When a cat has a frothy vomit or throws up bile, it is almost always because there was acid or bile build-up in an empty stomach. And if kitty has stomach acid and/or bile sitting in the stomach in anticipation of a meal, that meal will most likely be regurgitated. This usually happens with breakfast, being the longest period between meals, but it can happen at any meal. When feeding homemade food, this is rarely the result of eating too quickly, which is often suggested as the cause. What you're witnessing is a kitty transitioning to a protein-based, species appropriate diet, and their bodies are returning to their more natural, more acidic state. It is not cause for alarm, and it is easily managed.
A whitish foam is stomach acid. Yellow liquid is bile.
Of course, if your cat is vomiting repeatedly, projectile vomiting, not keeping anything down (not even water), or you suspect the cat ate or drank a poisonous substance or swallowed something inedible, please take kitty to the vet. And if throwing up acid frequently continues despite these recommended steps, if your cat has not had blood work done recently, please get this done. This can be a symptom of chronic kidney disease or hyperthyroidism.
But a cat vomiting stomach acid, bile, or breakfast is not unusual in cats new to set meal times or raw/homemade food. As explained by Lyn Thomson, BVSc DipHom writing for the Feline Nutrition Foundation in Gastric Acidity - What, How and Why
"Cats need a highly acidic stomach in order to properly digest their food. But, the carbohydrates in many processed foods make the stomach less acidic. Meat protein stimulates stomach acidity by triggering the production of hydrochloric acid in acid-secreting cells within the stomach. A complex cascade takes place when a cat or dog ingests food. Put simply, 80% of the gastric juices secreted are a direct result of chemoreceptors in the stomach detecting the presence of meat-based proteins. This keeps the stomach at a very low pH of around 1-2. A low pH means high acidity. This low stomach pH is important because digestive enzymes work best in an acidic environment and the acidity in the stomach will sterilize ingested pathogens, bacterial or fungal.
When a cat or dog swallows a commercial pet food that is high in carbohydrate and plant protein and low in meat protein, acid-secreting cells in the stomach are not stimulated to produce much hydrochloric acid. The pH within the stomach rises to around 4-5 and a high pH means low acidity. The acidic chyme leaving the stomach is the trigger for the next stage of digestion in the small intestine. The acidity encourages the flow of bile and the flow of pancreatic enzymes necessary to continue the digestive process. If the stomach contents are not sitting at a pH of around 1-2, then digestion is impaired throughout the rest of the digestive tract as well."
Cats normally, naturally, have very acidic systems. Their stomachs secrete powerful digestive enzymes with about 10 times the amount of hydrochloric acid than that of a human. As Dr. Thomson pointed out, the pH of a carnivore’s stomach is around 1-2 (highly acidic) - but they are able to maintain that highly acidic environment even with (high protein) food in the stomach. For humans, the pH ranges from 4 – 5 with food in the stomach. Thus it is that when we start feeding our cats a diet that is similar to what they naturally eat, their bodies can take some time to adjust. And we can help them.
How Do I Fix Stomach Acid/Bile Vomiting?
When to Seek Vet Care When Your Cat is Vomiting
Please note, cats with chronic kidney disease (CKD) or cats with hyperthyroidism (Hyper-T) can experience acid/bile vomiting as a side-effect of the disease. When was the last time your cat had blood work? How old is your cat? Is your cat drinking more water than normal? Acid / bile vomiting, especially in combination with increased water drinking, can be a symptom of chronic kidney disease or of a hyperthyroid. These conditions require a different form of management. Altering meal times and putting the stomach acid / bile to work can certainly help manage the symptoms. But these alone may not resolve the problem when there is CKD or Hyper-T.
Please also seek vet care for your cat that is throwing up along with other symptoms, such as
Adjusting meal times in combination with putting the stomach acid/bile to work is usually all it takes to resolve the overnight vomiting or breakfast/meal regurgitation. Add a third (or fourth) meal, make the last meal later, and if possible, the first meal earlier. Give your kitty a few freeze dried meat treats overnight if you can, and certainly 15 - 20 minutes before the morning meal. These simple steps are usually all it takes to resolve the problem!