If you’ve ever wondered why someone else can eat 4000 calories a day and be lean and you can barely go over 2000 without straining the button on your pants, there’s a good reason.

Back in December, I shared my thoughts on making “good holiday gains” when most people let their diet slip a little (or a lot) during the holidays and how to use those extra carbs and calories towards something positive. This type of “strategy” is much like the idea of looking at G-Flux or nutrient partitioning – or basically where all the calories (protein, fat, and carbs) you eat end up going depending on how active you are and what types of activity you do.

Most people know as we age, our metabolic rate drops. We lose muscle mass (unless we do something about it), tend to move less, and end up eating less (or at least we should). The end result is a body with more fat mass and less muscle mass, resulting in the inability to eat as much as we used to (*don’t remind me!*) without gaining weight. Sorry, but it’s science.

Muscle tissue burns/uses more calories than fat tissue, and active muscles (being used for consistent exercise) obviously use more calories than inactive muscles. The graph below is a representation of the drop in metabolic rate from Precision Nutrition’s article on G-Flux.

As the sad realization hits you, you may be thinking, “So, I can’t eat as much food and my body is going to turn to mush? Why is the world so cruel??”

Luckily, the idea of capitalizing on your G-Flux can help you out.

If you’ve ever wondered why someone else can eat 4000 calories a day and be lean and you can barely go over 2000 without straining the button on your pants, there’s a good reason. You can eat more and end up leaner than you are now, but in order to reap those benefits, you also need to move more.

Just think about the amount of food a person eats if they lift weights 5 hours per week compared to the person who walks or jogs 5 hours per week (if you can’t imagine it, find someone who fits the bill and ask them). The weight lifter will be using the same amount of time to exercise but will typically end up having more lean body mass and need to eat more food to maintain/build it.

The body will consume and use energy. A body that uses its muscles by staying consistently active will tend to be leaner and need to consume more calories. A body that moves less and contains less muscle mass will tend to consume less. This is why one person who moves a little can eat and burn 2000 calories a day and maintain the same weight, and another person can eat 5000 calories a day and use 5000 a day and maintain the same weight.

Moving a lot doesn’t necessarily mean you can go hog-wild and it’ll turn to lean muscle mass, though. Eating a well-rounded diet is key to making sure you’re fueling all of that activity, staying “in balance” with your G-Flux, and moving towards a leaner and stronger body.

Skim this article on G-Flux from Precision Nutrition to learn more.