A lot of people take their macros as a “set in stone” type of thing. You shouldn’t worry about hitting the mark every single day to the dot. If you’re a few calories over some days, a few calories under on others – it’s fine. Everything will even itself out in the end. It’s all about a long term plan that can work for you, and not the other way around.
The hypothalamus is the brain's main center responsible for hunger/satiety (H/S) control. In the theory that Mayer proposed more than 60 years ago, he assigned a central role to glucose levels in the H/S control: the so-called “glucostatic theory” (Mayer, 1955). Mayer suggested that depletion of carbohydrate availability leads to hunger, and the hypothalamic centers with receptors sensitive to glucose levels might be involved in the short-term regulation of energy intake (Mayer, 1955). The “feeding center” in the lateral hypothalamic area (LHA), according to the glucostatic theory, reacts to the between-meal fall of blood glucose and stimulates food intake. The LHA contains glucose-inhibited neurons that are stimulated by hypoglycemia, a process crucial to mediating the hyperphagia normally induced by hypoglycemia. The subsequent post-prandial hyperglycemia activates the “satiety center” in the ventromedial hypothalamus (VMH), which contains glucose-excited neurons and inhibits both “feeding center” and food intake.
The second type of cellular fuel comes from fat and fat metabolism products called ketone bodies.   The average sized human body can store hundreds of thousands of calories in the form of fat, so we could say that this system of energy is almost unlimited, depending on how long one goes without food.  Eventually, it would get used up, but people have been known to fast for months and live through it.
When you’re eating the foods that get you there (more on that in a minute), your body can enter a state of ketosis in one to three days, she adds. During the diet, the majority of calories you consume come from fat, with a little protein and very little carbohydrates. Ketosis also happens if you eat a very low-calorie diet — think doctor-supervised, only when medically recommended diets of 600 to 800 total calories.
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