FROM:
Br J Nutr. 2010 (Apr); 103 (8): 1185–1194 ~ FULL TEXT
Joseph Pizzorno, Lynda A. Frassetto, and Joseph Katzinger
Bastyr University,
Seattle, WA 98165, USA.
drpizzorno@salugenecists.com
The concept of diet-induced 'acidosis' as a cause of disease has been a subject of interest for more than a century. The present article reviews the history of our evolving understanding of physiological pH, the physiological support for the concept of 'acidosis', the causes of acidosis, how it is recognised, its short-term effects as well as the long-term clinical relevance of preventative measures, and the research support for normalisation of pH. Further, we suggest differentiation of the terms 'acidosis' and 'acidaemia' as a way to resolve the conflation of these topics which has led to confusion and controversy. The available research makes a compelling case that diet-induced acidosis, not diet-induced acidaemia, is a real phenomenon, and has a significant, clinical, long-term pathophysiological effect that should be recognised and potentially counterbalanced by dietary means.
From the FULL TEXT Article:
Historic overview
The study of acid–base equilibrium and its relationship to the
diet and disease has been a subject of considerable speculation
for at least several centuries. But, before the 19th century,
little was known about the concepts of acids and bases, and
no means were available to quantify the acid or alkaline
load of foods, or of the pH of physiological processes. Not
until the second half of the 19th century did nutritional science
began to develop, and the chemical components of food start
to be analysed. Henry Clapp Sherman’s book in 1919, [The]
Chemistry of Food and Nutrition, describes the history of
nutritional studies performed up to that date (Sherman was
the first to quantify nutritional acid load in 1912).
During the next half century, a number of clinical
practitioners outside of academia began to observe health
improvements in their patients from consuming unprocessed
raw fruits and vegetables, instead of the processed food
which were increasingly becoming the standard fare, and
developed the concept that life is an equilibrium between
acid and base. However, it was not until the 1960 s that
input–output acid–base balance studies were performed for
the first time in healthy adults and patients with chronic
renal acidosis. [1, 2] There is still a great deal of debate on
how to define acids and bases. In the 1980s, Peter Stewart
challenged traditional thought and mathematically determined
that hydrogen ion and bicarbonate concentrations were
dependent variables. He considered the independent variables,
which by definition would determine the dependent variables,
to be the strong ion difference (the difference in the net
charge of cations and anions fully dissociated in solution),
the partially dissociated weak acids (albumin, phosphate),
and the partial pressure of carbon dioxide (PCO2) of the
solution.
While a full discussion of the implications of
the differences between theories is beyond the scope of the
present paper, of significance is the lack of consensus in the
fundamental understanding of the mechanisms, as well as
the practical applications of acid–base chemistry in physiological
systems. A lengthy review published in January 2008
emphasises this point [3], and excellent reviews of acid–base
history are available. [4, 5]
Definitions: acidosis, acidaemia and diet-induced acidosis
For the purposes of the present review, some definitions are
in order, though it should be clear that most definitions are
not universally accepted. The term acidosis is often used interchangeably
with the term acidaemia, with the latter referring
to a blood pH of less than 7·35. Correctly used, the term
acidosis refers to a process, or a trend toward acidaemia,
without necessarily reaching a pH of less than 7·35, or
actual acidaemia. The concentration of Hþ in blood plasma
and various other body solutions is among the most tightly
regulated variables in human physiology. Acidosis only
becomes acidaemia when compensatory measures to correct
it fail. To illustrate the difference between acidosis and
acidaemia, take the following example: two processes
occurring simultaneously in the same individual, such as a
respiratory acidosis combined with a metabolic alkalosis. In
this case, if the respiratory trend toward acidosis is greater
than the metabolic trend, a pH of less than 7·35 may be
reached, and would be considered acidaemia, despite the
presence of a metabolic alkalosis. The intensity of each
‘process’ will determine the pH, but the terms themselves
(acidosis, alkalosis) do not indicate a certain pH.
