Protein After 70: What a New Tracer Study Reveals About How Older Bodies Actually Absorb Amino Acids

The study, conducted by researchers in the Netherlands, used what nutrition scientists consider the gold-standard technique for measuring amino acid digestibility in humans: the dual isotope tracer method. Ten healthy young adults (average age 22) and ten healthy older adults (average age 73) consumed test meals containing 20 grams of protein from one of three sources — milk, sorghum, or black beans — on three separate days. The protein in each meal was labeled with deuterium (²H); a reference mixture of amino acids labeled with carbon-13 (¹³C) was ingested alongside. By comparing the ratio of the two isotopes in blood plasma hours later, the researchers could calculate how much of the dietary protein's amino acids had actually crossed into circulation. The trial focused on two indispensable amino acids in particular: lysine and threonine, both critical for building and maintaining muscle.

The result that will travel furthest is also the most specific. For sorghum, the plasma-to-meal isotope ratio for lysine and threonine was 19 percent lower in older adults than in young adults — a statistically significant gap. For milk and for black beans, however, the difference between the age groups did not reach statistical significance. In other words, the aging gut did not uniformly underperform. It underperformed on one particular plant protein.

To make sense of this, it helps to know what the dual tracer method is really measuring. It is not measuring how full you feel, or how much nitrogen leaves in stool, or how strong your grip becomes after twelve weeks of eating more eggs. It is measuring something narrower and more mechanistic: of the indispensable amino acids you swallowed, how many showed up in your bloodstream over the next several hours? That number is the raw material your body has to work with — for muscle protein synthesis, for enzymes, for immune cells, for repair.

Milk and black beans appeared, in this trial, to deliver their lysine and threonine roughly as efficiently to older adults as to young ones. Sorghum did not. That pattern fits a long-standing hypothesis in protein nutrition: that the matrix of a food — the way its protein is packaged inside cell walls, starch granules, and fiber — can make some plant proteins harder to fully digest, and that age-related changes in the digestive tract may amplify that difficulty. The authors are careful in their conclusion, noting that lysine and threonine digestion "might be lower in older compared with young adults, but the effect differed among protein sources."

That qualifier matters. It is the difference between "older adults absorb protein worse" — a sweeping claim the data do not support — and "older adults may absorb certain plant proteins less efficiently, depending on the food." The second sentence is less satisfying. It is also closer to what was measured.

The strengths of this work are real. The dual tracer method is widely regarded as the most direct way to quantify amino acid absorption in living humans, and a crossover design — in which each participant serves as her own comparison across all three foods — controls for many of the individual differences that muddy nutrition research. The fact that the protein doses, the test conditions, and the blood-sampling windows were standardized across groups gives the comparison real bite.

The limits are equally real, and worth naming. With ten participants per age group, this is a small trial. The older adults were, by design, healthy seventy-somethings, not the broader population of women navigating menopause, polypharmacy, reduced appetite, or gastrointestinal conditions that can further alter digestion. The study measured plasma appearance of two amino acids, not downstream outcomes like muscle protein synthesis, strength, or function. And it tested single foods in isolation — not the mixed meals most of us actually eat, in which a splash of milk in coffee or a handful of seeds on a bean stew can change the amino acid profile considerably.

This is why the editorial evidence rating here is moderate, not strong. The method is rigorous; the sample is small; the implications for daily eating are suggestive rather than prescriptive.

If you are a woman over fifty-five thinking about protein, the practical takeaway is not to fear plants. Black beans, in this trial, held their own. Milk did too. The more useful framing is variety and quality: leaning on a single plant protein — particularly one like sorghum, which is less common in Western diets but a staple elsewhere — may not deliver the same amino acid yield in an older body as it does in a younger one. Mixing protein sources across a day, and pairing plant proteins with complementary foods, is a reasonable response to a finding like this. So is talking with a clinician or registered dietitian if you have concerns about meeting your protein needs, especially around menopause, bone loss, or recovery from illness.

What this study does not justify is panic, overhaul, or the purchase of yet another powder. It is one careful measurement in a long conversation about how aging bodies handle food. The conversation is finally being conducted in numbers rather than assumptions — and that, more than any single result, is the real news.

The protein-after-seventy debate will not be settled by twenty participants and three test meals. But it will be moved forward by them — and by the steady accumulation of trials that bother to measure rather than assume. For now, the most honest answer to how does an older body handle protein? is the one this study quietly offers: it depends on the food.