When to see a vet urgently
Seek veterinary assessment promptly if any of the following occurs:
- Your cat staggers, has seizures or holds its head unusually bent downwards.
- Your cat is not eating and becomes noticeably weak.
- Your cat has unexplained bleeding, bruising or very pale mucous membranes.
- Your cat has received a large amount of a vitamin product or ingestion is suspected.
You can find a vet near you. Do not try to treat neurological signs or bleeding at home by experimenting with vitamins.
Overview
Vitamins are biologically active substances needed for normal body function. They take part in metabolism and support growth, the immune system, the nervous system and cardiovascular function. A deficiency of each vitamin appears differently: for example, a lack of vitamin A affects vision and skin, a lack of vitamin D weakens bones, and B vitamin deficiencies affect nervous system function and blood formation.
Vitamins are divided into the fat-soluble vitamins A, D, E and K, and the water-soluble B vitamins and vitamin C. Requirements and the mechanisms of deficiency vary. Cats, for example, need preformed vitamin A in their diet because they do not convert plant beta-carotene into it in the same way as dogs.
Dog food is therefore unsuitable as a cat's sole diet. Conversely, even suitable food does not prevent every deficiency if a cat eats too little or a disease impairs nutrient utilisation. Adding a single vitamin does not correct an entire unbalanced diet.
Causes
The main causes of vitamin deficiency in cats include:
- an unbalanced diet, such as meat alone, porridge or human food without accounting for feline nutritional needs
- inexpensive prepared foods that do not contain enough vitamins
- chronic gastrointestinal diseases that impair nutrient absorption
- prolonged fasting or strict diets
- increased requirements in growing kittens and pregnant or nursing cats
A prepared food's suitability is assessed from its intended use, nutrient content and quality assurance. Complete food means food intended to meet an animal's nutritional requirements on its own. Complementary food alone is insufficient. Product category, packaging pictures or a single ingredient do not establish that a food is suitable as a cat's sole diet.
Food processing can also affect vitamin availability. A diet containing large amounts of raw fish carries a risk of thiamine, or vitamin B1, deficiency because some fish contain an enzyme that breaks down thiamine. This does not mean every complete cat food containing fish causes deficiency. The preparation method and balance of the whole diet are decisive.
Risk factors
- Cats fed only homemade food without vitamin supplementation.
- Kittens and older cats with increased nutritional requirements.
- Cats with chronic liver, pancreatic or intestinal disease.
- Animals recovering from a long illness or surgery.
Age or the recovery phase alone does not establish vitamin deficiency, and not every older cat needs the same supplement. The nutritional requirements of growing, pregnant and nursing cats differ from adult maintenance needs. In an ill cat, the amount eaten and nutrient absorption are also investigated.
Prolonged failure to eat and severe diseases of the intestines, pancreas, liver or biliary tract can be associated with particular vitamin deficiencies. Obstructed bile flow can impair the use of fat-soluble vitamins. In these cases, simply choosing another ordinary food cannot resolve the deficiency; the underlying disease also needs treatment.
Symptoms
Symptoms depend on which vitamin is lacking:
- Vitamin A: dry skin, a dull coat, impaired vision and slow growth in kittens.
- Vitamin D: weak bones, abnormal limb alignment and muscle weakness.
- B vitamins: lethargy, seizures, anaemia and neurological signs.
- Vitamin E: muscle weakness and reduced fertility.
- Vitamin K: a tendency to bleed and poor blood clotting.
This list describes possible associations, rather than a diagnostic method for use at home. Many other diseases cause similar weakness, coat changes or bleeding. Bone problems, for example, can be affected by mineral amounts and their balance as well as vitamins.
Thiamine deficiency can particularly damage brain function. A cat may stagger, tremble or bend its neck so the head droops down. Severe cases can involve seizures. Dietary history and response to thiamine treatment given by a vet are useful in assessing the diagnosis, but these symptoms also require investigation of other causes.
Vitamin E deficiency in cats has been associated with inflammation of fatty tissue, or pansteatitis, for example with a diet consisting largely of tuna and lacking variety. Signs may include loss of appetite, subdued behaviour and abdominal tenderness. Do not repeatedly press a painful abdomen to try to examine a cat with symptoms.
General signs of vitamin deficiency include lethargy, poor appetite, a coat and skin in poor condition, and reduced resistance to disease.
Possible complications
Without treatment, vitamin deficiencies can lead to:
- chronic liver and gastrointestinal diseases
- neurological signs
- anaemia and bleeding
- weakened bones and muscles
- reduced immune defences and more infections
Excessive vitamin intake can also cause serious damage. Long-term high vitamin A intake, for example from liver or supplements, can lead to painful excessive bone growth and restricted movement. Some of these skeletal changes can remain permanent even after the diet is corrected.
Signs of excessive intake, such as a poor coat, weakness and weight loss, can resemble deficiency symptoms. A supplement started “just in case” based on symptoms can therefore worsen the situation. Excess vitamin D can disrupt calcium balance and damage soft tissues. The combined use of several products is assessed before they are given.
