Authors: Registered Dietitians
(HealthCastle.com) Recently, I went to the annual Dairy Farmers' of Canada symposium on nutrition and health held at the Carlu, one of Toronto's more historical and unique special events venue. The Dairy Farmers' have been putting these symposia on since 2005 and have included a variety of speakers on equally varied topics from high fructose corn syrup and health, food psychology and mindless eating, the introduction of a newly proposed nutrient-rich foods index and more. It was at this symposium in 2010 that the attendees learned about the robust data that essentially kibosh the long-standing theory of saturated fat's role in cardiovascular disease. In fact, the data demonstrated the opposite: saturated fat found in dairy foods offers protection against heart disease and stroke. The symposium always manages to provide high caliber speakers, leading edge research and topical subjects. This year's theme was no different. Of the four presentations, I'm going to review one of them.
I was really looking forward to Tanis Fenton PhD, RD's presentation when I saw the symposium outline. Her main area of study and publication include critical appraisal in nutrition research, nutrition and bone health, and neonatal nutrition and growth. Her presentation looked at the scientific evidence behind the popular claim made by The China Study, as well as numerous books and websites, that certain foods, including dairy, meats and wheat, increase the acidity of blood (or lowers it's pH) which in turn leads to increase losses of bone tissue increasing the risk for osteoporosis, as well as contributing to most of the degenerative diseases of the Western world.
The main argument of the acid-base, acid-ash or alkaline diet hypothesis is that various foods increase acid production in the blood which leads to a decrease in pH, inappropriately referred to 'metabolic acidosis'. The body in turn needs to use calcium from bone to buffer the alleged increase in blood acidity. Those who promote this theory say you can test how acidic the body is by simply testing the pH of urine with the use of pH strips.
Dr Fenton pointed out a few flaws of this overly simplistic hypothesis, not the least of which is that the fact that the body has many protective mechanisms to keep blood pH in a tight range between 7.38 - 7.42; a pH slightly more alkaline than distilled water. These mechanisms involve the lungs, pancreas, kidneys and stomach and any slight deviation from the normal blood pH range would result in either true metabolic acidosis or metabolic alkalosis - two life threatening conditions reserved for the intensive care unit or dialysis unit of any hospital. The point is, there's nothing in the diet that can change the pH of blood in any meaningful clinical way.
What about those urine pH strips?
Of note, Dr Fenton pointed out that during her review of books and websites that promote the idea of an acid producing property of dairy foods, 83% of them did not provide any references to support the claim, no research whatsoever. Of the 17% that did provide a reference, they all used Remer & Manz's 1995 study from the Journal of the American Dietetic Association. The authors devised a theoretical formula to create the PRAL (Potential Renal Acid Load) equation using food composition tables versus actually analyzing foods; the higher value, the more acid producing the food. According to their own work, milk is 0.7 and yogurt 1.5 or 'near neutral'. Cheeses on the other hand can be between 14.6 to 29.6 or 'acid producing'. [For those with a better understanding of organic chemistry, Dr Fenton pionted out that the theoretical equation uses 'conjugate acids & bases', an approach that is blatantly incorrect].
The real question is whether or not foods of any kind can change the measured pH of blood. It's true that certain foods and beverages can temporarily change the pH of urine, but that does not represent, nor measure, what's happening with the blood. Remember, a slight deviation in blood pH would result in hospital stay in the ICU. Buclin 2001 and Maurer 2003 showed that the pH of urine can be modified with diet [hence those using urine pH strips will see changes], significantly so, but that the corresponding changes in blood pH were 0.014 and 0.007, or not in a significant or metabolically meaningful way, i.e. zero. This was also demonstrated by Heaney 2001 who showed that net acid excretion [more acid in the urine] was 2.6 times higher with colas, 0.84 times higher with plain water but -0.08 with milk, in other words, milk did not increase urin acid excretion at all.
But acid producing foods increases calcium losses from bone
Or do they? There's no denying that certain foods can increase the amount of calcium in the urine but that doesn't automatically mean the extra calcium is being lost from bone. In fact, by simply increasing the amount of protein or vitamin D in one's diet, more calcium can be measured in urine but because more dietary calcium is being absorbed by the gut. Studies using radioisotopes of calcium allow for precise measurement of where the increase in urinary calcium is coming from, and it's not from bone but rather from food. Calcium balance studies, those that measure the amount of calcium consumed with what's lost in both urine and feces are the gold standard; simply looking at the calcium in urine is not and calcium balance studies confirm that higher urinary acid has no relationship with total body calcium balance or the amount of calcium in bones.
Enough theory, give me the straight goods!
The bottom line is this: no food or beverage is able to change the pH of blood, also known as systemic pH, and food cannot cause metabolic acidosis or metabolic alkalosis; life threatening conditions that can only result under extreme conditions. Calcium balance and therefore bone health, is not affected by 'net acid excretion', or the amount of acid that is lost in urine. The alkaline diet hypothesis is not supported by scientific evidence.