Description: Why do some people burn more energy at rest? Compare muscle, organs, body size and genetic influences—and find out whether protein shakes make fat loss easier.
Someone sees a surprisingly high “basal metabolism” number on a smart scale and thinks: “Great. I must have lots of muscle.”
Then the explanation gets crowded. The liver walks in. So do the brain, kidneys, body size and genes.
Hang on. Which of those settings can I actually change?
That is the useful question. Resting energy use is partly tied to body composition, partly to physiology and partly to inherited differences. A single scale reading cannot tell you which contributed how much.
What explains the differences?
Basal metabolic rate is the energy needed to keep the body operating at rest; walking and workouts are extra. Most home scales estimate this number rather than measuring it directly.
In a 2005 study of 150 adults, fat-free mass statistically explained 63% of between-person variation in basal metabolic rate, fat mass 6%, and age 2%. Roughly 26% remained unexplained by that model [1].
Fat-free mass includes muscle and organs, bones, blood and other tissue. That 63% is neither the share of calories used by muscle nor a promise that gaining muscle will raise your metabolic rate by 63%.
The muscle-versus-liver plot twist
Approximate resting energy use per kilogram of tissue per day is 13 kcal for skeletal muscle, 4.5 for fat, 200 for liver, 240 for brain, and 440 for heart or kidney tissue [2].
Your liver has never posted a gym selfie and is still doing outrageous numbers.
Muscle matters because there is a lot of it. Yet under a simplified model where everything else stays equal, five extra kilograms of muscle correspond to only about 65 kcal per day of additional resting expenditure. Actual changes depend on the rest of the body too. The idea that a few kilos of muscle automatically create a massive calorie-burning furnace is overstated.
Is the genetic lottery real?
Yes, genetics contribute. A 1989 family-and-twin study estimated that genetic differences accounted for about 40% of the remaining variation in resting metabolic rate after adjusting for age, sex and fat-free mass [3].
Notice the word remaining. It does not mean that 40% of anyone’s metabolism is permanently locked by DNA. Heritability is a population-level estimate for the conditions studied, not a personal destiny meter.
So what can you control?
Muscle can grow with resistance training, adequate food and recovery. A review combining 49 studies and 1,863 participants found that protein supplements modestly improved muscle and strength gains alongside resistance training [4]. If food already provides enough protein, a shake has no special powers.
And if the real goal is easier weight management, distinguish resting metabolism from total daily energy expenditure. Movement, exercise, everyday activity and food intake matter. Strength training also improves strength and capacity for activity—benefits far bigger than a single basal-calorie number.
The takeaway
You cannot assign your liver a training split or reroll your genes. You can build some muscle, move more and manage what you eat.
The genetic lottery affects the starting screen. Your best controls are not limited to that screen.
References (4)
- Johnstone AM et al. (2005). Factors influencing variation in basal metabolic rate. American Journal of Clinical Nutrition. https://pubmed.ncbi.nlm.nih.gov/16280423/
- Wang Z et al. (2010). Specific metabolic rates of major organs and tissues across adulthood. American Journal of Clinical Nutrition. https://pubmed.ncbi.nlm.nih.gov/20962155/
- Bouchard C et al. (1989). Genetic effect in resting and exercise metabolic rates. Metabolism. https://pubmed.ncbi.nlm.nih.gov/2657322/
- Morton RW et al. (2018). A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. British Journal of Sports Medicine. https://pubmed.ncbi.nlm.nih.gov/28698222/
