On glycogen depletion after lifting: Resistance training does reduce muscle glycogen, but not to "depletion." A single bout of resistance exercise may deplete muscle glycogen by 24%–40%, depending on exercise intensity, duration, and the number of muscle motor units recruited. That means most glycogen remains after a typical lifting session.

On fat oxidation when glycogen is low: There is real physiology here. Endogenous carbohydrate oxidation was lower and fat oxidation was higher when exercise is initiated with low muscle glycogen. Similarly, glycogen depletion after postabsorptive exercise underlies an increase in accumulated fat oxidation over 24 h. Moreover, physiological responses to exercise shift toward greater fat use in this context. However, this is a shift in fuel proportion, not a magic "window" that only exists after lifting.

On the "fat-burning zone": The premise that walking in a specific low-intensity heart-rate zone maximizes fat loss is widely challenged. It is true that at a low intensity your body uses more fat for energy than it would at higher intensities. But "fat" here is referring to the type of nutrient, not the extra belly fat you want to get rid of. And while it is true that the body burns fat during low-intensity workouts, the total number of calories burned is lower. You would need to exercise for a longer period to burn the same amount of calories as you would at higher intensities. Total energy balance, not fuel percentage in the moment, drives fat loss. A controlled trial found that one trial was preceded by a rest day, and the other was preceded by a bout of exercise (60 min at 50% of V̇o2peak) with full replacement of the calories expended during exercise. There was no difference in total fat oxidation across 24 hours when calories were matched, undermining the idea that fuel-source manipulation alone drives fat loss.

On cardio ordering: There is some, but not conclusive, evidence favoring lifting-before-cardio. A recent trial reported that participants who performed weight training before cardio lost significantly more fat and became more physically active throughout the day compared to those who did cardio first. This is one study on 45 young men, not established consensus.

On cortisol and belly fat: The relationship is more nuanced than the meme presents. The idea is that weight gain is linked to high cortisol levels, often due to chronic stress. But there's little scientific evidence that cortisol is a primary cause of belly fat in most people. Chronic cortisol elevation is associated with visceral fat, but this is different from acute exercise-induced cortisol spikes, which are short-lived and generally beneficial.

On walking and cortisol: Some support exists that moderate walking is gentler on cortisol than high-intensity work. Cardio exercises like brisk walking, light jogging, swimming, or cycling for about 30 minutes daily can reliably reduce cortisol. Intensity should feel energizing, not exhausting. Consistency is key. However, cortisol also rises acutely during any meaningful exercise, including cycling: In a study examining 12 active men, researchers found that moderate- to high-intensity cycling workouts triggered spikes in cortisol. Acute cortisol spikes are not inherently harmful.

On HIIT causing fat gain via cortisol: The claim that HIIT and fasted early-morning cardio are counterproductive is not supported by strong evidence. HIIT has been shown in many trials to improve body composition. The idea that a single HIIT session's cortisol response "won't budge belly fat" conflates acute hormonal fluctuation with chronic pathophysiology.