What are the effects of sugar in cat food?

You got most of the points right. The sugar is added to cat food for the benefit of the manufacturer, not for the cat.

Cats cannot taste sweet

You need 2 specific genes in order to have receptors for sweet tastes on your tongue, but cats lack one of those genes.

[Joe Brand, biochemist and associate director at the Monell Chemical Senses Center in Philadelphia] and his colleague Xia Li first discovered the pseudogene after decades of anecdotal evidence—such as cats showing no preference between sweetened and regular water, unlike other animals—testifying to their indifference to the sweet stuff.

This is affirmed by this meta study that I highly suggest reading because of the varied information it contains.

Neurophysiologic studies of the facial nerve demonstrated responses to salt, bitter, sour, amino acids and nucleotides taste stimuli [59,60]. No neural responses to sucrose and other sugars were detected in cats [61,62]. The sense of taste in cats is therefore similar to that of other mammals, with the exception of insensitivity to sweeteners. The molecular basis for this feline sweet blindness is the lack of a sensory system to detect sweet stimuli. Li et al. characterised the sweet receptor genes of cats and concluded that Tas1r3 is expressed and functional, but Tas1r2 is an unexpressed pseudogene. The functional sweet receptor heterodimer (T1R2/T1R3) can therefore not form, resulting in an inability to taste sweet taste stimuli [63,64]. This was confirmed in lions.

But there seem to be some cats who can sense high amounts of sugar.

Why is there sugar in cat food?

First of all, small amounts of different sugars are contained naturally in the other ingredients of the food, mostly in vegetables and grains.

Most cat foods don't contain the white sugar we are used to having in our kitchen. Instead, the sugar is caramelized to give it a deep brown color. If mixed with wet food, this caramel gives the food the color of gravy and fried meat and adds a juicy sheen to the sauce. This is only done to appeal to the humans that buy the food, not for the cats.

If mixed with dry food, the caramel also adds a rich brown color (while the kibbles would look pale grey without it) and makes the food more crispy.

It's hard to find any independent sources about this topic. The big brands only offer the kind of nonsense you've found by Purina. I did find several articles by sugar-free pet food companies:

• Purr and Miaow health food for cats
• The Honest Cat Food Company

Is sugar in cat food really bad?

I'm not a scientist, but the data seems to suggest that any amount of sugar in cat food is too much sugar.

We as humans are very good at digesting carbohydrates and sugars, so good in fact that it causes obesity and diabetes on a large scale. When we taste sweet food, our body produces a number of different enzymes that break down the longer molecule chains of carbohydrates into smaller bits, starting in our mouth and continuing the work in the small intestine. The long chains are called starches, the smaller bits sugars, and the chains need to be broken down to a length of 1 - 2 sugar units to be absorbed into the blood stream. Since our bodies are so good at breaking down the chains and absorbing them into the blood, our blood gets flooded with sugar after a carb rich meal, which can cause diabetes.

This study found that, despite the evolution of cats as carnivores, they seem to be just as good at digesting carbohydrates as humans are.

Early studies by Morris et al. observed that adult cats could efficiently digest all carbohydrates added to a meat-based diet, despite the described evolutionary adaptations along the feline gastrointestinal tract [28]. The total apparent digestibility of starch is reported to be 40–100%, depending on source and treatment [28,29,30], which proves that cats can digest and absorb carbohydrates.

AND cats are actually better at absorbing sugars of different chain lengths into the blood stream, which may cause diabetes:

Anatomically it may seem as cats have a limited ability to absorb monosaccharides because of the limited length of the small intestine. However, partial compensation for this relative deficit in absorptive surface occurs. First, the mucosal area per unit serosal area in the cat’s small intestine is greater compared to rats and dogs [24]. Second, the d-glucose transport across the feline intestinal brush border appears to have a considerably higher capacity in cats than in cattle and rabbits, despite the similar amounts of glucose reaching the small intestine when these species are fed a natural diet. Although omnivorous species can adapt glucose transport to dietary carbohydrate intake, it was found in kittens that the glucose transport system in the intestinal brush border does not adapt to varying levels of dietary carbohydrate.

But since cats cannot regulate the amount of digestive enzymes to the amount of carbohydrates in their digestive tract, they cannot metabolize big meals:

Wild cats eat small vertebrate prey with low carbohydrate content (NFE 1–2%ME) [1,48], and must catch 8–12 of these small rodents every 24 h to provide their energy requirement [53]. Similarly, domestic cats in an experimental setting also spread their daily intake of food and macronutrients over 12–20 small meals, evenly spread between light and dark periods [53,54]. Because of the limited intestinal enzyme capacity, consumption of excessive amounts of digestible carbohydrates will not lead to a massive increase of glucose absorption, but will provide substrate for microbial fermentation in the colon, and cause gastrointestinal adverse effects.

Does sugar in cat food cause diabetes?

This is still subject to debate. Several studies have been conducted that either found no clear correlation or contradict each other.

An excerpt from the aforementioned meta study:

The effect of diet type remains a topic of debate. Slingerland et al. failed to find evidence that the energy percentage of dry food in the cat’s diet is a risk factor for the development of feline diabetes [121]. Also, in the study by Sallander et al. healthy controls consumed a higher portion of dry food compared to diabetic cases [117] and McCann et al. observed that cats eating a combination of dry and wet food are at higher risk compared to cats fed only dry or only wet food [111]. More recently, dry food feeding has been associated with increased risk of diabetes mellitus in normal-weight cats, while diet type was not found to be a risk factor in obese cats [110]. Body condition was, however, reported by the pet owners in this study, which may have skewed the body condition classification, as previous research has shown that pet owners underestimate their cat’s body condition [86,90,98,100,108]. As with the epidemiological research on risk factors for obesity, dietary macronutrient content and food intake were not assessed, so conclusions relating to dietary carbohydrate content, which are often made in these publications, cannot be drawn. Other dietary factors increasing the risk for diabetes include greedy eating behaviour and free choice feeding [110], which could be linked to higher energy intake and obesity.

The main risk factors for diabetes in cats are:

• Breed (Tonkinese, Norwegian Forest, Burmese Russian Blue, and Abyssinian)
• Age (older than 7 years)
• Sex (male and neutered cats have a higher risk)
• Housing and activity (inactive indoor cats are at higher risk, obese cats at much higher risk)