commonly was the next tested level above
the AI. The UL of 2,300 mg of sodium daily is not a recommended intake. There
is no benefit to consuming sodium in an amount that exceeds the AI. For members
of groups that are most sensitive to the blood pressure effects of increased
salt intake (that is, middle- and older-aged persons, blacks, and individuals
with hypertension, diabetes, or chronic kidney disease), it is advisable to consume
an amount of sodium that is less than the UL. These groups also have higher levels
of blood pressure
Positions Taken by Other Policymaking Groups. Numerous
policymaking organizations have recommended a reduced salt intake as a means
to lower blood pressure in the general population. In the United States, the
National High Blood Pressure Education Program set a sodium intake goal of 100
mmol (2,300 mg) per day as a means to prevent hypertension in nonhypertensive
individuals (Whelton et al., 2002) and as first line and adjuvant therapy in
hypertensive individuals (Chobanian et al., 2003). The American Heart Association
set an intake of 6 g of salt (2,400 mg of sodium) per day as a recommended upper
limit for healthy Americans. In Great Britain, the Scientific Advisory Committee
on Nutrition in 2003 conducted an independent review of available evidence and
also set an upper limit of 6 g of salt (2,400 mg of sodium) per day. Recently
published Canadian recommendations for the prevention and treatment of hypertension
are to restrict salt intake to 65 mmol to 100 mmol (1,500 mg to 2,300 mg) per
day in hypertensive individuals and to 100 mmol (2,300 mg) per day in normotensive
individuals at risk for becoming hypertensive (Touyz et al., 2004). Note that
in the United States, 90 percent of adults will develop hypertension (Vasan et
al., 2002). In its report, Diet, Nutrition and the Prevention
of Chronic Diseases (2003), the World Health Organization set an upper limit of 70 mmol (1,600 mg)
of sodium per day as a means to lower blood pressure.
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Sodium Intakes
According to data from NHANES III (IOM, 2004), the median intakes of sodium
among adult men and women age 31 to 50 are 4,300 mg and 2,900 mg of sodium per
day, respectively. One quarter of adult men exceed 5,200 mg of sodium per day,
and one quarter of women exceed 3,500 mg per day. Approximately 95 percent of
adult men and 75 percent of adult women exceed the UL of 2,300 mg of sodium per
day, and 100 percent exceed the AI of 1,500 mg of sodium per day. On average,
blacks and non-blacks consume similar amounts of sodium. The reported sodium
intakes probably are underestimates of total sodium intake because the NHANES
III did not ask about discretionary salt intake.
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QUESTION 3: WHAT ARE THE EFFECTS OF POTASSIUM INTAKE ON HEALTH?
Conclusion
Diets rich in potassium can lower blood pressure and lessen the adverse effects
of salt on blood pressure, may reduce the risk of developing kidney stones, and
possibly decrease bone loss. In view of the health benefits of potassium and
its relatively low intake by the general population, a daily potassium intake
of at least 4,700 mg is recommended. Blacks are especially likely to benefit
from an increased intake of potassium.
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Rationale
Review of the Evidence
Effect of Potassium on Blood Pressure and Salt Sensitivity.
Supportive evidence for the conclusion that an increased potassium intake lowers
blood pressure appears in the IOM report (IOM, 2004), as follows:
Most trials tested pill supplements, typically in the form of potassium chloride
(Tables 5-4 and 5-5, IOM, 2004). Three meta-analyses of these trials document
that, on average, increased potassium intake lowers blood pressure in nonhypertensive
and hypertensive individuals (Cappuccio and MacGregor, 1991; Geleijnse et al.,
2003; Whelton et al., 1997). In the meta-analysis by Whelton et al. (1997), average
net systolic/diastolic blood pressure reductions from a net increase in urinary
potassium excretion of 2 g per day (50 mmol per day) were 4.4/2.5 mmHg among
hypertensive individuals and 1.8/1.0 mmHg among nonhypertensive individuals.
No dose-response trial tested the effects of more than two levels of potassium
intake on blood pressure.
Relatively few trials tested the effects of potassium as provided in foods (Table
5-3, IOM, 2004). The potassium in fruits and vegetables is accompanied by bicarbonate
precursors rather than chloride. In the initial DASH trial, a diet rich in fruit
and vegetables (and therefore rich in potassium) lowered blood pressure (Appel
et al., 1997). Another trial documented that increased fruit and vegetable consumption
can significantly lower blood pressure (John et al., 2002), but that trial did
not report the potassium intake of participants on the fruit and vegetable intervention.
Because virtually all trials used potassium chloride supplements while observational
studies assessed dietary potassium intake from foods (paired with nonchloride
anions), the effect of potassium on blood pressure appears to result from potassium
rather than its conjugate anion. No single trial tested the effects of three
or more levels of potassium intake on blood pressure; hence, the dose-response
relationship is unclear. Still, blood pressure reductions from supplemental potassium
occurred when baseline intake was low (e.g., 1.3 to 1.4 g of potassium per day
in Brancati et al., 1996) and when baseline intake was much higher (> 3.1
g of potassium per day in Naismith and Braschi (2003)).
Evidence from the observational studies and clinical trials
has demonstrated heterogeneity in the blood pressure responses
to potassium intake. Blacks and
hypertensive individuals are more sensitive to the effects
of potassium than their nonblack and normotensive counterparts,
respectively. Dietary salt intake
also modifies the effects of potassium on blood pressure.
Specifically, the effects of potassium on blood pressure
are greater when salt intake is high than when
salt intake is low (see Table D7-1).
Some trials have assessed the effects of increased potassium intake on salt
sensitivity, that is, the pressor response to increased salt intake. Study populations
included nonhypertensive predominantly black individuals (Morris et al., 1999;
Schmidlin et al., 1999) and hypertensive individuals (Morgan et al., 1984). These
trials are consistent in documenting that potassium blunts the pressor (blood-pressure
raising) effects of salt. One dose-response trial documented that increasing
potassium intake to 4.7 g per day reduced salt sensitivity in nonhypertensive
blacks (Morris et al., 1999). In aggregate, these trials highlight the potential
benefits of increasing potassium intake in blacks, a group of individuals with
a high prevalence of hypertension and of blood pressure-related cardiovascular-renal
disease.
To date, no trial has tested the effects of increased potassium intake on blood
pressure-related clinical outcomes. However, observational studies suggest that
increased potassium intake may prevent stroke and perhaps coronary artery disease
(see Table 5-6, IOM, 2004).
Effect of Potassium in Preventing Bone Loss and Kidney Stones.
A diet rich in potassium from fruits and vegetables favorably affects acid-base
metabolism because these foods also are rich in precursors of bicarbonate (Sebastian,
et al., 1994, 2002). Acting as a buffer, the bicarbonate-yielding organic anions
found in fruits and vegetables neutralize acids generated from meats and other
high-protein foods. In the setting of an inadequate intake of bicarbonate precursors,
excess acid in the blood titrates bone buffer. This results in demineralization
of the bone. Increased bone breakdown and calcium-containing kidney stones are
adverse consequences of excess acid derived from the diet. Therefore, diets rich
in potassium with its bicarbonate precursors might help prevent kidney stones
and bone loss.
To date, two observational studies have documented that high intakes of potassium
(median of 4.0 g per day in men and 4.7 g per day in women) are associated with
a reduced risk of incident kidney stones (Curhan et al., 1993, 1997). In a third
observational study conducted in Finland, the relationship was statistically
nonsignificant, perhaps because of the much higher usual levels of potassium
consumed in this population (Hirvonen et al., 1999). In addition, one trial (Barcelo
et al., 1993) documented that approximately 3.6 to 4.7 g of supplemental potassium
citrate reduced the risk of recurrent kidney stones. The potassium added to processed
foods and the potassium in supplements typically has chloride as the conjugate
anion. Since chloride cannot neutralize excess acid in the body, this form of
potassium is not expected to help prevent kidney stones or bone loss.