What should also be defined, at least broadly, is the net acid
load of the diet, the primary topic of the present paper. Diet
net acid load can be estimated from measurements of urinary
excretion of ammonium, titratable acids and bicarbonate
(called net acid excretion; NAE), or can be calculated from
dietary constituents (called net endogenous acid production;
NEAP). The techniques for measuring or quantifying
the acid load of the diet will be discussed in a later section,
but, in general, food contributes a net acid or base effect
due to the balance between the acid-forming constituents,
such as sulfuric acid produced from the catabolism of
methionine and cystine in dietary proteins, and the baseforming
constituents, for example, bicarbonate, produced
from the metabolism of the K salts of organic anions
in plant foods. As predicted by the Stewart hypothesis,
sodium chloride appears to affect systemic acid–base status
independently of the net acid load of the diet, perhaps by
affecting renal excretion of Cl–/NH4, or by a strong ion
effect. [6] The effects of sodium chloride are especially
relevant, given the high salt content of the typical diet in
industrialised countries.
With an increasing understanding of acid–base chemistry
has come recognition of the significant differences between
contemporary diets and diets more typical of Homo sapiens
ancestors. Although of course we do not know exactly what
our hominid ancestors ate, studies in hunter–gatherer tribes
suggest a relatively high intake of plant foods compared
with modern-day humans. [7] In a recent study estimating the
net acid load (NEAP) of 159 hypothetical pre-agricultural
diets, 87% were found to be base producing, with an
estimated mean NEAP of negative 88 mEq/d. In comparison,
calculations from the US Third National Health and Nutrition
Examination Survey (NHANES III) found the average
American diet to be acid producing, with an NEAP of positive
48 mEq/d. [8] This represents a switch from the net baseproducing
diet we ate for the majority of our evolutionary
history to the net acid-producing diet we have been eating
for only several thousand years. Much of the current research
highlights this change and the potential long-term physiological
consequences of a chronic low-grade metabolic
acidosis among those eating the typical Western diet, and
the effects of reducing or eliminating this diet acid load by
altering the diet or giving base supplements.
Is acidosis a real physiological phenomenon?
The human body tends to maintain a tightly controlled
pH of about 7·40 in the extracellular fluid by respiratory
excretion of carbon dioxide and renal excretion of
non-carbonic (non-volatile) acid or base. [9] Everyday metabolism
produces acid as non-volatile sulfate from amino acid
catabolism, non-metabolised organic acids and phosphoric
and other acids. The kidney reabsorbs all of the filtered
bicarbonate (HCO3–) and generates new bicarbonate in the
collecting duct. Under normal steady-state conditions, the
net quantity of acid secreted and the consequent renal
generation of new bicarbonate equals the rate of metabolic
proton generation, preserving pH balance. In metabolic
acidosis, either non-volatile acid accumulates, or HCO3– is
lost (for example, in diarrhoea) and this can be happening
even when the plasma HCO3– is within the range considered
to be normal (24–28 mmol/l). [10]
While acute acid loading may only temporarily disrupt
acid–base equilibrium, a chronic perturbation occurs when
metabolism of the diet repeatedly releases non-carbonic
acids into the systemic circulation in amounts that exceed
the amount of base released concomitantly (for example,
bicarbonate from combustion of organic acid salts of K in
vegetable foods). [11] The size of the discrepancy between
acid and base production determines the NEAP rate. To
maintain equilibrium when there is a net retention of acid (H+), at least three compensatory physiological responses
are activated: buffering, increased ventilation, and increased
renal reabsorption and generation of HCO3–. The major
reservoir of base is the skeleton (in the form of alkaline
salts of Ca) which provides the buffer needed to maintain
blood pH and plasma bicarbonate concentrations. To some
degree, skeletal muscle also acts as a buffer. [12] Respiratory
ventilation increases within minutes if the acidosis is great
enough, and the kidneys compensate by increasing HCO3–
reabsorption, H+ secretion, and production of the urinary
buffer ammonia, all in response to an acidic load.
