Protein matters. Leucine matters more.
By Dr. Jerry Yoo, DPT, CSCS | NL Physio
Most active adults over 40 who feel stuck aren’t training wrong. They’re just not eating for the demands they place on their bodies.
I see it every week. Clients “volume-maxxing,” more classes, more miles, heavier weight on the bar, and ending up with the same body AND constant injuries. How frustrating. HYROX prep, strength days, Orange Theory six days a week, a long run on Sunday. Hats off to them for the work ethic.
(side bar- you know how I feel about rest days, especially once you’re over 40!)
BUT here’s the part almost everyone skips.
The training alone doesn’t build muscle, aid recovery, improve performance, or protect your body. Eating enough protein > carbs > fats to let the training, and the recovery, actually do its job is a must.
This doesn’t feel like a nutrition problem. Rather, it feels like age catching up with you, or bad luck.
You’re showing up, you’re doing the work, so when strength stalls or muscle seems to disappear despite the effort, most people assume it’s something they can’t control.
Almost nobody thinks to blame their fork.
Which means…you can add another class, another long run, another hard week, but if you’re still not eating enough of the fuel you need, you’re just going to end up more tired and more often injured.
Most people try to fix this the way they fix everything else. They Google it, download a macro app, and get a number that feels impossible to hit. Or they eat “healthy” and assume that covers it, salads, grilled chicken here and there, a shake after leg day, and figure that’s enough.
It usually isn’t. Eating clean and eating enough protein are not the same thing.
Remember, training doesn’t fix a feeding problem, whether you’re trying to lose fat or gain muscle.
Then there’s the more dialed-in crowd, the ones who’ve read that you need 25 to 30 grams of protein per meal to build muscle, so they obsess over hitting that number three or four times a day. Weighing chicken breasts. Timing shakes around workouts. Stressing about breakfast.
Here’s the part that surprised even me when I dug into it. That’s not actually where the science points anymore, and even where it once did, gram-counting per meal was aiming at the wrong target the whole time.
Long story short, a frequently cited 2009 paper is where the “25 to 30 grams per meal” rule came from.
It found that amount maximally stimulates muscle protein synthesis in a lab setting, and claimed the response gets blunted below about 20 grams per meal, or when protein is eaten alongside a lot of carbohydrate. BUT it was a short acute study, measuring a few hours of response after a single meal, not what actually happens to your muscle over a real day of eating.
A more telling test came later. Researchers put healthy older adults, 52 to 75 years old, through a controlled trial using amino acid tracers, comparing the RDA amount of protein against roughly double the RDA, at either an even spread across three meals or a heavily uneven one.
The result:
total daily protein amount drove muscle protein synthesis (MPS) and net protein balance. Meal-to-meal distribution pattern showed no measurable effect at all.
Which means...stop stressing over hitting a precise gram count at breakfast, lunch, and dinner. Focus on hitting your total daily number.
Here’s the piece almost everyone skips entirely. Not all protein grams are equal, because not all protein has the same amount of leucine, and leucine is the specific amino acid that flips the switch on muscle protein synthesis (MPS).
Think of total protein as the raw materials and leucine as the signal that tells your body to actually start building.
This matters more as you age. A controlled trial comparing young and older adults gave both groups the same essential amino acid mixture, once at a standard leucine ratio and once enriched with extra leucine.
In the young group, muscle protein synthesis increased either way.
In the older group, the standard ratio produced no response at all. Only the leucine-enriched version triggered MPS.
You see, older muscle needs a bigger leucine signal to trigger the MPS response. That could be one big reason training harder isn’t translating to muscle for a lot of my 40+ clients.
Separately, a randomized trial that directly measured leucine requirements in adults over 60 using tracer methodology found their actual leucine requirement was roughly double the current recommended intake built for younger adults. If you’re only eating to the old number, you’re very likely under-shooting, even if your total protein looks fine on paper.
For dosing, the commonly cited trigger threshold for a meal to maximally stimulate muscle protein synthesis is around 2.5 grams of leucine. Given that older muscle needs a stronger signal to respond, getting closer to 3 grams of leucine per meal may be the better number.
This is where it gets interesting, and explains a debate you’ve probably seen play out in the research: does whey beat soy, does animal protein beat plant protein, and why do different studies seem to disagree?
The honest answer is leucine density. Whey protein is unusually concentrated in leucine, roughly 10 to 11 percent of it by weight, which is part of why it shows up so often in muscle-building research.
A meta-analysis in the British Journal of Nutrition, pooling ten studies and 596 participants, found whey protein produced a significant increase in lean body mass while soy supplementation alone produced no measurable change in body composition at all.
