Your Body is Burning Carbs, NOT Fat Cells (here’s how to tell)

DeLauer explains that the crossover from fat to carb burning is dictated by the balance of energy demand and oxygen availability. When oxygen can't keep up, the body shifts to glycolysis, which produces lactic acid. The body buffers this acid with bicarbonate, generating CO2, which must be exhaled. This causes a sudden increase in breathing rate — the ventilatory threshold. Research in Frontiers of Psychology found that during maximal effort, the crossover occurs at ~76% VO2 max, but at submaximal effort it happens at just 50-58% VO2 max. So for many people, activities like climbing a flight of stairs or walking up a hill trigger this shift and heavy breathing. By tuning into this sensation, you can infer that you're burning carbs. DeLauer cautions that illness can also cause heavy breathing, but under normal health, it's a reliable marker. Long-term low-carb dieters might notice this even more acutely after eating carbs because their bodies are carbohydrate-intolerant and switch gears rapidly.

DeLauer draws on research showing that the initial exercise response triggers a cortisol surge that immediately pulls from glycogen stores. He extends this to everyday stress: even the momentary fear of a police car siren can spike cortisol and flip the metabolic switch. Adrenaline and norepinephrine might promote fat oxidation over longer periods, but the acute stress window is carb-driven. Chronic stress compounds this by keeping glycogen use high and promoting gluconeogenesis from muscle tissue and glycerol, meaning you stay in a carb-burning, catabolic state. This protocol is about mental tagging: when you feel that stress, you can assume you're not burning fat at that moment. It's a tool to understand why someone under constant stress might stall in fat loss, even if diet is optimized.

DeLauer explains that when fat utilization is high, the body upregulates oxidative enzymes and mitochondrial density, particularly in slow-twitch muscle fibers. This allows sustained energy output without heavy lactate buildup. He cites the feeling of being on 'cruise control' — where you could keep walking or jogging almost indefinitely. This is a hallmark of high fat oxidation. In contrast, when you're relying on carbs, glycogen stores deplete quickly and lactate accumulates, causing muscular fatigue. He gives the example of elite runners like Zach Bitter, who can sense the shift from fat to carbs. For the average person, this manifests as the point where a walk on flat ground feels sustainable forever, but a slight increase in pace makes you feel like you can't continue. That transition indicates carb entry.

DeLauer ties mental clarity to network stability. In a fat-burning state, the brain's networks are calmer and communicate more efficiently, sharpening executive function and memory. A high-carb meal, however, often leads to a decline in that clarity — the classic post-lunch fog. He suggests using this as a subjective indicator: if you eat and feel a mental slump, you've probably pushed yourself out of fat oxidation. This doesn't demonize carbs, but it offers a biofeedback tool: when mental clarity is high, you can assume fat is your primary fuel.

DeLauer cites a meta-analysis from Neuroscience and Biobehavioral Reviews that aggregated 31 studies giving carbohydrates or placebo. There was no significant positive mood change attributable to carbohydrates. In fact, the studies showed decreased energy and decreased alertness after carb consumption. He explains this with 'rebound hypoglycemia' — a rapid insulin spike drops blood glucose below baseline, causing fatigue. Moreover, he argues that ATP production is not simply increased by dumping more glucose into the system; energy output is determined by cellular demand. If you just increase glucose availability without a matching activity demand, the body downregulates fat oxidation instead of creating extra energy. The only scenario where carb intake consistently provides sustained energy is during continuous high-volume consumption, like Tour de France athletes who eat constantly to fuel massive output. For the average person, eating carbs likely reduces fat burning without delivering extra usable energy. He acknowledges FGF-21 as a nuance but sets it aside.

DeLauer references a study from 'Food Components to Enhance Performance' that examined the initial exercise response. At the start of exercise, a big cortisol release occurs, which immediately increases carbohydrate utilization and taps into glycogen stores. He extrapolates this to any acute stressor: the moment you feel an adrenaline and cortisol surge (e.g., seeing a police car and worrying about a ticket), your body pulls from glycogen and upregulates gluconeogenesis to ensure glucose availability. This means even non-physical stress pushes you out of fat oxidation. For those chronically stressed, this becomes a persistent state, where carb utilization stays high and fat utilization is suppressed. He also notes that while adrenaline and norepinephrine typically promote fat oxidation, the short-term stress response overrides that with glycogen use. Additionally, gluconeogenesis from both amino acids and the glycerol backbone of triglycerides increases, further elevating blood sugar during stress.

DeLauer describes 'network stability' as the ability of different brain regions to communicate effectively. When the body isn't flooded with exogenous carbohydrates, the brain operates in a calmer, less erratic mode, which actually speeds up processing. He cites the saying 'a slow brain or a calm brain is actually a fast brain.' This state improves fluid intelligence, executive function, and even episodic memory. He contrasts it with the post-carb meal fog many experience, noting that mental clarity often drops after a high-carb lunch. This provides a subjective cue that one is in fat-burning mode: if you feel mentally sharp and your mind isn't sluggish, you're likely metabolizing more fat. This ties into the broader idea that fat oxidation supports sustained, low-stress energy supply to the brain, unlike the peaks and troughs from carb-driven glucose spikes.