Diagnosis at the vet
Investigations include:
- taking a history and assessing the diet
- clinical examination
- blood tests
- biochemical tests for particular vitamin deficiencies where necessary
Your vet assesses whether a deficiency is due to inappropriate feeding or diseases that prevent nutrient absorption.
The dietary history includes all foods, treats, liver products, oils and supplements, and how they are actually used. Bring packaging information and the exact contents of any homemade recipe to the appointment. Also report changes in appetite, weight loss, diarrhoea, vomiting and medicines used. This allows a possible deficiency or excess to be considered in the overall context.
Blood tests are selected according to symptoms. One general test does not investigate every vitamin deficiency. Vitamin B12, or cobalamin, may be measured particularly with intestinal or pancreatic disease. Measuring methylmalonic acid can complement assessment of cobalamin status and monitoring of treatment response.
A low B12 result alone does not explain why the vitamin is lacking. It may be associated with chronic intestinal disease or insufficiency of the part of the pancreas that produces digestive enzymes. Vitamin replacement is therefore combined with any other necessary investigations and treatment of the underlying disease. Correcting a laboratory number alone does not remove every possible cause of symptoms.
If excess vitamin A and skeletal changes are suspected, X-rays may be needed alongside the dietary history. Blood vitamin A levels and organ values can contribute to the assessment. The investigation pathway therefore differs from that for B12 deficiency associated with intestinal disease, for example.
Treating deficiency and monitoring
Your vet selects the vitamin needed and how to give it according to the confirmed deficiency and your cat's condition. A patient with severe symptoms or vomiting may initially need injections. For B12 deficiency, both oral and subcutaneous treatment can work, but the choice is individual.
Cobalamin treatment has been studied in cats with chronic gastrointestinal disease or exocrine pancreatic insufficiency. As well as blood B12 levels, metabolic changes have been monitored using methylmalonic acid. Follow-up therefore assesses both laboratory results and your cat's symptoms. Do not change or stop a vitamin product yourself because of one good day.
Dietary correction is planned at the same time. A homemade recipe can be balanced with help from a vet with expertise in animal nutrition, or a suitable complete diet can be selected. Disease may require specific nutritional support. The aim is adequate energy and every essential nutrient, rather than simply adding one missing substance.
At home, monitor eating, weight changes, stools, movement and alertness. Contact the clinic if your cat does not eat, deteriorates or develops new neurological signs. The prospects for recovery depend on the cause of deficiency and severity of damage. The consequences of long-term inappropriate feeding do not necessarily resolve as soon as the contents of the food bowl change.
Prevention
- Use balanced complete foods.
- When feeding homemade food, add vitamin and mineral supplements recommended by your vet.
- Arrange regular veterinary health checks.
- Pay particular attention to feeding kittens, pregnant cats and older cats.
Check both the species and life stage on the packaging. Food for growth and reproduction and adult maintenance food are designed for different requirements. Treats and complementary foods do not form a cat's main diet. Vitamin mixtures are not automatically added for a healthy cat eating a complete food normally.
In a homemade recipe, supplements must suit the specific ingredients and their amounts. Changing one ingredient or omitting a supplement can alter the overall balance. Do not use a human vitamin product to correct your cat's nutrition on your own initiative. Tell your vet about supplements already in use too, so overlapping intake can be assessed.
This text provides general information and does not replace veterinary care. Contact a vet if your cat develops symptoms.
Frequently asked questions
Does a healthy cat need a vitamin supplement as well as complete food?
Usually not automatically. A suitable complete food is intended to meet nutritional requirements. Supplement needs are assessed separately, for example because of illness, failure to eat or a confirmed deficiency.
Does a poor coat definitely indicate vitamin deficiency?
No. Similar changes occur in many diseases and with excessive intake of some vitamins too. The diet and cat are assessed before a supplement is started.
Can a cat get too many vitamins?
Yes. High vitamin A or D intake in particular can cause serious harm. A suspected large overdose is a reason to contact a vet immediately.
Is B12 deficiency always caused by the wrong food?
No. A deficiency can be associated with intestinal or pancreatic disease even when the food contains the vitamin. Both the deficiency and its underlying problem are then addressed in treatment.
This article is for general information only and is not a guide to treatment. If you notice the first signs of illness, contact your vet.
Paw4care editorial team
Sources
- Nutritional Requirements of Small Animals, Merck Veterinary Manual, accessed 2026-11-03.
- Nutritional Disorders of the Spinal Column and Cord in Animals, Merck Veterinary Manual, accessed 2026-11-03.
- Nutrition in Hepatic Disease in Small Animals, Merck Veterinary Manual, accessed 2026-11-03.
- Effect of oral or subcutaneous administration of cyanocobalamin in hypocobalaminemic cats with chronic gastrointestinal disease or exocrine pancreatic insufficiency, Journal of Veterinary Internal Medicine, PubMed, accessed 2026-11-03.
- Koirien ja kissojen ruoka, Ruokavirasto, accessed 2026-11-03.
- Complete and Balanced Pet Food, U.S. Food and Drug Administration, accessed 2026-11-03.
- Vitamin A Poisoning in Cats, VCA Animal Hospitals, accessed 2026-11-03.