“I want to look like Giselle!”
“Why does my friend stay skinny when he/she eats whatever they want and I eat perfectly and I can’t get lean?”
I hear these comments and questions almost every day. Assuming training programs and diet are equal , there are genetic factors that limit potential for fat loss.
Genetic variation can explain significant differences in metabolic rate among individuals. These genetic differences mean that certain individuals, sometimes entire populations (for example, Pima Indians and Samoans), are extremely energy efficient.
The easiest way to explain this is to compare a Toyota Prius to a Porsche Carrera. With 1 gallon of fuel, a Toyota Prius can travel about 50 miles, the Porsche about 22 miles. For the same amount of fuel , the Prius can do twice as much work! The same concept applies to human metabolism. How efficiently we burn our fuel (calories) varies between individuals.
Person A may be a Porsche — extremely inefficient with fuel and using 100 calories to perform a mile run. Person B may be a Prius; in that same mile run, they are so efficient they only expend 50 calories. Unlike a car, where fuel efficiency is considered a good thing, being more efficient when burning calories actually predisposes you to gaining fat more easily and not losing fat quite so easily.
In a study performed by Bouchard, twelve pairs of identical twins were put into 1,000-calories-per-day surpluses for 84 days over a 100-day period (84,000 excess calories, a projected 24lbs fat surplus). During this 100-day period, all subjects remained sedentary. Results showed that within a set of twins, fat gain and distribution was VERY similar; however, between different sets of twins, it varied significantly. The average weight gain was 17.86 pounds, but the range was 9.48 pounds to 29.32 pounds. Yes, some sets of identical twins gained 3x the amount of fat in a 100-day period with the same eating and activity routine as the other sets.
These differences in fuel usage apply during exercise and at rest. Individuals with these genetic predispositions to burn fewer calories are influenced to a greater extent by over-consumption. It does not mean they can’t lose weight by counting calories — it just may require VERY different efforts for some to lose weight, and different end points will result. For this reason, do not confuse variation in size with variation in effort. Excess body fat is not necessarily a sign of laziness or lack of desire. It simply may be a sign of genetic differences.
Environmental factors also play a role in differing fuel efficiencies between individuals and within the same person over time. One environmental factor is dieting. Extreme calorie deficits cause our bodies to become more efficient. For more information on this phenomena read Mike’s blog on adaptive thermogenesis.
To be clear, these differences do not completely explain the obesity epidemic at the population level, and not every overweight person suffers from these traits. Fifty years ago, humans were the same as they are now from a genetic and metabolic perspective. Yet today obesity rates are MUCH higher. The majority of obesity today is caused by overconsumption of nutrient-void / calorie-dense foods, which causes these traits to manifest themselves.
Genetics can play a significant role in fat gain and fat loss. While much attention is paid to the aesthetic changes exercise and healthy eating bring , don’t forget the health benefits of being more fit. Individuals who are fit, but have excess fat, are better off than those who are thin, but not fit!