Lactose intolerance in dogs: symptoms after dairy products

Overview

Lactose is a carbohydrate found almost exclusively in dairy products. Lactase on the small intestinal surface breaks it into glucose and galactose. These sugars can then be absorbed for use by the body.

Finger-like villi on the small intestine's inner surface increase the nutrient absorption area. Surface cells on the villi help both digest food and transfer nutrients into the body. Tiny microvilli on these cells form the brush border.

Intolerance is a problem with food utilisation that is not necessarily an allergic reaction. In milk protein allergy, the immune system reacts to milk proteins. A reaction to milk may therefore involve its sugar, protein or another dietary factor. Tolerating lactose also does not mean dairy is a nutritionally necessary part of your dog's food.

Causes

Undigested sugar stays in the intestine

Lactose utilisation depends on intestinal lactase activity. When lactose remains unabsorbed, the osmotic effect of intestinal contents can retain water in the intestine. Loose stools or diarrhoea may result. Intestinal bacteria use undigested carbohydrates and produce gas.

Lactase activity is generally lower in adult dogs than puppies. Activity of 3.3 units per gram of tissue protein in adults and 96 in puppies has been described. These measure enzyme activity rather than safe milk quantities for dogs. Lactase activity usually begins declining after weaning.

Intestinal disease can reduce previous tolerance

Damage to the small intestinal lining can cause secondary lactose intolerance. Dairy may therefore worsen signs in a dog already experiencing diarrhoea. Broader malabsorption can relate to chronic inflammation, intestinal parasites or another intestinal disease.

Giardia, for example, is an intestinal pathogen associated with chronic intestinal disease and broader impairment of food utilisation.

Genetics may also influence lactase regulation. The LCT gene contains the information needed to produce lactase. Adaptation associated with a variant increasing lactase expression has been identified in European dogs. All adult dogs therefore cannot be assumed to have a complete lack of lactase.

Risk factors

Tolerance depends on both the dog and the dairy product eaten. Large amounts of milk may cause signs in adult dogs because of reduced lactase activity. In puppies, milk lactose is normally an important dietary sugar.

Declining lactase activity has been described from around 2 months old. Large milk intakes can cause diarrhoea in puppies older than 2–3 months. These ages are not precise tolerance cut-offs for every dog. Canine milk differs in composition from cow's milk. Dog and cat milk contain on average around 30% less lactose than cow's milk. A puppy's ability to use its mother's milk therefore does not mean large amounts of cow's milk are equally suitable. Intestinal disease can impair lactase even in an animal that previously tolerated dairy.

Typical symptoms

A typical pattern is digestive signs after milk or dairy products. Severity can vary greatly between dogs.

• Stools become loose or your dog develops diarrhoea.
• Your dog farts more than usual.
• The abdomen swells or seems gassy.
• Your dog seems to have abdominal pain or discomfort.
• Intestinal rumbling becomes louder than usual.

A nauseous dog may drool, lick the lips or leave food uneaten. Rumbling and nausea are general gastrointestinal signs rather than specific indicators of lactose intolerance.

Itching, paw licking or repeated ear infections may fit food allergy. Weight loss, persistent diarrhoea or poor appetite requires investigation of other causes too. Goat's milk can also cause lactose-related signs.

Complications and consequences

Ordinary food intolerance primarily causes gastrointestinal signs. Profuse diarrhoea can cause dehydration and electrolyte losses. Puppies dehydrate particularly quickly.

If intestinal disease underlies the signs, removing dairy alone may not resolve the whole problem. Widespread malabsorption can also cause weight loss and nutrient deficiencies. Longstanding malabsorption can involve cobalamin, or vitamin B12, deficiency. This is a consequence of intestinal disease needing its own nutritional assessment. Excluding dairy need not produce a nutritionally inadequate diet if your dog receives complete food appropriate for their life stage.

Diagnosis at the vet

Dietary history and symptom timing are central

Your vet reviews all foods and treats and any access to other products. The dairy product type, amount eaten and timing of signs help identify a possible trigger. Other skin, ear and intestinal signs are also considered. A reaction after dairy alone cannot distinguish poor lactose tolerance from a milk protein reaction.

Removing the food causing signs and monitoring response form part of investigating intolerance. Suspected food allergy may require a planned elimination diet followed by a challenge. Blood, hair and saliva tests do not reliably diagnose canine food allergy. An allergy dietary trial in a dog with skin signs may last 4–12 weeks. This investigates allergy rather than simply measuring milk sugar digestion.

Other intestinal diseases are investigated according to signs

Persistent diarrhoea or weight loss may need a blood count, biochemistry, urinalysis and a faecal sample. These look for diseases outside the intestine and possible parasites. Pancreatic insufficiency is a different cause of poor food utilisation from low lactase activity at the intestinal surface. It is investigated by measuring blood TLI. Weight loss despite a good or increased appetite fits broader malabsorption.