Thus the loss of bone mass due to acidosis has generally
been considered a passive process, a physico-chemical
dissolution of the matrix. However, bone dissolution is more
than just a passive process, but rather is one of active
resorption by osteoclasts, with extracellular H+ being a key
inducer of osteoclastic activity. Indeed, extracellular H+ has
been suggested to be the ‘long-sought osteoclast activation
factor’, partly because in vitro experiments show osteoclasts
to be almost inactive at a pH above 7·4, with pH reductions
of ≤ 0·1 sufficient to cause a doubling of resorption pit
formation, removing both mineral and organic components
of bone. [13] A number of in vitro studies have shown that
even very small changes in pH have an effect especially on
osteoclastic resorptive activity and on osteoblastic activity,
with acidosis exerting a selective, inhibitory action on
matrix mineralisation. [14–16] Most recently, Frick et al.
have demonstrated that ovarian cancer G protein-coupled
receptor-1 (OGR1) is the proton-sensing receptor on the
osteoblast that leads to osteoclast activation. [17]
In vivo studies have generally supported the in vitro findings
that acid-promoting diets are associated with both increased
Ca and increased bone matrix protein excretion (used as a
marker for estimating bone loss), and that neutralising the
acid intake with diet or bicarbonate supplements decreases
urine Ca and bone matrix protein excretion. [18–21] In a trial
of 170 postmenopausal women, for example, potassium
bicarbonate supplementation reduced daily urinary Ca
excretion, and one could predict which women would benefit
most – those with the greatest urinary Ca loss. [22]
Additionally, the same mechanism may be involved in Ca
nephrolithiasis. [23–25] A common risk factor for Ca stone
formation appears to be hypocitraturia, which has been
associated with a low urinary K level and a more acidic
urinary pH, both of which can be predicted by dietary
intake. [26, 27] However, it appears that dietary acid load is
a better predictor of urinary citrate than the intake of most
individual nutrients, including dietary K. In a trial of 187
patients with renal Ca stone disease, an inverse correlation
(–0·18) was found between daily potential renal acid load
(PRAL; defined below) and daily urinary citrate (P < 0·01).
PRAL was also inversely related to urinary pH and K, and
although dietary protein and K were correlated with citrate
excretion, these relationships did not reach statistical
significance. [28] A low-salt, low-animal protein diet has also
been shown to reduce risk of stone recurrence (relative risk
0·49) with men followed over 5 years. [27] While there are
several possible explanations for this effect, one is that the
reduced salt and/or animal protein diet is likely to result in
a lower acid load.
Finally, metabolic acidaemia has been associated with
increased protein breakdown in rats [29] and possibly subjects
with end-stage renal disease. [30] Mitch et al. demonstrated
that increased activation of a ubiquitin–proteasome pathway
(via an insulin- or insulin-like growth factor-1-mediated
transporter [31]) increases the production of amino acids
which are then made into glutamate in the liver, transported
to the proximal renal tubular cells, and then excreted as
ammonia (NH3) and complexed to H+. [29] This mechanism allows the kidneys to dramatically increase the amount of acid excreted daily.
What are the causes of acidosis?
The causes of metabolic acidosis include increased
consumption or generation of organic acids, as well as either
insufficient production of bicarbonate, or renal and/or gastrointestinal
loss of bicarbonate, such as that seen in renal
disease, diarrhoea, pancreatic drainage and biliary fistula.
Causes of renal tubular acidosis include Sjogren’s syndrome,
systemic lupus erythematosus, urinary tract obstruction,
fever, aldosterone deficiency and glucocorticoid administration. [10] Many of these medical conditions are typically
associated with overt laboratory abnormalities; either a frank
acidaemia, a decrease in plasma bicarbonate, or an increase
in the anion gap.
Figure 1
|
In comparison, diet-induced ‘low-grade’ metabolic acidosis
has only very small decreases in blood pH and plasma
bicarbonate within the range considered to be normal.
