Hydrolysed formula for babies: what it is and what it's for

Hydrolysed formula is an infant formula in which the cow's milk proteins have been "cut" into very small pieces so that the baby's immune system does not recognise them or can digest them more easily. It is mainly used in babies with cow's milk protein allergy and in some digestive conditions, and it should always be prescribed by the paediatrician. Our midwives explain the types available, what each one is for, its side effects and how to help your baby accept it.

Types of hydrolysed formula for babies

Type	What it is	What it is used for
Extensively hydrolysed (hypoallergenic or semi-elemental)	Cow's milk protein broken down into very small fragments	First choice for cow's milk protein allergy and in some digestive conditions. Always prescribed by the paediatrician
Partially hydrolysed (hypoantigenic)	Protein only slightly broken down, similar to a first infant formula	Some anti-colic or "comfort" milks. It does not treat allergy, and current guidelines do not recommend it to prevent it
Elemental or amino acid-based	No whole protein: free amino acids	Severe allergies or anaphylaxis, babies who do not tolerate extensively hydrolysed formulas and severe digestive conditions
Hydrolysed rice protein	Broken-down rice protein, with no cow's milk	An alternative for cow's milk protein allergy, depending on the paediatrician

If your baby is breastfed and is diagnosed with cow's milk protein allergy, there is usually no need to stop breastfeeding, which is still the best option: normally the mother cuts dairy out of her diet, always on the paediatrician's advice.

Protein hydrolysate infant formula

In these formulas the protein has been "pre-digested" through enzymatic hydrolysis, heat treatment and ultrafiltration. The starting protein can be cow's milk casein, whey protein or both; the two are often mixed so that the amino acid composition resembles that of breast milk. Depending on the degree of hydrolysis, they can be hypoallergenic or semi-elemental formulas (extensively hydrolysed) or hypoantigenic formulas (partially hydrolysed).

Hypoallergenic or semi-elemental hydrolysed formulas

Hypoallergenic infant formula has a high degree of hydrolysis: it is an extensively hydrolysed formula, in which between 85 and 100% of the peptides (short chains of amino acids) have a molecular weight below 5,000 daltons (the dalton is the unit used to measure the mass of molecules), and in some, below 1,500.

This greatly reduces its ability to cause an allergic reaction, but does not eliminate it completely; that is why they are called hypoallergenic and not "allergen-free". Some of these peptides may retain epitopes (the part of a molecule recognised by the immune system) capable of causing a reaction. The vast majority of babies with cow's milk protein allergy tolerate them well, but not all.

The rest of their composition (fats, carbohydrates, vitamins and minerals) is practically the same as that of first infant formulas. In cases of anaphylaxis (a severe allergic reaction) to cow's milk, starting with an elemental amino acid formula is recommended; hydrolysed rice protein is another alternative. Soya formulas are not recommended before 6 months.

Do hypoallergenic infant formulas contain lactose?

Many contain no lactose or very little: the carbohydrates are provided as dextrin-maltose or glucose polymers. The reason is that, in cow's milk protein allergy affecting the gut, the intestinal lining may be damaged and digest lactose less well. Today there are also extensively hydrolysed formulas with lactose, which many babies tolerate well; the paediatrician decides which to use. Some of the fat is provided as medium-chain triglycerides (MCT), in a proportion below 40% of the total, together with an oil rich in essential fatty acids.

Poo in babies fed hypoallergenic formula

Babies who take hypoallergenic formula usually have runnier, greener and stronger-smelling stools. This is normal: these formulas contain less long-chain saturated fat and free amino acids that speed up gut movement, so less water is absorbed and stools come out softer.

When hypoallergenic infant formula is recommended

Hypoallergenic formula is a special type of infant formula that must be prescribed by the paediatrician. It is indicated for babies with cow's milk protein allergy, in digestive conditions with poor fat absorption and in cases of severe intestinal involvement (short bowel syndrome, enteritis due to radiotherapy or chemotherapy, malabsorption syndromes or severe infant diarrhoea).