Further investigations may include ultrasound or intestinal biopsies. Cobalamin is absorbed mainly in the final small intestine, while folate is absorbed in the initial part. These levels may help assess intestinal disease. The intestinal disease underlying secondary lactose-related signs is investigated within this overall assessment.

Breath hydrogen can be used to investigate carbohydrate absorption. Measured hydrogen comes from intestinal bacteria because mammalian cells do not produce it. Some bacterial gas is absorbed from the intestine and exhaled. Breath testing can also reflect food transit from the mouth to the start of the colon. Abundant colonic bacteria make it difficult to determine which intestinal section produced the measured hydrogen. Technical difficulties and interpretation limitations restrict the method's use in animals.

Treatment

Remove the dairy product causing signs

If a dairy product causes digestive signs, stop giving it. An otherwise suitable, balanced diet can remain the basis of your dog's nutrition. Removing multiple foods without nutritional planning can make the diet inadequate.

Fluid therapy need in a dog with diarrhoea is assessed according to condition and dehydration. Mild acute diarrhoea in an otherwise well dog generally does not need antibiotics. Underlying intestinal inflammation or another disease also needs its own treatment. Your vet decides whether medication is needed.

Newborn feeding needs separate planning

A newborn puppy's colostrum contains immune proteins and other essential nutrients as well as sugar. If maternal milk is insufficient, milk replacer is needed to meet the puppy's nutritional requirements. However, milk replacer does not reproduce every property of colostrum.

Lactose-free and dairy-free mean different things

Lactase treatment breaks lactose into glucose and galactose. It does not itself remove milk proteins to which an allergic dog may react. Suitability of lactose-free dairy therefore cannot be determined simply from a suspicion of allergy. A dairy product may also contain fat affecting digestive signs.

An allergy dietary trial may use extensively hydrolysed protein or a novel-protein food. Selection depends on previous feeding and signs. Changing dietary protein and breaking down lactose are therefore different treatment principles.

Fermented products are not automatically lactose-free. In a separate product comparison, kefirs contained around 29.81 and 31.75 grams of lactose per litre. The lactase-treated comparison milk contained around 2.12 grams per litre. These figures concern the tested products rather than all kefirs or lactase-treated milks.

The kefir trial involved 12 healthy adult dogs. Kefirs did not improve stool quality compared with the tested milk. Results in healthy dogs do not establish kefir as a treatment for lactose intolerance.

Prevention

Avoiding dairy products that previously caused signs reduces repeated exposure. Assess the product's whole composition, because protein and fat may matter as well as sugar. A lactose-free label therefore does not replace assessment of individual tolerance.

Poor milk sugar tolerance does not mean the same digestion problem affects every carbohydrate. Pancreatic amylase, for example, helps digest starch, while lactose needs lactase on the intestinal surface. A milk sugar reaction alone therefore does not require changing the diet to avoid all carbohydrates.

Do not give dairy products sweetened with xylitol. Large amounts of extra sugar are also unnecessary in a dog's diet. Maintain nutritional balance even when excluding dairy.

Important for dog owners

Record the dairy product's name or composition, quantity eaten and timing of signs. Include all other foods, treats and any new supplements. Monitor drinking, eating, stools and energy in the same diary. Tell your vet if signs continue after stopping dairy or your dog loses weight.

Do not keep giving a product that causes signs as an experiment. Agree any dietary trial with your vet, particularly if itching or ear signs are also present. Even mild diarrhoea needs veterinary assessment if it does not improve within 48–72 hours. Blood, vomiting, refusal to eat or lethargy warrants earlier contact.

Frequently asked questions

Can I give my dog lactose-free milk?

Breaking down lactose can reduce the milk sugar load. However, lactose-free dairy may still contain milk protein or fat unsuitable for your dog. If dairy causes digestive signs, stop giving it. Complete food appropriate for your dog's life stage can provide their basic nutrition.

Is goat's milk better for a lactose-intolerant dog?

Goat's milk can also cause lactose-related diarrhoea, bloating and abdominal discomfort. Switching from cow's milk to goat's milk therefore does not by itself resolve the cause. Review the full dietary history and the timing of digestive signs with your vet.

Are yoghurt and kefir always lactose-free?

Fermentation does not guarantee a product contains no lactose. The tested kefirs contained around 29.81 and 31.75 grams of lactose per litre. Individual dogs can react differently to dairy. Do not continue a product causing signs as an intestinal treatment.

Is milk protein allergy the same as lactose intolerance?

In lactose intolerance, the problem is milk sugar breakdown. In allergy, the immune system reacts to food protein. Allergy may cause itching or ear infections as well as digestive signs. Investigation may require a veterinary-planned dietary trial followed by a challenge with the original food.

Does an adult dog need milk for nutrition?

Complete food appropriate for your dog's life stage supplies the nutrients needed. Dairy therefore does not need adding to ensure adequate nutrition. If a dairy product causes diarrhoea, gas or abdominal pain, stop giving it. When planning a broader home-made diet, ensure it is balanced.