Within that range, this means that the system equilibrates
nearer the lower end of normal rather than the higher end
of normal (see Figure 1). But, if the duration of the acidosis
is prolonged or chronically present, even a low degree of
acidosis becomes significant. This less severe but more
chronic ‘low-grade’ acidosis is thought to be brought about
primarily by two factors: advancing age with a consequent
decline in renal function, and diet, which may promote
acidosis by both its net acid load, as well as its sodium
chloride content. With age, the severity of diet-dependent
acidosis increases independently of the diet, most likely due
to a decline in kidney functional capacity with age. [20, 32, 33]
Renal insufficiency contributes to a metabolic acidosis by
reducing conservation of filtered bicarbonate and excretion
of acid.
Diet’s contribution to an acidotic state is now well
documented. In the European Prospective Investigation into
Cancer and Nutrition (EPIC) study of over 22,000 men and
women, after adjusting for age, BMI, physical activity and
smoking, a more alkaline diet (as calculated by PRAL
(a diet-only-based estimate of the production of endogenous
acid) was significantly associated with a more alkaline urine,
both before and after adjustment for age, physical activity
and smoking, and after exclusion for urinary protein,
glucose and ketones, and for those diagnosed with high
blood pressure and/or diuretic medication use (all factors
known to influence urine pH). [34]
How is acidosis diagnosed?
Several approaches have been used to estimate the acid–base
equilibrium of the body, as well as the net acid contribution
from the diet. As discussed previously, this is an area without
general consensus, and one that lacks a definitive marker of
acid–base status. Because plasma pH is so tightly regulated,
without acidaemia an acidosis may not be detected, despite
the stress on the body’s buffering capacity. Additionally, it
has been shown that plasma bicarbonate level may be
normal even when acidosis is present, and so neither plasma
bicarbonate nor the serum anion gap is a sensitive indicator
of acidosis. [10, 35] This situation is analogous to the impaired
glucose tolerance that precedes frank blood sugar elevation.
Waiting for acidaemia before recognising acidosis is not a
sound clinical strategy.
Table 1
|
NEAP represents the amount of net acid (acid minus
base) produced by the metabolic system every day (i.e. a
combination of cellular metabolism and exogenous acid and
base loads from the diet). NEAP can either be measured by
quantifying the inorganic constituents of diet, urine and
stool, and of the total organic anion content of the urine
(calculated as the sum of urinary inorganic sulfate and organic
acid salts minus dietary organic anions less faecal organic
anions) [1], or from estimations of acid or base production
from the constituents of the diet. Several methodologies
have been proposed to estimate NEAP from dietary
components. [11, 36, 37] To avoid confusion, when the NEAP is
estimated (which it usually is), it is identified as ‘estimated
NEAP’, and the algorithm used for the estimation clearly
specified (see Table 1). [38]
The net amount of acid produced daily (NEAP) is closely
tied to the net amount of acid excreted (NAE) by the kidneys
daily. NAE can also be measured by quantifying the amount
of acids in the form of ammonium and titratable acid in the
urine and subtracting the amount of base (bicarbonate). [34]
When measured directly, nearly 90% of the variance in
renal NAE among subjects is accounted for by differences
in NEAP, and because of this closeness in value, they are
often considered equivalent. [20]
Another test often used to estimate the NEAP is the
24 h urine pH. Urinary pH represents an index of the dietdependent
NAE (correlation with calculated NAE, r 0·83;
P < 0·001) [39], as well as the PRAL. [34, 40] Additionally, urine
pH can be adjusted to a target pH based on PRAL calculations
for dietary intake. However, pH strip measurement of the first
voided urine was not found to be predictive of the NEAP
reflected by the 24 h urine NAE. [41]
Lastly, while urinary Ca is not considered a specific marker
for acid–base equilibrium, it has been used as a marker of
bone turnover, and elevated levels are a risk factor for
kidney stone formation. [42–45] As discussed below, it does
not necessarily indicate increased bone Ca loss, however. It
could, for example, represent a change in intestinal absorption.
In summary, analysed NAE appears to be the closest
approximation of ‘true’ NEAP, and is more easily determined.
