The biggest and most important differences between vegetarian and omnivorous diets is they’re lower in calories and saturated fat, and higher in fiber and phytonutrients. Taken together this usually means lower weight and longer lifespans. Sign me up.

However an underappreciated difference between vegetarian and omnivorous diets is the difference in protein composition.

The protein in animal diets is typically considered superior since it has a more “complete” amino acid  profile and it’s generally assumed that more protein is always better.

I’ve written before about the flaws in the latter two points, but the differences in composition of animal and plant proteins deserves its own consideration because it can have a large impact on your health.

 How Animal Protein Harms Your Health

Consuming significantly more animal protein than vegetable protein can cause the following problems:

• it’ll raise your cholesterol
• it’ll mess up your kidneys
• it’ll weaken your bones

Let’s discuss these three points in some detail so you can understand the long term health risks of consuming too much animal protein.

How Protein Works

Proteins are the building blocks of cells. Practically every cellular molecule is made up of different proteins that your body synthesizes from 20 different amino acids. (There’s technically more than 20, but the others are esoteric and not relevant to our discussion). The twenty amino acids comprise all dietary proteins, regardless of whether or not they come from plants or animals.

The Difference Between Animal And Plant Proteins

So we know all proteins are made of the same basic stuff, which is all used to build our cells. So why would there be a big difference between different types of proteins? It boils down to the following point:

• animal proteins contain the amino acids in different proportions than plant proteins

Specifically…..

• animal proteins have more sulfur containing amino acids
• sulfur amino acids are metabolized into sulfuric acid
• your body is naturally alkaline, and thus has to make adjustments to reduce dietary induced acidity
• too many of these adjustments without corresponding changes in other nutrients creates metabolic imbalances which result in the health problems I listed above

So with that in mind, let’s take a look at how excessive amounts of animal protein can change the way your body works.

1). Animal Protein Weakens Your Bones

As I said before, animal proteins tend to be acidic and your body is naturally alkaline. When acidic compounds enter your body it releases a buffer solution to bring the pH back to its normal state. This buffer solution comes from your bones and it causes a lot of calcium to be excreted into your blood stream.

We all know that calcium is the building block of strong bones. So if your bones are getting rid of calcium that’s not a good thing for bone health.

Most evidence more or less confirms this intuition.

For instance, here’s a useful chart from a study published in Calcified Tissue International that compared the relationship between animal protein and hip fractures across different cultures:

Other dietary factors play a role, like the amount of phosphorus in the proteins digested and the presence of other minerals which affect how much calcium is excreted, but the general relationship still applies.

2). Animal Protein Disturbs Kidney Function

A fairly famous paper published in Nature found that animal protein causes the kidney to behave very differently than vegetable protein. When you eat animal protein your body begins to secrete two chemicals called glucagon and Prostaglandin 1 (PGF1), which change the hormonal balance in your kidney, which causes more blood and other stuff to pass through it.

More stuff passing through the kidney is called an increase in the Glomerular Filtration Rate (GFR) which causes the kidney to become swollen and can lead to kidney hypertension. Your kidney exists to effectively filter stuff out of the blood. So it’s not cool if it stops filtering things very well.

The relationship between animal protein and kidney function has become so strong that reducing animal protein is practically always considered the first step in treating a kidney disease of almost any sort. It’s possible that over time high amounts of animal protein can also lead to renal cancer, but that’s still up in the air.

Gout, Kidney Stones, and Arthritis, Oh My!

Anyone who’s ever had a kidney stone knows they’re a royal pain in the ass. They cause acute pain that comes in waves and leaves the body just as quickly as it storms in. It makes taking a piss (perhaps the most basic of all human functions) an act of pain tolerance.

The most common form of kidney stone is made of a compound called calcium oxalate.

Remember what animal protein causes your body to secrete more of? That’s right….calcium.

It also increases the excretion of uric acid and citrate, which are other precursors of kidney stone formation, and reduces your body’s ability to dissociate oxalate crystals in the blood.

Want to stop that burning sensation when you take a leak? Stop eating meat.

And It Doesn’t End There

Closely related to the problems of kidney stones is gout. Gout is the most common form of inflammatory arthritis, and people who suffer from kidney stones are twice as likely to suffer from gout as people that don’t.

Animal protein causes your body to excrete more uric acid. Increased excretion of uric acid and a diet rich in purines are the biggest precursors to gout, and both are closely associated with large doses of animal protein.

3). Animal Protein Raises Your Cholesterol

The idea that protein intake influences your cholesterol might seem novel, but it’s actually been known for over 100 years when a scientist known as Mr. Ignatowski published evidence in 1908 that his barn rabbits suffered from increased rates of athersclerosis when they were fed animal protein instead of egg protein.

The link between animal protein and cholesterol was eclipsed by the role of fat in the 1920’s, but increasing evidence has suggested that animal protein is atherogenic. Clinical studies in humans have found that serum cholesterol is lowered when casein protein is replaced with soy protein. Longer term clinical studies done on pigs have showed the same effect. When it comes to cholesterol pigs are a very good subject because their bodies metabolize cholesterol in a way that’s very similar to humans, so the evidence there is very solid.

It’s not clear exactly why animal protein raises cholesterol, but there are currently two good guesses:

• Vegetable proteins have an increased arginine to lysine ratio
• Animal proteins reduce the rate of cholesterol turnover in the cells, which causes them to build up

Damn, So Is Animal Protein Terrible For You?

Hopefully this article has scared you about gorging on animal protein.  However, it shouldn’t scare you too much.

In the overall pantheon of nutrition, how important is the difference between animal and vegetable protein?

Not a whole lot.