Hypoantigenic hydrolysed formulas

Hypoantigenic formulas have a low degree of hydrolysis: they are partially hydrolysed formulas. They contain high-molecular-weight peptides, which can still cause allergic reactions, and are otherwise similar to first infant formulas. This is the type of protein found in many anti-colic or "comfort" milks.

When hypoantigenic formulas are recommended

For years they were used to try to prevent cow's milk protein allergy in babies with a family history of allergy. Current guidelines from the European allergy and paediatric gastroenterology societies no longer recommend them for this purpose, because studies have not shown that they achieve it.

Nor do they treat allergy: as they are not free of proteins capable of causing a reaction, they must not be used in babies with cow's milk protein allergy, in whom severe reactions and even anaphylaxis have been reported.

Elemental or monomeric formulas

Elemental or monomeric formulas are synthetic formulas based on free amino acids, dextrin-maltose, MCT (approximately 40% of the fat) and long-chain fatty acids to prevent essential fatty acid deficiency. Some have a low calcium and phosphorus content.

They are the least likely to cause allergy, even less so than protein hydrolysates. In addition, their components are absorbed without needing to be digested, which allows the gut to rest and reduces the number and volume of bowel movements.

When elemental or monomeric infant formula is used

Their indications are very specific, because they have a high osmolality (which can cause osmotic diarrhoea and dehydration) and there are few long-term studies on the absorption of amino acids, vitamins and minerals.

That is why they are reserved for babies with severe allergies or anaphylaxis, babies for whom other formulas have failed and cases of severe malnutrition: malabsorption in pancreatic insufficiency and short bowel syndrome, inflammatory bowel disease, diarrhoea that does not respond to usual treatments, preparation for diagnostic tests of the colon, transition from parenteral to enteral nutrition, and cow's milk protein allergy that does not improve with hydrolysates.

Side effects of hydrolysed formula

Hydrolysed formula is safe and usually well tolerated. What families notice most is:

• Runnier, greener and stronger-smelling stools, as we explained above. This is normal.
• A more bitter taste and smell than a standard formula, because of the protein fragments and free amino acids. Some babies take a few days to accept it.
• A higher price. When the paediatrician prescribes it for an allergy or illness, ask them about any help or funding that may be available in your region.

Talk to your paediatrician if your baby vomits often, refuses it completely, is not gaining weight or you see blood in their stools. If hives, swelling of the lips or eyelids or difficulty breathing appear, go to A&E.

How to help your baby accept hydrolysed formula

• Don't add anything to the milk to mask the taste (sugar, cocoa, juice): it alters the formula and may not be safe.
• Make it up exactly as the packaging says and offer it calmly, at a time when your baby is hungry but not too unsettled.
• Be patient: younger babies, especially before 4–6 months, usually accept it better than older ones.
• If the paediatrician advises it, the switch from their previous milk can be made all at once or gradually: always follow their guidance.

Conclusion: hydrolysed formula for babies

If you are reading this article, your baby is probably already taking hydrolysed formula, or you have noticed that they don't tolerate milk well. Some babies like it from the very first day, while others take a little while because of its taste. Breastfeeding is the best option for feeding your baby; if they need a special formula, your paediatrician will tell you which one and how to give it. Use a bottle your baby is comfortable with, be patient and remember that it is a phase that will pass. And if you have any questions, we are here to help. If you'd like to know more about formulas in general, read what is the best infant formula.

Sources

• Vandenplas Y et al. Management of Cow's Milk Allergy in Infants: ESPGHAN position paper. Journal of Pediatric Gastroenterology and Nutrition, 2023.
• Halken S et al. EAACI guideline: Preventing the development of food allergy in infants and young children (2020 update). Pediatric Allergy and Immunology, 2021.
• Asociación Española de Pediatría (AEP, Spanish Paediatric Association). Protocolos diagnósticos y terapéuticos: alergia a proteínas de leche de vaca.