Marilyn Harris:
> > Again, the acid level in the stomach is a different topic than the
> > acidifying effect of certain foods on urine.
> 
> I think I see what you are saying now..... :-)

Thanks, I'm glad I was able to finally communicate the message clearly. It
helped when I remembered what had confused me when I first learned about it.

Marilyn:
> But the food mass is tremendously broken down by the time it reachs the
> intestines (it is soupy mixture) - one wonders how there could be an
> acidifying effect? It's no longer the intact food it was prior to
> eating.

It's even more broken down by the time it reaches the kidneys (it's liquid
urine at that point). Nonetheless, it can be either acidic or alkaline at
that point. The pH range of urine is actually the widest of any bodily fluid
(I have seen ranges quoted from as low as 4.0 to as high as 8.5), so when
critics talk about a "narrow range" I think they are probably usually
talking about either blood pH or stomach acid pH rather than urine pH upon
reaching the kidneys.

Marilyn:
> Do you know how this effect is caused?

I know the basic process that Cordain explained but I don't understand all
the details of the biological processes because I am not a biologist. The
basic process is that acidifying foods result in an acidic urine, which is
then buffered with calcium and bicarbonate in the kidneys. If there is not
enough free calcium and bicarbonate to buffer the urine it is leached from
the bones. Chronic acidification results, which can be part of a destructive
process known as acidosis (aka metabolic acidosis). Acidosis of bodily
fluids is one of many symptoms associated with diabetes and both are
symptoms of modern food syndrome (aka diseases of civilization). It used to
be that diet was not recognized as a contributing factor to either diabetes
or acidosis. We now know better.

Acid/alkaline balance is a popular subject with vegetarians, because they
use it as an argument against eating meat and dairy. Because of that I think
some Paleo dieters dismiss it because they dismiss the messenger. It turns
out there does appear to be evidence backing up the acid/alkaline theory,
but Paleo diets do not necessarily have to be acidifying if they include
fruits and vegetables. 

The question remains as to whether Eskimo and Plains Indians suffered high
rates of acidosis and osteoporosis. There is disagreement on this and I
haven't seen much evidence to support the claims of either side of the
argument. It seems to me that the mostly-carnivorous Indians must have had a
practical or biological way of dealing with the acidifying nature of the
meats and seafood in their diets because I find it unlikely that they would
have thrived for thousands of years despite chronic acidosis.

Here are some more excerpts on the subject:


FAQs
Loren Cordain
http://www.thepaleodiet.com/faqs/

"Do you have any thoughts on whether long-term use of a traditional
ultra-low-carb diet can interfere with thyroid function?

I would be strictly speculating here because I believe there is no direct
clinical evidence. However, one of the major problems of the ultra-low-carb
diets are that they yield a net metabolic acidosis because there is
insufficient alkaline base (only derived from fruits & veggies) to
neutralize the net renal acid load that the kidney must deal with from a
high-meat diet. As I have pointed out in question 2, there is some evidence
that a net metabolic acidosis may elicit a mild primary hypothyroidism and
hyperglucocorticoidism."


Urine pH
http://www.rnceus.com/ua/uaph.html

"A highly acidic urine pH occurs in: 

Acidosis 
Uncontrolled diabetes 
Diarrhea 
Starvation and dehydration 
Respiratory diseases in which carbon dioxide retention occurs and acidosis
develops 

A highly alkaline urine occurs in: 

Urinary tract obstruction 
Pyloric obstruction 
Salicylate intoxication 
Renal tubular acidosis 
Chronic renal failure 
Respiratory diseases that involve hyperventilation (blowing off carbon
dioxide and the development of alkalosis)"


Potassium Bicarbonate Reduces Urinary Nitrogen Excretion in Postmenopausal
Women 
L. Frassetto et al
http://jcem.endojournals.org/cgi/content/full/82/1/254

Abstract

"Previously we demonstrated that low grade chronic metabolic acidosis exists
normally in humans eating ordinary diets that yield normal net rates of
endogenous acid production (EAP), and that the degree of acidosis increases
with age. We hypothesize that such diet-dependent and age-amplifying low
grade metabolic acidosis contributes to the decline in skeletal muscle mass
that occurs normally with aging. This hypothesis is based on the reported
finding that chronic metabolic acidosis induces muscle protein breakdown,
and that correction of acidosis reverses the effect. Accordingly, in 14
healthy postmenopausal women residing in a General Clinical Research Center
and eating a constant diet yielding a normal EAP rate, we tested whether
correcting their "physiological" acidosis with orally administered potassium
bicarbonate (KHCO3; 60–120 mmol/day for 18 days) reduces their urinary
nitrogen loss. KHCO3 reduced EAP to nearly zero, significantly reduced the
blood hydrogen ion concentration (P < 0.001), and increased the plasma
bicarbonate concentration (P < 0.001), indicating that pre-KHCO3,
diet-dependent EAP was significantly perturbing systemic acid-base
equilibrium, causing a low grade metabolic acidosis. Urinary ammonia
nitrogen, urea nitrogen, and total nitrogen levels significantly decreased.
The cumulative reduction in nitrogen excretion was 14.1 ± 12.3 g (P <
0.001). Renal creatinine clearance and urine volume remained unchanged. We
conclude that in postmenopausal women, neutralization of diet-induced EAP
with KHCO3 corrects their preexisting diet-dependent low grade metabolic
acidosis and significantly reduces their urinary nitrogen wasting. The
magnitude of the KHCO3-induced nitrogen-sparing effect is potentially
sufficient to both prevent continuing age-related loss of muscle mass and
restore previously accrued deficits."


Estimation of the net acid load of the diet of ancestral preagricultural
Homo sapiens and their hominid ancestors1,2,3 
Anthony Sebastia et al

American Journal of Clinical Nutrition, Vol. 76, No. 6, 1308-1316, December
2002

http://www.ajcn.org/cgi/content/full/76/6/1308

"Results: ... The historical shift from negative to positive NEAP was
accounted for by the displacement of high-bicarbonate-yielding plant foods
in the ancestral diet by cereal grains and energy-dense, nutrient-poor foods
in the contemporary diet—neither of which are net base-producing. 

Conclusions: The findings suggest that diet-induced metabolic acidosis and
its sequelae in humans eating contemporary diets reflect a mismatch between
the nutrient composition of the diet and genetically determined nutritional
requirements for optimal systemic acid-base status. Am J Clin Nutr
2002;76:–16.

Discussion: ... substituting cereal grains for the other plant-food groups
in the preagricultural diet converts the diet from a net base-producing
[one] to a net acid-producing one. ... Because the steady state plasma
bicarbonate concentration is a continuous inverse function of NEAP over a
broad range of positive and negative values (16 , 36 , 56 , 60 ), the
findings in the present study also suggest, from an evolutionary
perspective, that a mild systemic metabolic alkalosis resulting from chronic
dietary net base loading is the natural and optimal systemic acid-base state
of humans. To our knowledge there have been no measurements of systemic
acid-base equilibrium in modern hunter-gatherers living in the wild. Denton
(61 ) summarized the preliminary studies of MacFarlane et al of a New
Guinean hunter-gatherer tribal group living in "the primitive feral
condition," noting that "urine pH of adults was usually between 7.5 and 9.0
because of potassium bicarbonate and carbonate excretion." Such high urine
pH values are difficult to achieve, except with large mild
alkalosis-producing bicarbonate inputs."