Essentials: The Science of Eating for Health, Fat Loss & Lean Muscle | Dr. Layne Norton

Andrew Huberman opens this Essentials episode by asking Dr. Layne Norton a basic question that turns out to be complicated: what happens when we eat food, and how does it become energy? He frames this as the starting point for talking about weight loss, weight maintenance, weight gain and body composition.

Norton's answer starts with a common complaint. People call "calories in, calories out" too simplistic, but he argues that the parts that make up each side of the equation are "actually very complicated." A calorie is a unit of heat energy. In food, it refers to the potential chemical energy stored in the bonds of macronutrients. Digesting, absorbing and metabolizing those nutrients mostly produces ATP, which he calls "your body's energy currency."

He then sketches how each macronutrient feeds that process. Protein is broken into amino acids. These can be used for muscle protein synthesis or protein synthesis in other tissues, converted to glucose through gluconeogenesis, or, for some ketogenic amino acids, used in other ways. Fatty acids go through beta oxidation, which removes two carbons at a time to make acetyl-CoA. Acetyl-CoA enters the Krebs cycle and produces hydrogen ions that power ATP production.

On the energy-in side, Norton names two sources of error. First, food labels are less accurate than people assume:

food labels, which we like to think as being from upon high, can have up to a 20% error in them.

Second, he separates a food's energy from its metabolizable energy. Insoluble fiber from plant material is largely indigestible. Carbohydrate, and even some protein, can be bound up in the plant structure where digestive enzymes cannot reach it. That adds bulk to stool but lowers the energy actually absorbed. He adds that there is "some evidence" that people's gut microbiomes may make some of them better than others at extracting energy from fiber.

Norton expects the objection that a 20% label error makes calorie tracking pointless, and he disagrees. His reasoning is that "typically if it's off, it's going to be consistently off." If you track the same way every time, you will eventually get a usable sense of what you are eating, even if the absolute numbers are wrong.

Norton calls the energy-out side "way more complicated" and splits it into several buckets. The largest is resting metabolic rate (RMR), which he puts at 50% to 70% of total daily energy expenditure for most people. Sedentary people sit at the higher end of that range. For more active people the share is lower, "not because their metabolic rate is lower," but because physical activity accounts for more of their calories.

The thermic effect of food (TEF) is the energy needed to extract energy from food. He puts it at about 5% to 10% of total expenditure and calls it "very difficult to measure." Researchers usually just assume a constant value for it. Here he pushes back on the slogan that "not all calories are created equal":

That's not true because calories are just a unit of measurement. That would be like saying not all seconds on a clock are created equal.

He restates the point more precisely: all calories are equal as a unit, but "all sources of calories may have differential effects on energy expenditure and appetite." He gives approximate TEF figures for each macronutrient: