Lactose Intolerance in Cats. When Do They Outgrow Milk?


Lactose intolerance is common in cats and refers to the inability to properly digest lactose, a sugar found in milk and dairy products. Lactose requires an enzyme called lactase to be broken down and absorbed by the body. Kittens are able to produce lactase to digest their mother’s milk, but lactase production declines after weaning, leading to lactose intolerance in adulthood.

Understanding when cats become lactose intolerant is important because ingesting dairy can cause digestive upset and other problems. While giving milk to kittens is fine, cat owners should avoid feeding milk and other dairy products to adult cats who likely lack the lactase enzyme. Luckily, there are lactose-free milk substitutes made especially for cats.

What is Lactose?

Lactose is a type of sugar found in milk and dairy products. It is made up of two simple sugars – glucose and galactose. Lactose requires an enzyme called lactase to be broken down and absorbed during digestion. Kittens are born with the ability to produce large amounts of lactase in order to digest their mother’s milk which contains high levels of lactose. As kittens grow older, they begin to produce less lactase naturally after weaning, typically at around 6-8 weeks of age. This results in a decreased ability to digest lactose as adult cats. The lack of lactase leads to indigestion and gastrointestinal upset when cats consume dairy products, indicating they have developed a lactose intolerance. [1]

When Kittens Can Digest Milk

Kittens are born with the ability to digest lactose in milk because they have high levels of the enzyme lactase. Lactase breaks down lactose, the main carbohydrate found in milk, into simple sugars that kittens can then absorb and use for energy and growth. Kittens need to consume milk for proper nutrition and development in their early weeks of life. As this pet forum discussion notes, kittens initially get all of their nutrition from nursing milk from their mother. Mother’s milk provides kittens with a rich source of protein, fat, and calcium in addition to lactose. Kittens have especially active lactase enzymes so they can properly digest the lactose-rich milk.

Lactase Production Declines After Weaning

Kittens are born able to digest the lactose in their mother’s milk because they produce the enzyme lactase. Lactase breaks down lactose into simple sugars that can be absorbed and used by the body. Kittens need to be able to digest lactose in order to get nutrition from nursing milk.

Around 4-6 weeks of age, kittens begin the weaning process as they transition from milk to solid food. During this time, their small intestines gradually produce less lactase because milk is no longer the sole source of nutrition. By about 8 weeks old when kittens are fully weaned, lactase production has declined significantly. Mother cats’ milk production also begins to naturally dry up around this time

“The milk should be completely dried up after a couple of weeks but this is a gradual process. The teats will still be swollen initially and the kittens will try to suckle but little or no milk will be produced.”

The reduction of available lactase coupled with less milk for nursing signals the kitten’s digestive system to stop prioritizing lactose digestion. This metabolic shift leads to progressive lactose intolerance in early kittenhood.

By 6-8 Months Most Cats are Lactose Intolerant

Most kittens start to lose the ability to digest lactose around 6-8 months of age. As kittens are weaned and stop nursing from their mother, usually between 4-6 months old, their bodies begin producing less of the enzyme lactase that is needed to properly break down lactose. By around 6-8 months old, lactase production has declined to the point where most kittens are no longer able to effectively digest the lactose found in dairy products.

According to veterinarians, once cats reach 6-12 months of age, over 90% of them will be lactose intolerant to some degree 1. This means that after ingesting milk, cheese, yogurt or other dairy products, much of the lactose passes through their digestive system undigested. The undigested lactose in their intestines leads to uncomfortable digestive upset and symptoms of lactose intolerance.

Why Lactose Causes Problems

Lactose is a disaccharide sugar found in milk that requires the enzyme lactase to digest it. Kittens are born with high levels of lactase which allows them to properly digest their mother’s milk. However, as kittens are weaned off milk, their lactase levels drop significantly (1). Without sufficient lactase, cats cannot properly break down lactose in milk.

When undigested lactose reaches the large intestine, it causes an osmotic effect, drawing water into the intestinal lumen (2). This leads to diarrhea, gas, bloating, and other gastrointestinal upset as the undigested lactose essentially overwhelms the digestive system. The undigested lactose also serves as food for intestinal bacteria which ferment it and produce gas, contributing to flatulence and abdominal discomfort (3).

So in summary, an adult cat’s relative lactase deficiency prevents proper digestion of lactose, resulting in gastrointestinal distress. Providing lactose-containing milk to lactose intolerant cats overburdens their digestive system.




Signs of Lactose Intolerance

Cats who are lactose intolerant can exhibit several symptoms after consuming milk or dairy products. Common signs of lactose intolerance in cats include:

  • Diarrhea – Loose, watery stools are a classic symptom of lactose intolerance in cats. The undigested lactose draws water into the intestines, leading to diarrhea that may contain mucus or blood. Diarrhea from lactose intolerance can occur 8-12 hours after ingesting milk products according to Vets Now (
  • Vomiting – Some lactose intolerant cats may vomit after consuming milk or dairy. The indigestible lactose can irritate the digestive tract and lead to vomiting.
  • Gas and abdominal pain – The undigested lactose in the colon can lead to gas production and abdominal discomfort. A cat may seem restless or uncomfortable and may cry out in pain.
  • Decreased appetite and weight loss – Chronic diarrhea and vomiting can cause dehydration, electrolyte imbalances, and diminished appetite. This may lead to weight loss over time.

If a cat exhibits any of these symptoms after ingesting lactose, it’s a sign they may have lactose intolerance. Keeping dairy products away from the cat and switching to lactose-free options can help resolve symptoms.

Lactose-Free Options

Fortunately, there are many lactose-free cat food options on the market today to choose from. These foods are formulated to provide all the necessary nutrients cats need, without containing lactose that could cause stomach upset.

Some good lactose-free wet and dry cat food brands include Purina ONE Sensitive Systems, Hill’s Science Diet Sensitive Stomach & Skin, and Royal Canin Veterinary Diet Gastrointestinal.

For lactose-free treats, consider options like PureBites Freeze Dried Chicken Breast, Zuke’s Super Foods Cat Treats, and Whole Life Pet Simply Chicken Freeze-Dried Treats. These all contain single-protein sources without milk ingredients.

When transitioning to a lactose-free diet, gradually mix the new food in over 5-7 days to allow your cat’s digestive system to adjust. Provide frequent small meals at first as well. With the right lactose-free foods, most cats find relief from the symptoms of lactose intolerance.

When to See a Vet

In most cases, the symptoms of lactose intolerance in cats will resolve on their own within 12-24 hours after removing milk from the diet. However, it’s important to monitor your cat closely and contact your veterinarian if any concerning symptoms persist longer than a day or two.

Signs that require prompt veterinary attention include:

  • Severe vomiting or diarrhea lasting more than 24 hours
  • Lethargy, weakness, or lack of appetite
  • Dehydration – dry gums, sunken eyes, lack of skin elasticity
  • Breathing difficulties
  • Bloody stool
  • Weight loss
  • Fever

Prolonged diarrhea or vomiting can quickly lead to dehydration, electrolyte imbalances, and other secondary health issues in cats. Seeking timely veterinary care can help provide supportive treatment and avoid complications.

In some cases, your vet may prescribe medications to settle digestion, control nausea and vomiting, or provide rehydration therapy. Rarely, hospitalization with IV fluids may be needed for cats with severe dehydration.

Your vet can also rule out underlying conditions that may be causing or contributing to persistent gastrointestinal symptoms in your cat after milk exposure.

With appropriate veterinary care and avoiding dairy products going forward, most cats fully recover from episodes of lactose intolerance.

Key Takeaways

Cats can digest milk when they are young kittens because they produce an enzyme called lactase that breaks down the sugar lactose in milk. Lactase production starts to decline around 6-8 weeks of age when kittens are weaned. By 6-8 months, most cats have become lactose intolerant. Consuming milk and dairy products can cause digestive upset like diarrhea and gas in lactose intolerant cats. There are lactose-free milk alternatives made especially for cats that are safe options. If signs of lactose intolerance persist, it’s best to see a veterinarian. The takeaway is that kittens can have milk, but once cats reach 6-8 months they should avoid regular milk and stick to cat-formulated lactose-free products.

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