Carbohydrates

Neither the dog nor the cat has a physiological requirement for carbohydrates — the body produces all the glucose it needs on its own from protein and fat through gluconeogenesis [1]. That's why in BARF carbohydrates are not a question of "whether at all", but of "which and how much". And here lies the crux: carrot and rice are two metabolically different worlds. The label "total carbohydrates" covers three completely different fractions:

Starch and digestible sugars — enzymatically digested, raise blood glucose, provide calories (rice, sweet potatoes, potatoes, peas, bananas).
Soluble fiber — fermented by large-intestine bacteria into short-chain fatty acids (SCFA), which nourish the gut; a prebiotic role (pumpkin, psyllium). More in the article Fiber.
Insoluble fiber — non-fermented, increases fecal bulk (cellulose, peels, stalks).

20 g of carbohydrates from pumpkin and spinach is mainly fiber and water — almost energetically neutral. 20 g from rice is starch that raises blood glucose. Always look at the source, not just the number.

Species difference: Through domestication the dog developed more copies of the pancreatic amylase gene than the wolf and tolerates moderate amounts of starch [2]. The cat, as an obligate carnivore, is metabolically far less well adapted: it has constantly active gluconeogenesis, minimal hepatic glucokinase activity and poor regulation of postprandial blood glucose. The cat's natural diet contains below 2% carbohydrates in dry matter [3].

Where it's found: From vegetables, fruit and possible starchy additions. Low-starch vegetables (zucchini, spinach, cucumber) are almost energetically neutral; pumpkin and carrot provide some sugars; sweet potatoes, rice and potatoes are real calories from starch. The dog does not digest cellulose — raw, unblended vegetables pass undigested, so they need to be blended or lightly cooked.

Deficiency: Does not exist. A purely meat-bone-organ diet without vegetables is fully proper in both the dog and the cat — the calculator should not raise an alarm at a zero carbohydrate share [1]. The only niche exception is extremely hard-working sporting dogs, for which trainers add 5–10% starch on the days of greatest load.

Excess: A chronic excess of starch and sugars promotes obesity, insulin resistance, loose stools and undesirable fermentation in the gut. In the cat it is more dangerous — it is a recognized risk factor for type 2 diabetes; many cats with diabetes achieve remission after switching to a raw low-carbohydrate diet [3]. But the greatest risk of carbohydrates does not lie in themselves, but in what they displace: every percent of vegetables is a percent less of meat, organs and fat — and with them heme iron, zinc, taurine, B vitamins and omega-3.

Hierarchy of importance

Carbohydrates are a third-order topic. Before you optimize them, check the mineral balance (calcium, phosphorus, iodine, zinc), the fat balance (omega-6:3), protein and vitamins. A healthy, active dog tolerates 5–15% vegetables and fruit, but many dogs get none at all and function properly. In the cat the goal is a maximum of 3–5% carbohydrates in dry matter, and 0% is fully physiological. Pumpkin and psyllium are the best tools for intestinal problems — without significant starch.

Sources

National Research Council (NRC) (2006). Nutrient Requirements of Dogs and Cats — no specific carbohydrate requirement in dogs and cats. National Academies Press. https://nap.nationalacademies.org/catalog/10668/nutrient-requirements-of-dogs-and-cats
American Veterinary Medical Association (2019). Role of carbohydrates in the health of dogs. JAVMA 255(5):546. https://avmajournals.avma.org/view/journals/javma/255/5/javma.255.5.546.xml
Verbrugghe A, Hesta M (2017). Cats and Carbohydrates: The Carnivore Fantasy? Veterinary Sciences 4(4):55. https://doi.org/10.3390/vetsci4040055
Hand MS, Thatcher CD, Remillard RL, Roudebush P, Novotny BJ (2010). Small Animal Clinical Nutrition, 5th Edition. https://www.markmorrisinstitute.org/sacn5.html