For simplicity, however, various computational models for
estimating NEAP are often employed, each with its own
advantages and disadvantages (see Table 1). For a more
complete discussion of the pros and cons for each methodology,
the reader is referred to a recent review. [46]
How is acidosis normalised?
The normalisation of a low-grade chronic metabolic acidosis
has been accomplished by two methods: change in dietary
patterns and alkaline supplementation. Dietary factors that
affect net acid production include the quantity and type of
protein intake, fruits and vegetables and table salt (sodium
chloride). Alkali supplementation is generally in the form of
potassium or sodium bicarbonate or citrate.
Increased fruit and vegetable consumption, as well as K
and Mg alkali intake, is consistently associated with a baseproducing
diet. [47] It has been shown that a vegetarian diet
has a considerably lower NEAP than both a high and moderate
omnivorous protein intake. [36]
Reducing protein consumption down to the US dietary
recommended intake in a trial of thirty-nine healthy premenopausal
women has also been shown to reduce Ca excretion
and raise urinary pH, as well as reduce markers of bone
resorption. [48] It should be emphasised that this trial did not
evaluate a low-protein diet, but rather lowered what could
be considered a high protein intake to a level of 0·8 g/kg for
this population. Because renal NH4+ formation is dependent
upon adequate protein intake, an extremely protein-deficient
diet may also increase acidosis. [49] In fact, in a recent study
of 161 postmenopausal women, protein intake had a positive
association with lumbar bone mineral density, but only after
adjusting for the negative effect of the sulfur content of the
protein (sulfate), perhaps ‘reconciling reports of positive
impacts of dietary protein on bone health with reports of a
negative impact of the acid load from sulfur-containing
amino acids’. [50] In children, a greater protein intake has been
associated with greater bone strength, though this effect is
negated if alkalinising nutrients are lacking. It should be
noted, however, that clearly bone may be influenced by these
minerals in ways unrelated to acid–base chemistry. [51]
Finally, increasing sodium chloride intake dose-dependently
decreases blood pH and plasma bicarbonate levels [52],
independent of the partial pressure of carbon dioxide (PCO2),
creatinine clearance and dietary acid load. [6] This effect may
be due to a decrease in the strong ion difference, as total
chloride concentration increases relative to total Na concentration,
an effect that may increase H+ concentration. [53]
Subjects who are particularly sensitive to salt, generally
defined as an increase of 3 to 5mmHg for a given salt load,
have more of a metabolic acidosis than those subjects who
are salt resistant. [54] So, while everyone’s net acid load
would improve by lowering their dietary salt intake, some
individuals should benefit more than others from this dietary
intervention.
A number of supplemental interventions have also been
used. Salts of carbonic acid are available in a variety of
formats. These include sodium or potassium bicarbonate and
calcium carbonate. Alkali salts are also available as citrate,
acetate or hydroxides. As suggested above, giving Na salts
may be partly counterproductive, given their other effects,
and so most studies use K or Ca alkali salts. These salts
dose-dependently decrease NAE. [55, 56]
Caution using alkali therapy without careful consideration
and expertise in subjects with heart, lung or kidney disease
is needed. In congestive heart failure, sodium bicarbonate
impairs arterial oxygenation and reduces systemic and
myocardial oxygen consumption in these patients, which
may lead to transient myocardial ischaemia. [57] Additionally
there may be several simultaneous processes affecting acid–
base status among patients with congestive heart failure. [58]
Similarly, bicarbonate loading may worsen exercise response
in chronic obstructive pulmonary disease patients. [59] Finally,
subjects with kidney failure may develop elevated blood
K levels and potentially fatal cardiac arrhythmias if given K
alkali salts, or volume overload and breathing problems if
given Na alkali salts.
Is acidosis clinically relevant?
Finally, from a clinical standpoint, the most important
question is whether or not acidosis has pathophysiological
effects that could be ameliorated or abolished by reversing
the acidosis. These detrimental effects are associated with
long-term acidosis (years to decades), and offer significant
potential for prevention-based interventions. Areas of recent
interest on the effects of chronic low-grade metabolic acidosis
include effects on bone, muscle and kidney stone formation.
