Another common piece of advice is "eat protein." And yeah, that's true. We need protein to survive. It's probably the most essential nutrient in existence because we can't make it ourselves. But sometimes digging a little deeper pays off.
Not all protein is created equally. Protein is composed of up to 20 different amino acids. Every protein source contains some or all of those amino acids in different proportions, so each source of protein really is different. When we digest protein, what our body actually absorbs and utilizes are those amino acids. Each one plays a different role in the body, from building and repairing various tissues, performing vital metabolic processes, acting as progenitor for essential compounds, and even regulating gene expression. We need amino acids to live.
We need some amino acids more than others. We can synthesize or convert some of the amino acids we need, but there are 9 amino acids that we cannot make or convert. These are the essential amino acids, and we must consume foods that contain them.
Another category is the conditionally essential amino acids. These are the amino acids that we can synthesize or convert, but certain conditions and contexts increase our requirement for them. In many cases, people don't eat enough of these conditionally essential AAs. They are therefore essential for most people.
For the most part, animal-based protein contains adequate concentrations of all the essential amino acids. Furthermore, animal muscle meat is roughly identical in amino acid composition. Whether you eat chicken thighs, lamb chops, pork loin, salmon filets, or ribeye, you'll be getting the same basic pattern of amino acids in your diet—including all the essentials. The same thing goes for almost all animal-derived foods, like eggs and dairy. They're all complete proteins—they provide the essential amino acids.
Plant proteins are incomplete - they're usually missing one or more of the essential amino acids. That's why cultures that rely heavily on plant protein end up with staple food combos carefully curated to provide all the essential amino acids, like beans with rice or beans with corn.
Eating a variety of protein sources ensures you're getting all the amino acids you need to perform basic physiological processes. So here are a couple reasons why balancing your protein intake from different sources is important.
Methionine/Glycine
I've written about this before. The crux of the matter is this:
Most animal proteins are high in methionine, an amino acid critical for growth and development and overall robustness but also implicated in unchecked growth of cancer cells. The calorie restriction with optimal nutrition (CRON) crowd tends to avoid methionine like the plague, pointing to animal studies in which animals on high-methionine diets die earlier and get more cancer and other degenerative diseases than animals that restrict methionine.
What I and people like Denise Minger have suggested is that glycine—an amino acid found abundantly in connective tissues but not in muscle meat—can counter the anti-longevity effects of methionine. A study (the abstract of which is sadly no longer free to view; wonder why) from 2011 found that giving glycine to rodents on a high-methionine diet extends lifespan and emulates the effect of methionine restriction.
If that's true in humans, then expanding our protein intake to include both muscle meats (methionine) and connective tissue (glycine) will make us healthier.
And although scientists haven't looked at the topic very closely yet, we have inklings that glycine is important for humans. In one recent study, the relationship between red meat and diabetes was abolished after controlling for low-glycine status. People with low glycine levels and high meat intakes were more likely to have diabetes; people with higher glycine levels could have higher meat intakes without any issues. In another study, low circulating levels of glycine also predicted diabetes risk.
We do know that glycine is a conditionally essential amino acid. We can make it from proline, but evidence shows that we can't make enough to cover all the tasks glycine performs. So if we want to sleep better at night, maintain the structure and integrity of our ligaments, tendons, and cartilage, keep our skin taut and firm, and balance out our methionine intake, we'd better start eating skin, tendon, bone broth, and other gristly bits.
Increasing protein variety to include collagenous materials will balance out our meat intake and make us healthier.
Comment: For more information on the health benefits of Glycine:
Plant Protein
If we're gonna have to eat animal protein or assemble complex combinations of plant proteins that provide all the requisite amino acids, why eat plant protein at all? Why not just eat a few ounces of steak instead of the perfect proportion of rice and beans?
Variety can be good for its own sake. Some people get bored eating the same thing every day. Opening up an entirely different genre of protein—plants—will only increase variety.
And as I laid out a couple weeks ago, legumes—the most popular and dense source of plant protein—offer other advantages: prebiotic fiber, minerals like magnesium, copper, and manganese, and vitamins like folate and B1.
Comment: Legumes (beans) should be avoided due to lectins and other plant anti-nutrients:
Plus, plant protein is usually cheaper than animal protein. Obtaining a portion of your protein from plants offsets the cost and allows you to focus on quality protein from grass-fed and pasture-raised animals. A pound of steak doesn't offer any distinct advantages over 3/4 pound of steak with a half cup of black beans. If anything, the latter offers a bit more nutrient variety.
Increasing protein variety to include plant sources allows more freedom when planning meals, offers fiber, minerals, and vitamins we can't easily get from animals, and makes it easier to afford high-quality animal protein.
Nutrient Co-riders
Another reason to vary your protein intake is that different sources of protein are accompanied by different nutrients. A mussel might give you similar amino acids as a chicken thigh, or a cup of yogurt, but the similarities end there. The mussel provides manganese, selenium, a ton of B12, and some folate. The chicken thigh provides less B12, some niacin, a little more magnesium. The yogurt offers probiotics and calcium. You're better off eating some of all three rather than an equal amount of one.
Eating a variety of protein sources grants access to different co-riding nutrients.
Like for Like
Some traditional medicine systems have a concept called "like for like." If you want to improve your masculine vigor, you eat tiger penis. If you want to promote kidney health, you eat stir fried pork kidneys. That sorta thing.
Is there anything to it, or is that superstitious mumbo jumbo?
While I can't speak to the libidinous merits of consuming tiger penis, I can speak to the benefit of some other examples.
Livers are extremely high in folate and choline, two important nutrients for liver function.
A lamb brain is full of omega-3 fats. A 3 ounce portion of cow's brain has a full gram of omega-3s. Since we need omega-3s for optimal brain development and function, eating an animal's brain can help our brains.
Animal skin is made up of collagen, the densest source of glycine. Our bodies use glycine to build and repair collagenous tissues, including skin, cartilage, tendons, and other connective bits. Eating skin can improve the health and appearance of your skin.
A can of bone-in sardines contains easily-digested bone. Compare that to another bone friendly food, dairy. You're getting all of the bone, not just the calcium. If there are any nutrient co-factors that help bone mineral density, they're probably contained in the bone itself—the bone that you're eating. I haven't found any studies examining the effect of eating bone-in sardines on bone mineral density or osteoporosis, but I bet it helps.
If you're just eating the same cut of steak every day, you'll miss out on the "like for like" mechanism.
To sum up, protein variety is important for many reasons:
It helps you obtain all essential and conditionally essential amino acids.
It helps you balance out methionine intake with glycine.
It increases the range of co-riding nutrients you obtain.
It makes it easier to afford higher-quality, pasture-raised, and grass-fed animal products.
It increases food variety and makes your diet more enjoyable and sustainable.
It allows you to follow the "eat like for like" rule when applicable.
What are some other good reasons to vary your protein intake?
Thanks for reading, everyone. Take care!
"..eating an animal's brain can help our brains." Umm... like mad cow disease?