THE SHORT VERSION

  • Casein fragments can bind calcium in lab studies. In a 2006 trial of 15 adults, adding these fragments to a calcium drink did not increase the share of calcium absorbed.
  • Lactoferrin is an intact milk protein that carries iron. In 20 healthy young women, absorption of iron from a lab-made form did not differ significantly from absorption of iron from ferrous sulfate, a common supplement form.
  • The takeaway: binding a mineral in the gut and helping a person absorb more of it are different findings. Better absorption has to be demonstrated in people.
  • The question to ask: which preparation was tested, at what mineral dose, and in whom?

Turn a carton around and the nutrition panel gives you a calcium number. That number says how much calcium the serving contains. It does not say how much of it your body will take in.

Before a mineral can be used, it has to cross the lining of the gut. Some milk proteins, and the fragments cut from them, can bind minerals along the way. Researchers have therefore asked whether these interactions affect absorption.1,2

Two examples show how different the chemistry can be. One involves casein fragments that interact with calcium. The other is lactoferrin, an iron-binding protein in the whey part of milk.1,3

The calcium story begins with a fragment

Digestive enzymes cut casein into smaller pieces. Some keep phosphate groups attached to the amino acid serine. These fragments are called caseinophosphopeptides, or CPPs. The phosphate groups give them sites that can bind calcium.1,2

Calcium can form compounds that barely dissolve in the gut, and dissolved calcium is easier to take up. CPPs can help keep calcium in solution, offering a possible way to keep it available for absorption.1,2

Cell and animal experiments have tested this idea and reported greater calcium uptake or absorption under particular conditions.2

Keeping calcium available in the gut is an earlier step than absorbing it across the lining. The peptide can interact with calcium within the gut contents, without entering the bloodstream itself.1,4

Whether that interaction helps a person absorb more calcium is a separate question. It needs to be tested in people.4

When the mechanism was tested in people

In a 2006 randomized crossover trial, 15 adults tried three calcium drinks on separate occasions. One supplied 400 milligrams of calcium as calcium lactate. The other two contained the same calcium lactate plus different CPP preparations.4

The researchers measured absorption using calcium isotope tracers: labeled forms of calcium that can be followed through the body.4

The CPP drinks delivered slightly more absorbed calcium in total: about 117 and 121 milligrams, compared with 103 milligrams from the control. However, they also contained about 100 milligrams of extra calcium from the peptide preparations.4

The share of calcium absorbed did not improve. It was roughly 23–24% with the CPP drinks, compared with 26% for calcium lactate alone. The researchers judged the differences unlikely to be biologically significant.4

The comparison matters. Absorbing more calcium from a drink that contains more calcium does not show that the peptides made absorption more efficient. The laboratory mechanism and its nutritional value remain separate questions.4

Iron introduces a different kind of protein

Lactoferrin is an intact protein that holds iron. While the calcium example follows fragments cut from casein, lactoferrin can already carry a mineral in its original form. Its presence in milk has prompted research into iron handling and other biological functions.3,5

Holding iron tightly raises a fair question: can the body still use it? The human gut has receptors for lactoferrin, which is one reason researchers have studied it. A receptor alone, though, cannot tell us how a particular food will perform.5

A 2006 study tested this in 20 healthy young women. They ate standardized meals containing labeled iron, either bound to purified recombinant human lactoferrin—a lab-made form of the human protein—or supplied as ferrous sulfate. The order was randomized.5

Iron absorption did not differ significantly between the two sources: about 20% from lactoferrin and 19% from ferrous sulfate.5

The finding shows that the body could use iron from that lactoferrin preparation under the study conditions. It did not demonstrate better absorption or establish an effect from an ordinary whey drink.5

The preparation matters

In a stable-isotope study of infants in Kenya, iron absorption was greater when apo-lactoferrin was given with ferrous sulfate than when ferrous sulfate was given alone. Apo-lactoferrin is the form with little or no iron bound to it.6

Holo-lactoferrin, the iron-loaded form, showed no significant difference in absorption compared with ferrous sulfate.6

These studies highlight why the preparation, the people studied, and the question being asked all matter. Neither tested an ordinary whey drink.5,6

Each step needs its own evidence. Binding in the lab does not establish better absorption in people, and absorption at a meal does not establish long-term effects. Sources: mineral binding and availability,1,2,3 calcium absorption,4 and iron absorption in women.5

Each step needs its own evidence. Binding in the lab does not establish better absorption in people, and absorption at a meal does not establish long-term effects. Sources: mineral binding and availability,1,2,3 calcium absorption,4 and iron absorption in women.5

What the protein grams leave unanswered

The two stories show different roles for milk proteins. A fragment released during digestion can change the chemistry around calcium. An intact protein can carry iron. Each proposed function belongs to a specific molecule.1,3

A serving’s total protein grams do not tell you how much lactoferrin it contains or which CPP sequences will remain during digestion. Those answers depend on the ingredient and how it was prepared.1,3

A protein quality score rates amino acid nutrition. It cannot answer those questions either.3

For a mineral absorption claim, look for research on the relevant preparation, the mineral dose, and the people who consumed it. An absorption study also answers a different question from a study of long-term iron status or bone health.

Related reading

References

Educational content. The Fountain is written to explain, not to make health claims about any product. This article summarizes published peer-reviewed research. Nothing here is medical advice.

  1. Tenenbaum M, Deracinois B, Dugardin C, et al. Identification, production and bioactivity of casein phosphopeptides: a review. Food Research International. 2022;157:111360. https://doi.org/10.1016/j.foodres.2022.111360
  2. Sun S, Liu F, Liu G, et al. Effects of casein phosphopeptides on calcium absorption and metabolism bioactivity in vitro and in vivo. Food & Function. 2018;9(10):5220–5229. https://doi.org/10.1039/C8FO00401C
  3. Auestad N, Layman DK. Dairy bioactive proteins and peptides: a narrative review. Nutrition Reviews. 2021;79(Suppl 2):36–47. https://doi.org/10.1093/nutrit/nuab097
  4. Teucher B, Majsak-Newman G, Dainty JR, McDonagh D, FitzGerald RJ, Fairweather-Tait SJ. Calcium absorption is not increased by caseinophosphopeptides. American Journal of Clinical Nutrition. 2006;84(1):162–166. https://doi.org/10.1093/ajcn/84.1.162
  5. Lönnerdal B, Bryant A. Absorption of iron from recombinant human lactoferrin in young US women. American Journal of Clinical Nutrition. 2006;83(2):305–309. https://doi.org/10.1093/ajcn/83.2.305
  6. Mikulic N, Uyoga MA, Mwasi E, et al. Iron absorption is greater from apo-lactoferrin and is similar between holo-lactoferrin and ferrous sulfate: stable iron isotope studies in Kenyan infants. Journal of Nutrition. 2020;150(12):3200–3207. https://doi.org/10.1093/jn/nxaa226