While the data are generally consistent for the effects of
alkali salts on urine Ca, bone biomarker or urinary N excretion
(that is, shorter-term effects), the data are less consistent for
the longer-term effects of alkali salts on bone mineral density,
muscle function or fracture incidence. Positive correlations
have been demonstrated using epidemiological or cohort
studies, but results of prospective, randomised, placebocontrolled
studies on long-term outcomes are mixed.
Bone
In an examination of over 1000 women between the ages of
45 and 54 years, a lower dietary intake of acid-producing
foods correlated with greater spine and hip bone mineral
density, as well as greater forearm bone mass, after adjusting
for age, weight, height and menstrual status. [60] In the Study of
Osteoporotic Fractures Research cohort, over 1000 women
aged 65+ years were enrolled in a prospective cohort
study. Those with a high dietary ratio of animal to vegetable
protein intake (a marker for a greater NEAP) were found to
have more rapid femoral neck bone loss and a greater risk
of hip fracture than did those with a low ratio. [61]
A number of trials have shown that the bone loss can be
reversed by the addition of a base. [21] Potassium bicarbonate
has been shown to improve Ca and P balance, reduce bone
resorption rates, and mitigate the normally occurring agerelated
decline in growth hormone secretion. [20] Potassium
citrate combined with calcium citrate may be more beneficial
than either alone, as demonstrated in a cross-over trial on bone
turnover in postmenopausal women. [62] Urinary Ca excretion
and markers of bone health were improved with potassium
citrate, more so in those consuming a high-Na diet. [63]
A recent randomised cross-over trial enrolled postmenopausal
women, and introduced four well-controlled diets with varying
amounts of calcium and sodium chloride, and evaluated bone
Ca balance. They found that at low Ca intakes, the bone Ca
balance was negative at varying degrees of sodium chloride
intake, but at high Ca intakes, a positive balance was only
seen when Na intake was low. [64] Similarly, in a trial of men
with step-wise increases in sodium chloride intake, indications
of increased bone resorption and metabolic acidosis were seen
with increasing salt intake, and a change in blood pH change
averaging approximately 0·02. [52]
In a prospective, controlled, trial, 109 men and women with
kidney stones had a bone mass increase by increasing their
daily alkali intake (potassium citrate), an effect attributed
directly to alkalinisation because urinary Ca excretion did not
change. [65] Another prospective, blinded study using potassium
citrate in 161 postmenopausal women also demonstrated an
increase in bone mass over a 12–month period. The authors
concluded that ‘as a proof-of-principle study, it demonstrates
that neutralization of diet-induced endogenous acid production
increases BMD (bone mineral density), thereby proving the
concept that such dietary acid loads are detrimental to bone
mass and thus constitute a causative risk factor for bone loss in
postmenopausal women with osteopenia’. [66]
On the other hand, a recent prospective randomised placebocontrolled
trial by MacDonald et al. did not demonstrate a
difference in bone mineral density by dual-energy X-ray
absorptiometry after 2 years of potassium citrate supplementation
of either 55·5 or 18·5 mEq/d, or an increase in fruit
and vegetable intake by 300 g compared with placebo in 276
postmenopausal women. [67] Interestingly though, they found
a significant correlation between the amount of fruit eaten at
baseline and bone mineral density at the hip in the entire
cohort. And the authors report that ‘food diaries and checklists
suggested good compliance for most women, but the blood
measurements [plasma vitamin C, homocysteine, and whole
cell folate] did not corroborate this’.
