The Answer to a Colon Cancer Mystery

Colorectal cancer is the third most common cause of cancer death in the world. Thankfully, the good bacteria in our gut take the fiber we eat and make short-chain fatty acids, like butyrate, that protect us from cancer. We take care of them, and they take care of us. If we do nothing to colon cancer cells, they grow. That’s what cancer does. But if we expose the colon cancer cells to the concentration of butyrate our good bacteria make in our gut when we eat fiber, the growth is stopped in its tracks. If, however, the butyrate stops, if we eat healthy for only one day and then turn off the fiber the next, the cancer can resume its growth. So, ideally, we have to eat a lot of fiber-rich foods—meaning whole plant foods—every day.

What about the populations, like those in modern sub-Saharan Africa, where they don’t eat a lot of fiber yet still rarely get colon cancer? Traditionallly. they used to eat a lot of fiber, but now their diet is centered around highly refined corn meal, which is low in fiber—yet they still have low colon cancer rates. Why? This was explained by the fact that while they may be lacking protective factors like fiber, they are also lacking cancer-promoting factors like animal protein and fat. But are they really lacking protective factors?

If you measure the pH of their stools, the black populations in South Africa have lower pH, which means more acidic stools, despite comparable fiber intakes. That’s a good thing and may account for the lower cancer rates. But, wait a second. Low colon pH is caused by short-chain fatty acids, which are produced by our good bacteria when they eat fiber, but they weren’t eating any more fiber, suggesting there was something else in addition to fiber in their diets that was feeding their flora. And, indeed, despite low fiber intake, the bacteria in their colon were still churning out short-chain fatty acids like crazy. But if their bacteria weren’t eating fiber, what were they eating? Resistant starch. “[T]he method of cooking and eating the maize [corn] meal as a porridge results in an increase in resistant starch, which acts in the same way as fiber in the colon,” as a prebiotic, a food for our good bacteria to produce the same cancer-preventing, short-chain fatty acids.

As I discuss in my video Resistant Starch and Colon Cancer, “[r]esistant starch is any starch…that is not digested and absorbed in the upper digestive tract [our small intestine] and, so, passes into the large bowel,” our colon, to feed our good bacteria. When you boil starches and then let them cool, some of the starch can recrystallize into a form resistant to our digestive enzymes. So, we can get resistant starch eating cooled starches, such as pasta salad, potato salad, or cold cornmeal porridge. “This may explain the striking differences in colon cancer rates.” Thus, they were feeding their good bacteria after all, but just with lots of starch rather than fiber. “Consequently, a high carbohydrate diet may act in the same way as a high fiber diet.” Because a small fraction of the carbs make it down to our colon, the more carbs we eat, the more butyrate our gut bacteria can produce.

Indeed, countries where people eat the most starch have some of the lowest colon cancer rates, so fiber may not be the only protective factor. Only about 5 percent of starch may reach the colon, compared to 100 percent of the fiber, but we eat up to ten times more starch than fiber, so it can potentially play a significant role feeding our flora.

So, the protection Africans enjoy from cancer may be two-fold: a diet high in resistant starch and low in animal products. Just eating more resistant starch isn’t enough. Meat contains or contributes to the production of presumed carcinogens, such as N-nitroso compounds. A study divided people into three groups: one was on a low-meat diet, the second was on a high-meat diet including beef, pork, and poultry, and the third group was on the same high-meat diet but with the addition of lots of resistant starch. The high-meat groups had three times more of these presumptive carcinogens and twice the ammonia in their stool than the low-meat group, and the addition of the resistant starch didn’t seem to help. This confirms that “exposure to these compounds is increased with meat intake,” and 90 percent are created in our bowel. So, it doesn’t matter if we get nitrite-free, uncured fresh meat; these nitrosamines are created from the meat as it sits in our colon. This “may help explain the higher incidence of large bowel cancer in meat-eating populations,” along with the increase in ammonia—neither of which could be helped by just adding resistant starch on top of the meat.

“[T]he deleterious effects of animal products on colonic metabolism override the potentially beneficial effects of other protective nutrients.” So, we should do a combination of less meat and more whole plant foods, along with exercise, not only for our colon, but also for general health.


This is a follow-up to my video Is the Fiber Theory Wrong?.

What exactly is butyrate? See:

For videos on optimizing your gut flora, see:

Interested in more on preventing colon cancer? See:

If you’re eating healthfully, do you need a colonoscopy? Find out in Should We All Get Colonoscopies Starting at Age 50?.