At least not compared caloric intake, the sodium/potassium ratio, or the excessive levels of saturated fats that come with meat. All proteins are made of amino acids, and your body’s pretty good at getting rid of what it doesn’t need.

From what I can tell, the protein in animals doesn’t have an effect on the following conditions:

• blood pressure
• most forms of cancer
• metabolic disorders

Eating meat might contribute to these disorders….but it ain’t the protein that’s doing it.

Rethinking The Superiority of Animal Protein In the Human Diet

Protein in general is good for you, and that includes animal protein.

However, given the metabolic changes your body undergoes when it continually digests extremely large amounts of animal protein, eating a lot of it relative to vegetable protein is probably not a great idea.

It’s a common chestnut that you need complete proteins (aka animal) to be healthy, but there are a lot of reasons to think this is not the case.

Vegetable proteins from soy and hemp provide all the essential amino acids and don’t create the acidity in your body that animal protein does.

References:

Feskanich, Diane, et. al. “Protein Consumption and Bone Fractures in Women.”

http://aje.oxfordjournals.org/content/143/5/472.full.pdf

Sellmeyer, Deborah, et. al. “A high ratio of dietary animal to vegetable protein increases the rate of bone loss and the risk of fracture in postmenopausal women”

http://www.ajcn.org/content/73/1/118.full

Abelow, Benjamin, et. al. “Cross-Cultural Association Between Dietary Animal Protein and Hip Fracture: A Hypothesis”

http://www.vivalis.si/literatura/4a95.pdf

Munger, Ronald, et. al. “Prospective study of dietary protein intake and risk of hip fracture in postmenopausal women”

http://www.ajcn.org/content/69/1/147.full

Massey, Linda. “Dietary Animal and Plant Protein and Human Bone Health: A Whole Foods Approach”

http://jn.nutrition.org/content/133/3/862S.long

Hiatt, Robert, et. al. “Randomized Controlled Trial of a Low Animal Protein, High Fiber Diet in the Prevention of Recurrent Calcium Oxalate Kidney Stones”

http://aje.oxfordjournals.org/content/144/1/25.full.pdf

Taylor, Eric, et. al. “Dietary Factors and the Risk of Incident Kidney Stones in Men: New Insights after 14 Years of Follow-up”

http://jasn.asnjournals.org/content/15/12/3225.long

Bernstein, Adam, et. al. “Are High-Protein, Vegetable-Based Diets Safe for KidneyFunction? A Review of the Literature”

http://www.sciencedirect.com/science/article/pii/S0002822307000272

Choi, Hyon, et. al. “Purine-Rich Foods, Dairy and Protein Intake, and the Risk of Gout in Men”

http://ernaehrungsdenkwerkstatt.de/fileadmin/user_upload/EDWText/TextElemente/Ernaehrungswissenschaft/Gicht_Gout_Purin_rich_food_NEJM_2004.pdf

Walker, JD, et. al. “RESTRICTION OF DIETARY PROTEIN AND PROGRESSION OFRENAL FAILURE IN DIABETIC NEPHROPATHY”

http://www.sciencedirect.com/science/article/pii/S0140673689920321

Armstrong, B. “A RETROSPECTIVE REFERENCE STUDY OF RENAL CANCER WITH SPECIAL TO COFFEE AND ANIMAL PROTEIN CONSUMPTION”

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2024935/pdf/brjcancer00311-0004.pdf

Kritchevsky, David. “Dietary Protein, Cholesterol and Atherosclerosis: A Review of the Early History”

http://jn.nutrition.org/content/125/3_Suppl/589S.full.pdf

Carroll, K. K., et. al. “Hypocholesterolemic effect of substituting soybean protein for animal protein in the diet of healthy young women13”

http://www.ajcn.org/content/31/8/1312.full.pdf

Fernandez, Maria, et. al. “Hamsters and Guinea Pigs Differ in Their Plasma Lipoprotein Cholesterol Distribution when Fed Diets Varying in Animal Protein, Soluble Fiber, or Cholesterol Content”

http://jn.nutrition.org/content/129/7/1323.full

Forsythe, W.A., et. al. “Effects of Dietary Protein and Fat Sources on Plasma Cholesterol Parameters, LCAT Activity and Amino Acid Levels and on Tissue Lipid Content of Growing Pigs”

http://jn.nutrition.org/content/110/12/2467.full.pdf

Lichtenstein, Alice, et. al. “Lipoprotein Response to Diets High in Soy or Animal Protein With and Without Isoflavones in Moderately Hypercholesterolemic Subjects”

http://atvb.ahajournals.org/content/22/11/1852.full

Robertson, WG. “Diet and Calcium Stones”

http://www.ncbi.nlm.nih.gov/pubmed/3306314

Kramer, Holly J, et. al. “The association between gout and nephrolithiasis in men: The Health Professionals’ Follow-Up Study”

http://www.nature.com/ki/journal/v64/n3/abs/4493979a.html

Fam, Adel G. “Gout: Excess Calories, Purines, and Alcohol Intake and Beyond. Response to a Urate-Lowering Diet”

http://jrheum.com/subscribers/05/05/773.html

Alexander, Dominik, et. al. “Meta-analysis of animal fat or animal protein intake and colorectal cancer”

http://www.ajcn.org/content/89/5/1402.long

Chow, Wong-Ho, et. al. “Protein Intake and Risk of Renal Cell Cancer”

http://jnci.oxfordjournals.org/content/86/15/1131.abstract

Maclure, Malcolm, et. al. “A Case-Control Study of Diet and Risk of Renal Adenocarcinoma”

http://www.jstor.org/discover/10.2307/25759846?uid=3739600&uid=2&uid=4&uid=3739256&sid=47699126438447