Fruits and vegetables contain glutamic acid, which cats have a lower tolerance for than other species. The explanation in Comparative Nutrition of Cats and Dogs, Annu. Rev. Nutr. 1991. 11.’239~5, as an example, is that it is "probably due to the low activity of alanine aminotransferase in gut mucosa of cats", quoted from Rogers, O.R., Phang, J.M. 1985. Deficiency of pyrroline-5-carboxylate synthase in the intestinal mucosa of the cat. J. Nutr. 115:146-50
The information around avocado use is usually a little ambiguous which may help to explain why avocado is promoted in one particular pet food, however, it is not something I personally would feed my cats, based upon the following:
"Avocados contain a toxic component called persin," explains Jill A. Richardson, DVM, of the ASPCA Animal Poison Control Center (APCC), "which has been shown to produce cardiac tissue damage, respiratory distress and mammary gland damage in a variety of animals--including horses, goats, sheep, dogs, cattle, rabbits, fish and birds."
The above information is repeated frequently on the net if you do a search.
Ingestion of fruit, leaves, stems, and seeds of avocado has been associated with toxicosis in animals; leaves are the most toxic part. The Guatemalan varieties of avocado have been most commonly associated with toxicosis
If I had a cat that required tamoxifen I might feel comfortable adding avocado to the diet as Persin is supposed to compliment the chemotherapy action of this drug, however, I would never let avocado near a cat with a heart weakness:
Clinical signs: vomiting, diarrhea, death, inflammation of mammary glands of rabbits, goats, cattle, and horses- Cardiac failure in goats- Respiratory distress, generalized congestion, fluid accumulation around the heart.
In Ramesh Gupta's "Veterinary Toxicology, Basic and Clinical Principles" (2007), avocado is listed under Table 13.1, 'Plants affecting the heart', in reference to cardiac glycosides on page 196:
"Interefere with the Na+/K+-ATPase enzyme in cardiac fiberes resulting in decreased intracellular K+ and increased intracellular Na+ causing various degrees of heart block" This section was written by Steven I Baskin, Steven E Czerwinski, Jaime B. Anderson, and Manu M. Sebastian and, as I prefer to see, had reputable references sources for their comments including Akera et al (1973), for those who wish to dig further.
There are warnings regarding human consumption of avocado fruit:
These data suggest that consumption of avocado oil extracted from intact fruit may cause changes in liver metabolism.
Avocado ingestion may decrease the anticoagulant effect of warfarin. Patients taking warfarin should consult their health care provider before eating avocado or taking herbal products.
Avocado Adverse Reactions
Allergy to latex, bananas, melons, and peaches may result in a cross-sensitivity to avocado; if allergic, use products that contain avocado with caution.
The cranberry industry is targeting pet foods as a source of income, with no concern for the effect of benzoic acid, or other issues involved that affect felines. Even in the attached Wikipedia site they mention, "Cats have a significantly lower tolerance against benzoic acid and its salts than rats and mice".
Benzoic acid toxicity in felines is well documented online at sites such as the following:
Outbreaks of poisoning affecting 28 cats have followed ingestion of meat containing 2.39% benzoic acid. The effects were nervousness, excitability, and loss of balance and vision. Convulsions occurred and 17 cats either died or were killed. Autopsies showed damage to intestinal mucosa and liver. The sensitivity of the cat may be due to its failure to form benzoyl glucuronide and toxicity may develop with quantities greater than 0.45 g/kg single doses or 0.2 g/kg repeated doses (Bedford & Clarke, 1971).
In cats, glucuronidation is generally very low (Williams, 1967).
CHEMINFO Record Number: 374, states, "Ingestion:
Convulsions and death were observed following single oral doses of greater than 450 mg/kg benzoic acid to cats. Similar effects were observed following daily doses of greater than 200 mg/kg. The authors conclude that these effects are likely specific to cats.(12) "
Cranberries are sometimes recommended for reduction of crystal growth, but there have been studies showing that cranberries are not effective for this. As the first study here shows that it is the benzoic acid that is supposed to suppress crystals, this is not a healthy option for cats.
Cranberries also contain Salicylic acid that is very similar to aspirin. Aspirin is not good for pets.
Contain various sugars, and phenols that are not appropriate for felines. If your cat has allergies to pollen, you may find apples affecting this condition. Apples also contain Oxalic acid 500 µg that will contribute to crystal formation. Apple fibre also lowers the digestibility of proteins. Apples also contain phenols that are known to be irritants specifically to cats, (well known in cleaning products), and may cause dermatological discomfort at the least. If the seeds are included in the content included in a food, then amygdalin has been known "in rare cases to be fatal."
The Cornell Book of Cats 2nd edition, p. 383, Reference Guide: First Aid for Plant Poisoning states the following about the green peel on potatoes:
"Type of Illness: Lower Gastrointestinal
Vomiting, abdominal pain, bloody diarrhea, dry mouth, all after a latent period of 18-24 hours after plant was eaten. May proceed to nervous system stimulation followed by, i.e., trembling, salivation, and paralysis. May lead to cardiac arrest."
Cornell clarifies that the part of the plant they are describing are, "Nightshades, Jerusalem cherry, Potato (green parts and eyes)." While we should be able to have confidence in the term "human grade", it is very easy to see green parts on potatoes in supermarkets that humans are expected to prepare by removing the peal. I don't see that happening with pet foods, however, that is my personal opinion. The glycoalkaloids are also in bruised potatoes and flesh irritated by micro-organisms.
Then there are other sites online that you may access, such as here, and here, with the second site stating that these alkaloids are not removed with cooking.
Along with the site above you can access Cornell who include the "solanum-type glycoalkaloids" as part of their poison plant section. Lower on the same page, after clarifying that it is the greener tomatoes that will be a risk, they echo the fact, "These alkaloids are not destroyed by cooking or drying at high temperatures" Tomato Puree contains 16.4 grams of carbohydrate per 100 grams of tomato.