Bovine colostrum: what immunoglobulin G is
and how much of it is still intact at the end of the gut

First, what colostrum is
and why it is milk, not something else

Colostrum is the secretion the mammary gland produces in the first hours after birth, before it switches to ordinary milk. In cows, the transition is fast: the concentrations that make colostrum special drop visibly over the first three days of lactation.

That means it is milk. With everything that comes with it: milk proteins and lactose. If you are allergic to cow's milk proteins, colostrum is exactly the thing to avoid, however small the amount in a capsule. It is the first practical piece of information in this whole article, and almost nobody puts it first.

The difference in composition compared with mature milk, however, is large. In a review published in Frontiers in Nutrition in 2021, the averages look roughly like this: 140 mg of protein per millilitre in colostrum compared with 36 in mature milk, 64 mg of fat compared with 39, and, interestingly, less lactose, 27 mg compared with 49. Colostrum is fattier and much richer in protein than milk, but less sweet.

And with immunoglobulins, the difference is no longer one of proportion, it is one of order of magnitude.

Immunoglobulin G
a Y-shaped molecule with two different ends

An antibody is a protein, not an active substance in the usual sense of the word. Immunoglobulin G, IgG for short, weighs about 160 kilodaltons, which makes it enormous on the scale of molecules found in a supplement. For comparison, vitamin C weighs 176 daltons. A thousand times less.

Its shape is a Y. The two upper arms are called Fab fragments and each ends in a variable region, built to fit a single target, like a lock to a key. The stem of the Y, the Fc fragment, is the part that immune cells and receptors in the body recognise. The region connecting the arms to the stem is called the hinge, and it will matter two chapters from now.

In cow's colostrum, IgG is the dominant class: between 85 and 90 per cent of all immunoglobulins. And within it, the IgG1 subclass accounts for 80 to 90 per cent. The other two classes are far behind, with about 4.3 mg per millilitre of IgM and 1.66 of IgA. Alongside them comes lactoferrin, an iron-binding protein, at about 0.80 mg per millilitre, compared with 0.10 to 0.30 in mature milk.

So the figure on the label, 800 mg of colostrum, does not mean 800 mg of antibodies. It means a milk powder whose protein fraction is dominated by IgG, in the proportion set by the raw material and the drying process.

The 24-hour window
that a calf has and you do not

This is the part worth correcting if you have read otherwise until now. Colostrum does not put antibodies into your blood.

In a calf it works, and it works spectacularly, but within a very short window. A calf is born with practically no antibodies of its own in circulation, because the cow's placenta does not let them through before birth. The shortfall is made up by mouth. In the first hours of life, the cells lining the small intestine take up whole proteins through a non-specific mechanism called pinocytosis, meaning they absorb them as they are, without selecting them, and pass them into the circulation.

An invited review published in the Journal of Dairy Science in 2022 puts numbers on the closing of this gate: intestinal permeability to whole proteins starts to fall rapidly 12 hours after birth and stops completely at 24 hours. The apparent efficiency of absorption, that is, the percentage of IgG drunk that actually reaches the blood, is usually between 20 and 35 per cent, with wide variation between studies. And, to keep the proportions in mind: the recommendation for a calf is to receive 150 to 200 grams of IgG at its first feed.

One hundred and fifty grams. At the first feed. That is the dose that makes the window worthwhile.

In humans, after infancy, that gate has long been closed. What you drink does not reach the blood intact. So the right question is not how much is absorbed, but what happens to these molecules while they pass through the digestive tract, which is where they could actually do their work, locally.

What happens during digestion
and how much stays intact to the end

A swallowed antibody enters the stomach, an acidic environment full of pepsin, then the small intestine, where trypsin and the rest of the pancreatic enzymes are waiting for it. On paper, it should be cut into pieces like any other protein in food.