When regular starches are cooked and then cooled, some of the starch recrystallizes into resistant starch. For this reason, pasta salad can be healthier than hot pasta, and potato salad can be healthier than a baked potato. Find out more in my video Getting Starch to Take the Path of Most Resistance.

In health,
Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my free videos here and watch my live, year-in-review presentations:

Reduce Acid-Forming Proteins to Protect Kidney Function

Chronic kidney disease is a major public health problem affecting about one in eight Americans, increasing the risks of disease and death even among those with only mild decreases in kidney function. Low-cost, low-risk preventive strategies that anyone can do are needed to address the epidemic of kidney disease. I discuss some of these in my video Protein Source: An Acid Test for Kidney Function.

Diet plays a role in kidney function decline. “Specifically, diets higher in animal protein, animal fat, and cholesterol” may be associated with protein leakage into the urine, which is a sign of kidney damage, and, generally, “diets higher in fruits, vegetables, and whole grains, but lower in meat and sweets, may be protective” against kidney function decline.

In comparison to the diet eaten by our ancient ancestors, not only are we eating more saturated fat, sugar, and salt, we now also eat an acid-producing diet, as opposed to a base-producing, or alkaline, diet. Ancestral human diets were largely plant based and, as such, produced more base than acid.

“Dietary acid load (DAL) is determined by the balance of acid-inducing foods which is rich in animal proteins (such as meats, eggs, and cheese)” and offset by base-inducing foods, such as fruits and vegetables. In a national survey of 12,000 American adults, DAL was associated with kidney damage among U.S. adults.

Acid-inducing diets are believed to affect the kidney through tubular toxicity, damage to the tiny, delicate, urine-making tubes in the kidney via increased ammonia production. Ammonia is a base, so the kidney creates it to buffer the acid from the food we eat. This is beneficial in the short term to get rid of the acid; however, in the long term, all that extra ammonia in our kidneys day in and day out seems to exert toxic effects.

Our kidney function tends to decline progressively after our 30s, and, by our 80s, our kidney capacity may be down to half. “Perhaps, the so-called age-related decline in renal function is a result of damage induced by ammonia overproduction.” That’s just one theory, though. The acidic pH may increase the production of free radicals and damage the kidney that way, or add to scarring.

Not only is protein derived from plant foods accompanied by antioxidants that can fight the free radicals, but plant protein is also less acid-forming in the first place because it tends to have fewer sulfur-containing amino acids. One of the reasons plant foods tend to be less acid-forming than animal foods is because acid is produced by the sulfur in the protein, and there’s less in plant proteins.

“[T]he more important determinant of the effect of dietary protein on nephropathy [kidney disease] progression is the quality of the ingested protein (i.e., whether it induces acid-production like most animal protein or base production like most fruit and vegetable protein) when ingested rather than the quantity of protein ingested.”

American diets “are largely acid-producing because they are deficient in fruits and vegetables and contain large amounts of animal products,” so changing from a standard American diet to a vegan diet may improve acidosis in patients with chronic kidney disease. Under normal circumstances, a vegetarian diet is alkalinizing, whereas a nonvegetarian diet leads to an acid load. This was true even of vegetarians who consumed processed meat replacements such as veggie burgers.

Plant-based diets have been prescribed for decades for those with chronic kidney failure. They contain no animal fat, no cholesterol, and less acid formation, and help to lower blood pressure. Indeed, if you compare the kidney function of vegans with vegetarians and omnivores, the most plant-based diet was most associated with improved parameters for the prevention of degenerative kidney decline.


I was surprised to learn how powerfully diet can affect kidney function and structure. My kidney videos include:

And be sure to check out my overview video, How Not to Die from Kidney Disease.

Aren’t some plant foods acidic, though? Check out the chart in my video, How to Treat Kidney Stones With Diet.

Is there any way to test to see how acid-forming your diet is? Yes—and it’s fun! See Testing Your Diet With Pee and Purple Cabbage.

In health,
Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my free videos here and watch my live, year-in-review presentations:

The Effect of Animal Protein on the Kidneys

Between 1990 and 2010, some of our leading causes of death and disability remained the same. Heart disease was the leading cause of loss of life and health then and remains the leading cause today. Some things got better, like HIV/AIDS, but others got worse, like chronic kidney disease. We saw a doubling in the tens of thousands of deaths and the hundreds of thousands of patients whose kidneys failed completely, requiring kidney transplants or lifelong dialysis.