A recent study suggests that bicarbonate has favourable
effects on bone resorption and Ca excretion compared with
K (as potassium chloride), which calls for greater clarification
as to the active component in supplemental therapies. [68]
Unless Ca-balance studies are performed, rather than just
urinary Ca measurements, it cannot be certain that lower
urinary Ca or a decline in bone markers actually translates
into less bone loss. For example, decreased urinary Ca may
be due to reduced Ca absorption and not due to less bone
loss. This point is emphasised in a recent meta-analysis
(Fenton et al. [69]) which found that although there is a
linear association between urinary Ca excretion in response
to the changes in NAE, this does not necessarily mean that
the increased Ca in the urine is from bone. However, they
did find that over 20 years, the quantity of excess urinary
Ca is consistent with the loss of almost half of skeletal
Ca and severe osteoporosis, though their study did not show
causality(69). These same authors published a second metaanalysis
which assessed the effect of dietary changes in
NAE on urinary Ca, Ca balance, and N-telopeptide, which
found that a higher NAE does not reflect a net loss of whole
body Ca. [70] This analysis had several limitations – the studies
included did not directly measure bone health or the
progression of osteoporosis, and only studies which modified
protein intake were included. Thus, studies which examined
the effect of changes in NAE from either bicarbonate salts
or altered intakes of fruit and vegetables or grain foods were
excluded, and not assessed by this meta-analysis.
Kidney stones
Also of clinical significance is the role metabolic acidosis plays
in nephrolithiasis, and its potential connection to bone loss.
Urinary Ca excretion is directly proportional to NAE in both
stone-formers and normal subjects. [71] In a study of nearly
200 renal stone-formers designed to identify the greatest risk
factors for nephrolithiasis, it was the potential acid load of
the diet which had the strongest association with stone risk.
The authors suggest ‘that a diet with a very low potential
acid load should be encouraged in renal stone patients for the
prevention of recurrent stones. This result can be obtained by
the restriction of animal proteins but also by abundant
supplementation with vegetables and fruits’. [28] Potassiummagnesium
citrate has been shown to counter renal stone
formation associated with immobilisation and was associated
with a significant increase in urinary pH. [72] Though increased
urinary Mg, pH and citrate are associated with reduced stone
formation individually, their effect appears to be greater
when used together. [73] Potassium bicarbonate is more effective
than potassium chloride for preventing stone formation when
used with hydrochlorothiazide. [74] Potassium citrate is also frequently
used, and has been found to reduce bone degradation in
stone-formers [65] and reduce urinary Ca excretion. [63] The same
may be true of potassium and/or magnesium citrate and for
stone prevention – in a trial with normal volunteers and
stone-formers, potassium and magnesium citrate together had
a greater effect than either individually to reduce the ion
activity product index of calcium oxalate. [75] Potassiummagnesium
citrate was also shown to reduce the risk of calcium
oxalate stone formation by 85% during a 3–year period in a
randomised controlled trial [76], perhaps by correcting
hypocitraturia, a known risk factor for forming kidney stones
and a factor (as mentioned previously) inversely associated
with acid load calculated by PRAL. [28]
Unlike in the studies of healthy subjects, in both men and
women with a history of forming Ca stones, a 2–year treatment
with potassium citrate increased forearm bone mineral density
in idiopathic Ca stone-formers, with the speculation that it was
the alkali load that reduced bone resorption by buffering
endogenous acid production. [65]
Muscle
Another area of interest is the use of alkaline therapy for
improving muscle function, exercise capacity and reducing
age-related muscle wasting. Acidaemia has been shown to
increase muscle degradation in patients on haemodialysis. [77]
One epidemiological study of 384 healthy men and women
aged 65 + years found a higher intake of foods rich in K
(fruits and vegetables) was associated with greater lean
muscle mass. The authors speculated that ‘this association is
likely to result from the fact that the ingestion of potassiumrich
alkaline foods such as fruit and vegetables relieves the
mild metabolic acidosis that occurs with the ingestion of a
typical American diet’, and suggest that it is plausible that
age-related muscle mass decline and sarcopenia may be
prevented by the appropriate intake of alkaline K salts. [78]
Potassium bicarbonate has been shown to neutralise the
metabolic acidosis, and reduce urinary N wasting in postmenopausal
women. As faecal N excretion is typically very
low (<12%), and unaffected by alkali administration, the
authors speculated that this N-sparing may prevent loss of
muscle mass, and may even restore past deficits. [79] While it
cannot be certain that the decrease in N excretion is directly
due to less muscle degradation, it is consistent with previous
research suggesting that acidosis stimulates muscle proteolysis
by activating proteolytic pathways. [29]
Whether neutralisation of acidosis improves exercise
function is unclear, with studies demonstrating both positive
and negative results. Short-term intake of sodium bicarbonate
helped reduce the exercise-related drop in pH, improved
anaerobic performance in a dose-dependent manner [80],
improved intermittent sprint performance [81] and was of
ergogenic benefit in the performance of short-term, highintensity
work. [82] One possible mechanism of action is
an increase in plasma pH at rest, providing a delayed onset of
intracellular acidification during exercise [83], or by providing
additional bicarbonate for an increased buffering capacity, as
concluded in a trial involving sodium bicarbonate ingestion
by elite swimmers. [84] Other studies have not demonstrated
any effect of short-term bicarbonate supplementation. [85]
Longer-term studies on the effects of alkali neutralisation
on muscle function and mass are presently underway (LA
Frassetto, personal communication).
