Three Things You Probably Don’t Know About Protein

Last week in The Protein Pendulum, we saw the crucial macronutrient go from Victorian obsession to wartime ration to low-fat-era afterthought. Now the science has caught up, and unsurprisingly, it turns out the truth is more nuanced than any of those swings suggested.

Three things in particular matter more than most people realize:

• Not all protein is the same,

• Our muscles get more difficult to feed as we age, and

• Scientific debate over protein took a detour through longevity research that nearly led us off another cliff.

Understanding all three will change how you think about what’s on your plate.

You Can’t Just Go By What’s on the Label

Not all protein is created equal. Let’s ignore the tribalism that surrounds this topic and talk solely about the science:

Plant proteins are significantly less bioavailable than animal proteins.

Bioavailability basically means, “How much of this protein can your body actually use?” In short, animal proteins — meat, eggs, dairy, seafood — are about 93% digestible, meaning your body can break them down and use nearly all of the protein in them. Plant proteins in their whole-food form are around 80% digestible, and that’s being generous, because grains and nuts are often closer to 70%.

Why the discrepancy?

There are 20 amino acids your body requires and only 11 that it makes itself. The remaining nine are called essential amino acids (EAAs), and they’re called essential because you must get them through your diet.

Your body needs all nine EAAs in adequate amounts to build muscle. If you’re short on even one, the whole assembly line stalls. Plant proteins often lack or are low in certain EAAs. For instance, grains are typically low in lysine, legumes in methionine and cysteine.

And plants also contain what scientists politely call “antinutritional factors,” compounds like lectins, trypsin inhibitors, and phytates that actively interfere with protein digestion and absorption. These are the plant’s natural defense mechanisms. Animals run from predators; plants deploy biochemistry.

Effective for their survival, less great for you.

Humans have traditionally worked around this through soaking, cooking, and fermenting. Modern protein isolation also helps: this is why soy protein isolate is more bioavailable than whole soybeans, and why pea or rice protein powders can approach the digestibility of animal proteins.

But your handful of trail mix or smear of almond butter? Not even close to what the nutrition label suggests you’re getting.

The Leucine Factor

Here’s where we need to talk about an important subset of EAAs called branched-chain amino acids (BCAAs). Of the nine essential amino acids, three have a branched molecular structure: leucine, isoleucine, and valine. And leucine is the prime ignition switch for muscle protein synthesis.

Leucine activates something called the mTOR (short for “mechanistic target of rapamycin”) pathway — think of it as your body’s muscle-building control switch. When leucine levels hit a certain threshold in your bloodstream, mTOR fires up and signals your muscles to start synthesizing new protein. The simple equation is mTOR on = muscles building.

Without sufficient leucine, it doesn’t matter how much total protein you eat; you’re not effectively triggering the cascade that builds and maintains muscle.

Animal proteins are leucine-rich and protein-dense. A four-ounce serving of chicken delivers about 2.5 grams of leucine and clears the mTOR trigger at around 185 calories.

Plant proteins tell a different story. A cup of lentils, already a substantial serving at 230 calories, gets you to only 1.3 grams of leucine, roughly half the threshold. To actually hit 2.5 grams from lentils alone, you’d need close to two cups at nearly 460 calories. Peanut butter is worse: At 0.5 grams of leucine per two tablespoons, you’d need nearly ten tablespoons — about 950 calories — in one meal.

So Why Not Just Supplement It?

This is why the supplement industry has built a multi-million-dollar empire around BCAA powders. People figured out that leucine matters, so why not just take leucine?

I’ll tell you why.

Because taking BCAAs or leucine by itself is useless for building muscle. You need all nine essential amino acids present together, in the right ratios, for muscle protein synthesis to actually happen. Leucine may be the ignition key, but you still need all the other amino acids to fuel the car.

Taking isolated BCAAs doesn’t do that. In fact, some research suggests it might actually make things worse — triggering muscle protein breakdown to scavenge the other amino acids needed for synthesis.

The Protein Tax

One more thing the label doesn’t tell you: Protein is the most metabolically expensive macronutrient to process. Your body burns 20–30% of protein’s calories just digesting, absorbing, and converting it, compared to 5–10% for carbohydrates and a mere 0–3% for fat. Eat 100 calories of protein, and your body immediately spends 25 of those calories just dealing with it.

This is called the thermic effect of food. Your body resists burning protein for energy because protein is primarily a building material, not fuel. Think enzymes, hormones, antibodies, muscle tissue, collagen. These are jobs only amino acids can do. The metabolic inefficiency is your body’s way of protecting a precious resource.

And, bonus!, it also means higher-protein diets carry a built-in caloric discount — which is one reason they consistently outperform lower-protein diets for body composition, even when total calories are matched.

Why Older Muscles Get “Hard of Hearing”

If you’re over 40, we need to have a conversation about what’s happening behind the scenes with your muscles. It’s called anabolic resistance, and it’s one of the primary reasons we lose muscle mass as we age — a phenomenon so common it has a name: age-related sarcopenia.

Here’s how muscle protein synthesis normally works: You eat protein. Your blood amino acid levels rise. This triggers the mTOR pathway we talked about earlier. Your muscles receive that signal and ramp up protein synthesis for a few hours.

Young, healthy muscles are exquisitely sensitive to this signal. Feed them even modest amounts of protein — say, 20 grams — and they respond robustly. You remember those days, right?

But somewhere around your 40s, this system (like everything else, it seems) starts to break down. Your muscles become resistant to the anabolic (muscle-building) signals from protein intake. It’s not that they can’t respond; it’s that they require a much stronger signal to get the same response.

And just how much might shock you. Research shows that to achieve the same muscle protein synthesis response, older adults need about 68% more protein per meal than younger adults.

Where a 25-year-old might max out their muscle-building response with 20–25 grams of high-quality protein, a 65-year-old needs closer to 40 grams to hit the same peak. And even then, the peak might not be quite as high, and it doesn’t last as long.

What Causes Anabolic Resistance?

Researchers aren’t entirely sure, but several factors seem to play a role:

• Chronic low-grade inflammation, which increases with age (researchers call it “inflammaging”), interferes with the signaling pathways that control protein synthesis. Your muscles are trying to build, but inflammatory signals are jamming the communication.