In practice, it is only partly cut, and where it is cut matters. Pepsin preferentially attacks the hinge region, exactly the point connecting the arms to the stem. The result is not a soup of amino acids, but Fab fragments, those variable ends that recognise the target. A review published in Nutrition Journal in 2015, which brings together the studies in humans, notes that bovine Fab fragments that were still immunologically active were recovered from intestinal aspirate, of about 100 kilodaltons for pairs and 50 for single units, starting from a whole molecule of around 160.

The most cited figures come from a study published in The Journal of Nutrition in 1995, which used immunoglobulins labelled with nitrogen 15 in seven volunteers fitted with an intestinal tube. In the middle of the small intestine, in the jejunum, about 59 per cent of the immunoglobulins given were still immunologically active. At the end of the small intestine, in the ileum, about 19 per cent remained.

The other studies in humans give a wide range, and that is just as important to know: recovery from stool varies from under one per cent to over 30, depending on the formulation and on whether it was taken on an empty stomach or with food. One study in six adults reported between 0.6 and 32.7 per cent, and another, in six people, between 4 and 50, with the lowest value precisely for capsules with an acid-resistant coating.

Only one conclusion can be drawn from these figures, and it is one about mechanism: some of the swallowed molecules reach the gut whole or half whole, where they can bind what they were built to bind. What happens after that is another matter, and there the data is much thinner than articles listing benefits suggest.

What was measured in humans
and where nothing moved

The cleanest result comes from a study published in 2011 in the American Journal of Physiology: Gastrointestinal and Liver Physiology. Twelve volunteers, a double-blind crossover design, 14 days of colostrum or placebo before a standardised bout of exercise, followed by measurement of intestinal permeability using the lactulose to rhamnose ratio in urine.

In both arms, the exercise raised lactate, heart rate and core temperature, with an average rise of 1.4 degrees. The difference appeared in permeability. In the placebo arm, the ratio rose from 0.38 to 0.92, almost two and a half times higher. In the colostrum arm, it rose from the same starting value, 0.38, to 0.49. The increase was reduced by about 80 per cent.

It is a placebo-controlled result, on an objective marker, and that puts it above almost everything else circulating on the subject. But it is twelve people and a single bout of exercise.

At the other end is a meta-analysis published in Nutrients in 2020, which brought together 10 randomised studies with 239 participants, at doses between 10 and 25 grams a day and for durations from 8 days to 12 weeks. It looked for an effect on immune markers and did not find one. Lymphocytes, neutrophils, serum immunoglobulins A and G, secretory immunoglobulin A in saliva: none of the differences compared with placebo reached the threshold of statistical significance. For secretory IgA in saliva, for example, the effect size came out at 0.15, with a confidence interval that crosses zero.

Ten to 25 grams a day, for months on end, and the markers did not budge. That, for now, is what is known. It is not a disastrous picture, it is an incomplete one: intestinal permeability moved, the blood markers did not, and between the two there is a research gap that nobody has filled yet.

Where colostrum sits
in our capsules

At ROOTFUL, bovine colostrum is in Full-immunity, the morning capsule. The serving is two capsules, taken with a meal, and each serving contains 800 mg of bovine colostrum, the largest amount in the formula: the first secretion of the mammary gland, spray dried at low temperature precisely so that the protein fraction this whole article is about is preserved. Alongside it: 300 mg of vitamin C as calcium ascorbate, 100 µg of vitamin D3 from lanolin, which is 4,000 IU, 50 mg of zinc sulphate, 60 mg of selenium yeast, 100 mg of quercetin from Sophora japonica buds and 100 mg of Lion's Mane fruiting body extract.

What can be said clearly about what the formula does comes from the nutrients with authorised claims: vitamin C, zinc, selenium and vitamin D contribute to the normal function of the immune system. Colostrum is there for what you have read above, described as a source, a dose and a form, not as a promise.

Because it is derived from milk, the label declares it as such. If you are allergic to cow's milk proteins, this product is not for you, and that remains true however small the amount may seem.

And if the question behind your search is really what can be done in practical terms during a cold season, the subject is broader than a single ingredient. We have written about it separately, in how to boost my immunity fast.

Frequently asked questions
about bovine colostrum