Other
There may also be a connection between insulin resistance
and acid–base equilibrium, though this relationship is still
speculative. Insulin resistance has been associated with a
lower urinary citrate excretion, and hypocitraturic patients
show greater insulin resistance than normocitraturic Ca
stone-formers. [86] Type 2 diabetes mellitus has been shown
to increase the risk of uric acid stone formation, because it
causes a lower urinary pH due to impaired kidney ammoniagenesis. [87] This lower urinary pH cannot be entirely attributed
to a greater BMI or acid intake, though both are factors in
determining urinary pH. [88] Finally, in an evaluation of 148
adults with no kidney stones, participants with the metabolic
syndrome had a significantly lower 24 h urine pH than those
without, with an incremental reduction in pH associated
with the number of metabolic abnormalities present. [89, 90]
Also of clinical relevance may be the treatment of pain,
although it is less well studied. Local tissue acidosis has
been documented in patients with complex regional pain
syndrome, and leads to increased pain sensation. [91] The
mechanism for pain sensation may be mediated by acidsensing
ion channels (ASIC), with an increase in ASIC
activity in spinal dorsal horn neurons promoting pain by
central sensitisation, a mechanism documented in rats. [92]
ASIC activity may also be induced by NO [93], and appears
to be tightly regulated by pH. [94] This is an area that lacks
significant clinical research.
Conclusion
The lack of consensus for both qualitative and quantitative
aspects of acid–base chemistry in physiological systems as
well as measurement has caused considerable confusion
for both researchers and clinicians. This confusion has also
complicated the search for cause and effect and made clinical
application difficult and controversial. Nonetheless, the
available research makes a compelling case that diet-induced
acidosis is a real phenomenon, has significant clinical
relevance, may largely be prevented through dietary changes,
and should be recognised and treated.
Both dietary interventions (lowering protein and
increasing fruit and vegetable consumption) and nutritional
supplementation (with K and Mg salts) have been shown
to normalise acidosis, but with discordant results on whether
this is then associated with clinical improvement in
bone, muscle or other physiological or pathophysiological
conditions. A positive NEAP diet results in increased urine
Ca, N and bone marker excretion, and predisposes to kidney
stones. Whether or not, over the longer term, this translates
to lower bone density, increased bone and muscle loss with
ageing is unclear and requires further investigation.
You may refer to this
acid/alkaline food chart
for general guidance.
Acknowledgements
The present review was supported by partial funding from
pH Sciences (Seattle, WA, USA).
L. A. F. would like to acknowledge Anthony Sebastian
and R. Curtis Morris Jr, the two people she has worked with
for all of these years, who are responsible for the ideas
behind what she does.
J. P. and J. K. were responsible for data review and writing
of the initial manuscript. L. A. F. provided critical review and
editing of several drafts, and participated in final manuscript
writing.
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