More than 30 years ago, an idea was put forward that high colonic pH promoted colorectal cancer. A high colonic pH may promote the creation of carcinogens from bile acids, a process that is inhibited once you get below a pH of about 6.5. This is supported by data like this, showing those at higher risk for colon cancer may have a higher stool pH, and those at lower risk, a lower pH. There was a dramatic difference between the two groups, with most of the high-risk group, pH over 8, and most of the low-risk group, pH under 6.

This may help explain the 50-fold lower rates of colon cancer in Africa compared to America. The bacteria we have in our gut depends on what we eat. If we eat lots of fiber, then we preferentially feed the fiber-eating bacteria, which give us back all sorts of health-promoting substances like short-chain fatty acids, which have anti-inflammatory and anti-cancer properties. More of these organic acids were found in the stools of native Africans than African-Americans. More acids, so lower pH. Whereas putrefactive bacteria, eating animal protein, are able to increase stool pH by producing alkaline metabolites like ammonia.

The pH of the stools of white versus black children in Africa was compared. Children, because you can more readily sample their stools–particularly the rural black schoolchildren who were eating such high fiber diets (whole grains, legumes, nuts, vegetables, fruits and wild greens) that nine out of ten could produce a stool on demand. Stuffed from head to tail with plants, they could give you a stool sample at any time, as easy as getting a urine sample. Hard to even get access to the white kids, though, who were reluctant to participate in such investigations, even though they were given waxed cartons fitted with lids, and all the black kids got was a plate, and a square of paper towel.

What’d they find? Significantly lower fecal pH in those eating the traditional rural plant-based diets, compared to those eating the Western diet, who were eating far fewer whole plant foods than the black children. But remove some of those whole plant foods, like switch their corn for white bread for just a few days, and their stool pH goes up. And add more whole plant foods, like an extra five to seven servings of fruit every day, and their stool pH goes down even further, gets more acidic. Makes sense, right? What happens when you ferment plants–fruits/veggies/grains? They turn sour, like vinegars, sauerkraut, sourdough, because good bacteria–like lactobacillus–produce organic acids, like lactic acid. And those who eat a lot of plants have more of those good bugs in their system. So using the purple cabbage test, we want blue pee, but pink poo.

No surprise, then, if you compare the fecal samples of those eating vegetarian or vegan to those eating standard diets. Plant-based diets appear to shift the makeup of the bacteria in our gut, resulting in a significantly lower stool pH, and the more plant-based, the lower the pH dropped. It’s like a positive feedback loop. Fiber-eating bacteria produce the acids to create the pH at which fiber-eating bacteria thrive while suppressing the group of less beneficial bugs.

How long does it take to bring stool pH down on a plant-based diet? As little as two weeks. A dozen volunteers carefully selected for their trustworthiness, and randomized to sequentially go on regular, vegetarian, or vegan diets–and two weeks in, a significant drop in fecal pH was achieved eating completely plant-based.

But there are plant-based diets and then there are plant-based diets. Remember these two groups? Dramatically different stool pH, yet both groups were vegetarian. But the high risk group was eating mostly refined grains, very little fiber, whereas the low risk group was eating whole grains and beans, packed with fiber for our fiber-friendly flora to munch on.

Just as a reduction in high serum cholesterol contributes to the avoidance of coronary heart disease, so a fall in the fecal pH value may contribute to the avoidance of bowel cancer, and through the same means: eating more whole plant foods.

To see any graphs, charts, graphics, images, and quotes to which Dr. Greger may be referring, watch the above video. This is just an approximation of the audio contributed by Katie Schloer.

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