BUT a separate meta-analysis in the International Journal of Sport Nutrition and Exercise Metabolism, pooling nine studies and 266 participants, found no significant difference between soy and whey or other animal proteins for strength or lean body mass.
My read on why these disagree: when total protein and leucine content are actually matched between groups, the source matters much less. When they’re not matched, whey’s natural leucine density gives it an edge.
A large 2025 meta-analysis in Nutrition Reviews, pooling 43 randomized trials, backs this up.
Animal protein produced modestly greater muscle mass gains than plant protein overall.
BUT soy protein specifically performed just as well as milk protein, with no meaningful difference between them.
The gap only showed up against non-soy plant proteins like rice, oat, and potato protein, the ones that are naturally lower in leucine per gram.
And interestingly, that same review found the animal-protein advantage was strongest in younger adults, not older ones, where the difference wasn’t statistically significant at all.
So it’s not animal versus plant as some kind of moral hierarchy. It’s likely leucine density.
Best sources if you’re trying to hit your leucine target (these are typical values, not exact, so treat them as a general guide rather than a precise label):
- ✔ Whey protein isolate, the most leucine-dense option available, roughly 2.5 to 3 g of leucine per 25–30 g scoop
- ✔ Chicken breast or thigh, eggs, and lean beef or ground meat, roughly 2 to 2.5 g of leucine per 3 to 4 oz serving
- ✔ Fish like tuna or salmon, similar range to poultry and beef
- ✔ Cottage cheese and Greek yogurt, solid options, slightly lower leucine density than meat or whey but still meaningful
- ✔ Soy foods (tofu, tempeh, edamame), the best plant option, close enough to matching milk protein that it shouldn’t be dismissed as “just decent for a plant option”
- ✔ Lentils, chickpeas, and seitan, useful but lower in leucine density, so you typically need a larger portion to match an equivalent serving of meat, whey, or soy
Not sure what’s working against your training?
Book your Total Body Assessment now at nlphysio.com:
On any given day, I eat 1 to 2 portions of chicken thighs, fish, eggs, or ground meat at nearly every meal, with rice and veggies, and I don’t measure it out or think twice about it anymore.
I drink the rest of my protein.
Simple always wins when we’re talking about consistency.
I mix 3 scoops of whey protein isolate powder with water in a large water bottle and sip it all day long. That alone is 60 grams of protein and super low on calories, about 100 calories a scoop, and whey happens to be the most leucine-dense option there is.
My current favorite: Ascent Clear Whey Isolate. The pineapple/coconut version tastes like a piña colada. Deadly…
I often get asked whether you can hit your leucine target without meat or dairy. You can. It just takes more intention, and usually bigger portions.
A randomized trial in The Journal of Nutrition put the plant-based question to a real test. Young adults followed either an omnivorous or a vegan high-protein diet, around 1 gram per pound of body weight per day, through 10 weeks of progressive resistance training.
Muscle growth in the thigh came out identical between groups, 8.3 percent in both, along with comparable strength and muscle fiber gains. When total protein is high enough and includes leucine-dense plant sources like soy, the source stops being the limiting factor.
Here’s what worries me more than any single missed protein target. The active adults who ignore this for years are the ones who eventually show up in my clinic in their 50s and 60s with falls, fractures, and a level of frailty that didn’t have to happen.
Yes, more muscle does look good in the gym mirror. More importantly, it’s about staying mobile and independent.
And, I don't know about you, but I don’t intend on being the old guy who can’t run, golf, lift, or play on the floor with my future grandkids because his body wasted away…
I see this constantly with our OrangeTheory and HYROX clients across our North Jersey and North Carolina locations. A woman training hard four days a week at OTF who can’t figure out why her squat numbers have stalled for a year, and who eventually starts favoring one hip during sled work because the muscle around it never recovered strong enough to keep up. A guy deep into HYROX prep who keeps tweaking his hamstring during sled pushes, because his posterior chain doesn’t have the raw material to rebuild between sessions, and that tightness starts creeping into his low back on heavy weeks.
Both of them assumed it was a training problem. Neither of them was training their nutrition to support what they were demanding from their bodies.
PS-> If you’re training hard and still feel like you’re losing ground,
book your Total Body Assessment at nlphysio.com and let’s find the missing piece.
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Medical disclaimer: This content is for educational purposes only and is not medical advice. Consult a qualified healthcare professional before beginning any new exercise program, especially if you have a history
of heart disease, back pain, or other medical conditions.
About the Author
Dr. Jerry Yoo is the Founder of Next Level Physio.
He has worked with runners and lifelong athletes for over 25 years, and is a clinical running research partner with Rutgers University. Dr. Jerry is an expert at helping athletic men and women over 30 who just don't want to stop.
He can be reached directly at Jerry@NLPhysio.com