Dogs get energy primarily from fats and proteins, with carbohydrates playing a supporting role. Unlike humans, who rely heavily on carbohydrates as their default fuel source, dogs are built to run on a fat-and-protein engine. Their liver processes amino acids from protein at roughly three times the rate it processes glucose, making protein a central part of their energy system rather than a backup.
Fat and Protein: A Dog’s Primary Fuels
Fat is the most energy-dense nutrient a dog can eat, packing more than twice the calories per gram compared to protein or carbohydrates. When a dog eats a high-fat meal, its liver ramps up a process called ketogenesis, breaking fat down into molecules the body can burn for fuel. Dogs fed low-carbohydrate, high-fat diets show significantly higher levels of these fat-derived fuel molecules in their blood, confirming that their bodies readily shift toward burning fat when it’s available.
Protein serves double duty. Dogs use amino acids from digested protein to build and repair tissues, support immune function, and fuel growth. But their liver also converts amino acids directly into glucose through a process called gluconeogenesis. This is driven by the hormone glucagon and is a major energy pathway for dogs. In fact, when dogs eat a mixed meal, the liver absorbs about three times more amino acids for glucose production than it absorbs actual glucose. The byproduct of this conversion shows up as blood urea nitrogen, which is why healthy dogs naturally have higher levels of it compared to animals that rely less on protein for energy.
When a dog’s diet includes enough carbohydrates, the body can use those carbs for energy and spare protein for other jobs like tissue repair and immune defense. This is one reason balanced commercial dog foods include both protein and carbohydrate sources.
How Dogs Handle Carbohydrates
Dogs can digest starches more efficiently than their wolf ancestors. During domestication, dogs developed greater expression of the gene for pancreatic amylase, the enzyme that breaks starch into simple sugars. This adaptation to starch-rich diets was likely a crucial step in early domestication, as dogs living alongside humans had access to grain-based scraps and cooked foods that wolves never encountered regularly.
Once starch is broken down into glucose and absorbed through the small intestine, a dog’s body has three options: burn it immediately for energy, store it in the liver and muscles as glycogen for short-term use, or convert it to fat for long-term storage. The liver acts as a glucose thermostat. Early in a period without food, the liver breaks down its glycogen stores to release glucose into the bloodstream. During longer fasts, it switches to manufacturing new glucose from amino acids, lactate, and glycerol. Both pathways require the same final enzyme to release glucose into circulation, which is why disorders affecting that enzyme can be so serious.
From Food to Fuel Inside the Cell
No matter whether energy starts as fat, protein, or carbohydrate, it all converges on the same destination: the mitochondria inside each cell. Mitochondria are the structures that produce ATP, the molecule that powers nearly every physiological process in a dog’s body. Heart contractions, brain signaling, muscle movement, digestion: all run on ATP.
Mitochondria work by combining nutrients with oxygen in a tightly controlled chain of chemical reactions. The efficiency of this process varies between individual dogs and even between breeds. Long-lived dogs tend to show different patterns of mitochondrial performance compared to short-lived breeds, suggesting that how well cells produce energy influences overall health and lifespan. Mitochondria also generate heat as a byproduct, which is part of how dogs maintain their body temperature.
How Digestion Delivers Nutrients
After a dog eats, food passes through the esophagus in about 13 minutes and spends a long time in the stomach. In one study of Beagles, food remained in the stomach for an average of roughly 14 hours after a meal, being broken down by acid and enzymes before moving into the small intestine. The intestinal phase, where the real absorption happens, lasts another 14 hours on average. Total transit time from mouth to elimination averaged about 28 hours.
In the small intestine, each nutrient type has its own absorption method. Simple sugars and amino acids are actively transported across the intestinal wall using specialized carrier proteins. Fat absorption works differently: digested fats passively diffuse into the intestinal lining and enter the lymphatic system before reaching the bloodstream. Iron is absorbed in the first sections of the small intestine, while vitamin B12 is absorbed further down, in the ileum. This is why damage to different parts of the intestine can cause very specific nutritional deficiencies.
How Much Energy Dogs Actually Need
A dog’s energy needs start with its resting energy requirement, the calories needed just to keep basic functions running: breathing, pumping blood, digesting food, and maintaining brain activity. Veterinarians calculate this by multiplying the dog’s body weight in kilograms, raised to the three-quarter power, by 70. For a 10-kilogram dog (about 22 pounds), that works out to roughly 400 calories per day just to stay alive at rest.
Real life demands more than resting, though. The actual daily calorie need depends on life stage and activity level. A neutered adult dog typically needs about 1.6 times its resting requirement, while an intact adult needs about 1.8 times. Puppies under four months old need three times their resting energy to support rapid growth, dropping to about twice the resting level after four months. Dogs prone to obesity may only need 1.4 times the resting amount. That 10-kilogram neutered adult, for example, would need around 640 calories per day rather than the 400 calories calculated at rest.
When Energy Production Fails
When a dog’s blood sugar drops below normal levels, the results can be dramatic. Hypoglycemia in dogs is defined as a blood glucose concentration below 60 mg/dL, but visible symptoms usually don’t appear until levels fall below 40 to 50 mg/dL. At that point, you may notice changes in behavior and alertness, weakness, muscle tremors, stumbling, or even seizures and collapse. Some dogs show signs of a stress response instead: restlessness, rapid breathing, trembling, vomiting, or sudden ravenous hunger.
Small breed puppies and toy breeds are especially vulnerable because they have limited glycogen reserves and high metabolic rates relative to their body size. In adult dogs, persistent low blood sugar can signal an underlying condition affecting the liver, adrenal glands, or pancreas. The hallmark of true hypoglycemia is that symptoms improve when blood sugar is corrected, a pattern veterinarians refer to as Whipple’s triad.