Observational studies, including both cross-sectional studies and longitudinal
studies, suggest that increased potassium intake is associated with increased
bone mineral density (See IOM, 2004, Table 5-7). Trials also have documented
that supplemental potassium bicarbonate can reduce bone breakdown and increase
bone formation (Sebastian et al., 1994). However, no trial has tested the effect
of increased potassium or diets rich in potassium on bone mineral density or
on clinical outcomes related to osteoporosis.
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Recommendations for Potassium Intake
The IOM set the AI for potassium for adults at 4,700 mg per day. This level
of intake should maintain lower blood pressure levels, mitigate the adverse effects
of salt on blood pressure, reduce the risk of developing kidney stones, and possibly
decrease bone loss. At present, dietary intake of potassium by all groups in
the United States is considerably lower than 4,700 mg per day. In recent surveys,
the median intake of potassium by adults in the United States was approximately
2,900 to 3,200 mg per day in men and 2,100 to 2,300 mg per day in women. On average,
blacks consume less potassium than non-blacks. Among men, age 31 to 50 years,
median potassium intake was approximately 2,600 mg in blacks and 3,300 mg in
non-blacks. Corresponding figures in women were 1,900 mg and 2,400 mg, respectively
(see Table D1-7). Because blacks have a relatively low intake of potassium and
a high prevalence of elevated blood pressure and salt sensitivity, this subgroup
of the population would especially benefit from an increased intake of potassium.
In the generally healthy population with normal kidney function, a potassium
intake from foods that exceeds 4.7 g per day poses no potential for increased
risk because excess potassium is readily excreted in the urine. Hence, the IOM
did not set a UL for potassium (IOM, 2004). However, a potassium intake below
4.7 g per day is indicated for individuals whose urinary potassium excretion
is impaired. Adverse cardiac effects (arrhythmias) can result from hyperkalemia,
which is a markedly elevated serum level of potassium. Common drugs that can
substantially impair potassium excretion are angiotensin converting enzyme (ACE)
inhibitors, angiotensin receptor blockers (ARB), and potassium-sparing diuretics.
Medical conditions associated with impaired potassium excretion include diabetes,
chronic kidney disease, end stage renal disease, severe heart failure, and adrenal
insufficiency. As a group, elderly individuals are at increased risk of hyperkalemia
because they often have one or more of these conditions or take one or more of
the above medications.
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SUMMARY
Healthy persons who have routine access to fluids and who are not exposed to
heat stress consume adequate water to meet their needs. Thus, the Committee makes
no special recommendations concerning water intake. To help lower blood pressure,
the Committee recommends that individuals reduce their salt intake as much as
possible, aiming for less than 2,300 mg of sodium daily. The Committee recommends
a concurrent increase in potassium intake to 4,700 mg daily. In addition to helping
lower blood pressure and blunting the effects of salt on blood pressure, this
amount of potassium intake may reduce the risk of developing kidney stones and
possibly reduce bone loss. Blacks are especially likely to benefit from reductions
in sodium intake and increases in potassium intake.
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Figure D7-1. Dose-Response Relationship Between Systolic Blood Pressure
and Sodium Intake in Two Diets: Main Results From the DASH Sodium Trial (Sacks
FM et al., 2001)
 d
Control Diet represents the typical American diet. DASH diet emphasizes fruits,
vegetables, and low-fat dairy foods, includes whole grains, poultry, fish, and
nuts, and is reduced in fats, red meat, sweets, and sugar-containing beverages.
The 3 sodium levels are defined as higher (3,450 mg/d), intermediate (2,300 mg/d)
and lower (1,150 mg/d).
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Table D7-1. Trials That Assess the Main and Interactive
Effects of Salt and Potassium on Blood Pressure
Select icon to view table
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