About one in eight of us now has chronic kidney disease—and most don’t even know it: About three-quarters of the millions of people affected are unaware that their kidneys are starting to fail. This is “particularly worrisome given that early identification provides an opportunity to slow the progression and alter the course of disease.” So, what can we do about it?

In my video Which Type of Protein Is Better for Our Kidneys?, I discuss how the Western-style diet is a major risk factor for impaired kidney function and chronic kidney disease. Also known as “the meat-sweet diet or standard American diet,” it causes an impairment of kidney blood flow, inflammation, subsequent leakage of protein in the urine, and a rapid decrease in kidney function. Table sugar and high-fructose corn syrup are associated with increased blood pressure and uric acid levels, both of which can damage the kidney. The saturated fat, trans fat, and cholesterol found in animal fat and junk food negatively impact kidney function, as well.

The consumption of animal fat can actually alter the structure of the kidney, and animal protein can deliver an acid load to the kidneys, increase ammonia production, and damage the sensitive kidney cells. This is why restricting protein intake is recommended for preventing kidney function decline—though it may be animal protein in particular that may need restricting, not just protein in general. So, the source of the protein, plant versus animal, may be more important than the amount regarding adverse health consequences.

Animal protein intake profoundly affects normal human kidney function, inducing what’s called hyperfiltration, which increases the workload of the kidney. This may help explain why our kidneys fail so often. “Unlimited intake of protein-rich foods, now generally regarded as ‘normal,’ may be responsible for dramatic differences in renal function between modern human beings and their remote predecessors who hunted and scavenged for meat.” Indeed, our kidney reserves are continuously called upon by our sustained, rather than intermittent, excesses of protein. This causes a kind of unrelenting stress on our kidneys that can predispose even healthy people to progressive kidney scarring and deterioration of kidney function. It’s as if we’re always revving our engine into the red. Conversely, intake of an equal quantity of protein from plants doesn’t appear to have the same effects.

We saw in a study that eating meat increases the workload on the kidneys within hours of consumption, but, apparently, processing plant protein is a cinch. That particular study was done with beef, but any animal protein will do. If you eat a meal of tuna fish, pressure on the kidneys goes up within just hours of consumption. Then the researchers switched it up and found that if instead of having a tuna salad sandwich, you have a tofu salad sandwich (with the same amount of protein): no adverse effect.

The same thing happens with eggs and dairy protein—both in people with normal kidneys and people with diseased kidneys.

Short-term studies have indicated that “substituting soy protein for animal protein is associated with less hyperfiltration and [protein leakage]…therefore slowing deterioration of renal function.” However, the long-term effect had not been adequately studied, until 2014. A six-month double-blind, randomized, placebo-controlled trial compared the effects of soy and dairy protein on renal function and determined that “the consumption of whole soy tended to preserve renal function compared with milk placebo in individual with lowered renal function.” Similar results were reported in diabetics. Even just giving isolated soy protein appeared to make things better, compared to dairy protein which made things worse.

Once one’s kidneys have deteriorated to the point that they’re actively losing protein in the urine, a plant-based diet may help come to the rescue. Protein leakage on a standard low-sodium diet was compared to a special supplemented vegan diet. The researchers showed through diet alone, kidney dysfunction could be switched on and off like a light switch depending on what was going into their mouth. See the extraordinary graph in my video.

What is going on? Why does animal protein cause that overload reaction, but plant protein doesn’t? The renal reaction appears to be an inflammatory response triggered by the animal protein. We know this because administration of a powerful anti-inflammatory drug abolished the hyperfiltration,  protein leakage response to meat ingestion. Kidney stress has been shown to escalate in response to a meat meal, but that stress level remains low during the meal when supplemented with an anti-inflammatory drug. This confirms the role of inflammation in the impact of animal protein on our kidneys.


This is part of a new, extended series of videos on kidney health, which includes:

I’ve compared plant and animal protein in a variety of ways. See, for example,

Methionine Restriction as a Life Extension Strategy, Prostate Cancer Survival: The A/V Ratio, and Caloric Restriction vs. Animal Protein Restriction.

Perhaps the most dramatic demonstration was the study I featured in my 2015 annual presentation, Food as Medicine: Preventing and Treating the Most Dreaded Diseases with Diet.  

In health,

Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my free videos here and watch my live, year-in-review presentations: