I've read that cats cannot live on a vegan diet (they may even die). Is that true? Why is that? Is it the same with dogs?
There are commercially available vegan dog foods that can be safely fed to dogs. Brown et al. tested vegan dog food on a set of Siberian Huskies:
In 2009, Brown and colleagues  reported the results of a study of 12 sprint-racing Siberian Huskies fed either a commercial diet recommended for active dogs (n = 6), or a meat-free diet formulated to the same nutrient specifications (n = 6). The commercial diet contained 43% poultry meal, which was replaced by maize gluten and soybean meal in the meat-free diet. The dogs were fed these diets for 16 weeks, which included 10 weeks of competitive racing. Health checks were conducted by a veterinarian blinded to the dietary regimens. All dogs were assessed as being in excellent physical condition, and none developed anaemia or other detectable health problems.
No such product exists for cats as of now. Theoretically, there is nothing that cats require that can't be obtained form plants or synthesized, but creating such a diet is likely to be quite expensive.
My dog lives on a vegan diet. I'm buying my dog the Benevo food. My dog likes it and has no problems after two years of eating it.
The company produces both cat and dog food.
That's why I think this is not true. Dogs and cats can live on plants.
This is my non-expert opinion: dogs and cats have bodies that can process meat, while humans have bodies designed for a plant-based diet. But even cats and dogs can live healthily on a plant-based diet, simply by ensuring that they get all the necessary types of foods and nutrients, which are all available in the plant kingdom or in synthetic supplements.
I know from personal experience that dogs can live healthily on a veg diet. I once had the pleasure to dog-sit for a couple, friends of mine. They told me that their dog voluntarily turned vegan by refusing to eat the usual (meat-based) dog biscuits! I used to feed him the vegan food that they left prepared and he loved it! He was very active and looked very healthy to me. I have not come to know any vegan cats yet, although I hear that there are many.
Taurine, which is said to be essential for cats, may be found in some species of seaweed or in synthetic form, and a good vegan cat diet must include that. One should research carefully what their other dietary needs are in order to include vegan sources in their food, and watch their health closely with the help of a veterinarian, especially in the beginning. It would be irresponsible otherwise.
If you don't want to prepare your own food you can buy it ready made, which also gives you some assurance that it meets their nutritional needs, but again, please watch their health closely. They're available for both dogs and cats from a quick search, vegancats.com being one example. But I'd still suggest some lovingly homemade foods for the dogs as often as you can.
Professor Andrew Knight has published research on feeding dogs and cats a plant based diet, and from what I understand, he shows that if done properly it can actually improve their health.
There seems to be a consensus that dogs can be very healthy under a vegan diet, since they're omnivores like us. Of course it requires proper balance of macro and micronutrients. Cats are much more complicated, since they're carnivores, and I've heard conflicting reports. There's a fairly extensive list of references about vegan pets on the FAQ of r/veganpets and they include a section about vegan cats that I'll just copy and paste here:
Can cats be vegan?
The short answer is yes.
The long answer is that cats require many nutrients that typically come from meat. In regular kibble, many of these nutrients are stripped away during processing and added back in from synthetic sources. These same sources are used to fulfill missing nutrients in vegan diets. Studies and anecdotal evidence support vegan diets as a healthy diet for cats. Synthetic Taurine, for example, has been shown to fully fulfill cat's nutritional needs.
There is one point of concern though. Vegan diets are typically more alkaline/basic (high pH) than meat based diets. Basic diets can cause FLUTD (feline lower urinary tract disease), especially in male cats. To combat this their food must be acidified.
Most vegan and non-vegan cat kibble is acidified to prevent this condition. It's still a good idea to monitor your cat as it transitions foods to ensure its best health. On his website, Dr Andrew Knight says
Based on his experiences with thousands of vegan cats Gillen (2003) states that 85-90% of vegetarian cats do not require attention to dietary content; however, for the remaining 10-15%, urinary pH and dietary magnesium concentrations...require monitoring
The most relevant research has this to say on the matter
The normal pH of a cat’s urine is 5.5–7, and the normal range for a dog’s urine is pH 5–7 . A pH > 7 indicates alkalinity. A variety of dietary products (e.g., “Vegeyeast” from Harbingers of a New Age—see ) and additives can correct alkalinization, should it occur. Asparagus, peas, brown rice, oats, lentils, corn, brussel sprouts and yeast may be included in feline and canine diets, and are all urinary acidifiers . Vitamin C (ascorbic acid) is also a urinary acidifier. The British Small Animal Veterinary Association (BSAVA) Small Animal Formulary  recommends a dosage of 50–80 mg/kg every 24 h for cats and dogs. And for more serious cases, the amino acids methionine and cysteine may be used 9. The BSAVA Small Animal Formulary  recommends a dosage of 200 mg/cat every 8 h. More detailed advice about urinary alkalinisation and corrective strategies is available via www.vegepets.info, or within veterinary medical texts.
Increased urinary acidity, decreased urinary magnesium and increased water consumption all help to keep the urinary pH within a healthy acidic range, and help to prevent the formation of struvite crystals. However, acidifying nutrients, agents, or products should be used carefully, as excessive levels can lead to metabolic acidosis. Increased urinary acidity may also promote higher urinary excretion of calcium and lower excretion of magnesium, and magnesium is a natural inhibitor to the formation of urinary stones associated with calcium .
"Urinary pH is the most important factor in determining the SAP [struvite activity product, which can lead to FLUTD]. Acidification of urine causes deprotonation of phosphates and increases the total proportion of urine phosphate existing as trivalent anions, reducing the SAP.6 Urinary pH and SAP have been reduced with both dietary modification and administration of urinary acidifiers.7 The solubility of struvite is maximized when the urinary pH is <6.4 ... acidification of the urine to <6.29 may increase the risk of calcium oxalate urolith formation...[Urinary acidifiers] should be considered only when the urine pH is >6.5 with ad libitum feeding conditions...A general recommendation for prevention of urolithiasis is to increase water consumption...Diets with reduced magnesium that maintain a urine pH between 6 and 6.3 are recommended despite lack of evidence of efficacy...Monitoring urine pH is recommended to assess dietary compliance and efficacy. Values between 6.0 and 6.5 may reduce the incidence of calcium oxalate and struvite crystal formation. "
So what should you do?
- Read Dr Knight's recommendations
- Clean, fresh water should always be available and food should be wet or thoroughly soaked kibble
- Buy vegan cat foods that have been acidified to lower pH. You can check for acidifiers by looking in the ingredient list for sodium bisulfate, dl-methionine, ammonium chloride, Vitamin C and the others discussed in the above study. These are already commonly added to regular pet foods.
- Check your cat's urine pH 1-2 weeks after switching to vegan and then twice a year. The target urine pH is 6.3-6.5. Below 6.0 or above 7.0 is unhealthy.
- Buy and use your own acidifiers if your cat's urine is approaching pH 7.0.
- If your cat appears to be in pain when using the litter box, immediately contact your vet. FLUTD is a serious condition that can lead to death.
- If you make your own food, which is riskier than buying, it is important to properly acidify and blend/cook so it is digestible.
- If your cat has urinary issues, the author of Obligate Carnivore recommends taking these steps:
- For minor cases, enzyme supplements which include methionine, vitamin C, and/or cranberry extract will be sufficient. These limit both urinary alkalinisation and inflammation. They also aid digestion, and can result in increased vitality.
- For moderate cases, Gillen states that Vegecat pH, with added sodium bisulfate, may be sufficient.
- For severe cases Gillen recommends methionine pills. Severe cases also require a visit to the vet and possibly removing them from a vegan diet. Gillen estimates 85-90% of cats will never experience even a minor case.