Celebrity trainers and buff social media stars use terms such as “shred”, “burn” and “melt” to describe bodies responding to resistance training and cardiovascular exercise with rapid physical transformation.
In the kitchen, shredding a carrot takes just a few minutes and results in destruction of a solid into small, manageable parts. Burning involves heat and occasionally pain, and can occur in just seconds (especially if you turn your back). Solid fats melt into liquids that can be drained away.
But do these terms actually describe what’s going on when we exercise? A simple analysis of how our body uses, stores and mobilises energy says no, they don’t.
The currency of immediate energy
When we have a meal, the gut breaks down the food into carbohydrates, lipids (fats) and protein, which are released into the blood stream.
For immediate energy requirements, our bodies use multiple biochemical pathways to convert these components into a high energy compound known as ATP (adenosine triphosphate). The energy released is used to keep us awake, sustain breathing, work our brains and for some physical exercise.
In a sense, ATP is the “currency” the body uses to complete daily bodily functions and physical tasks. In its ready-to-be-traded-for-energy form, the total amount of ATP stored in cells at any given time only lasts about two seconds.
The rate of ATP production is adjusted constantly to the amount of energy we require at any given time. For example, when we are asleep we require less ATP than when we are in the middle of a workout on a treadmill or using weights.
So what happens if we have a meal and don’t require a lot of energy in the short term? Rather than the meal being converted into ATP, it is transformed into stored energy inside our body for later use.
Energy stored for use later
While our body doesn’t store a large amount of ATP, it does store nutrients away from the bloodstream so that we can access them in between meals and during the fasting hours at night time. When energy demands increase through exercise, we use these stored nutrients to respond.
Proteins are mainly used as building blocks for skeletal muscle, hormones and other compounds. Proteins only provide around 5% of energy required for exercise.
Carbohydrates are stored in the form of a complex molecule called glycogen in skeletal muscles and the liver.
Molecules known as free fatty acids are created from dietary fats, and are converted and stored as fat throughout the body if not immediately used. But body fat doesn’t just come from dietary fat: once we reach the maximum storage capacity for glycogen (carbohydrates), we convert the excess carbohydrates into body fat too.
Why do we tend to accumulate fat around our bodies so easily? Because it’s the most effective way to store energy, providing 10-15 times the amount of energy as glycogen. Accumulation of body fat can be substantial, and often higher than we might like for optimal health.
What do we use when we exercise?
During exercise, 95% of the energy we use comes from glycogen and body fat, and the proportion of each depends on the intensity of exercise.
Carbohydrates stored as glycogen offer a medium term source of energy: these can be mobilised to service approximately two hours of high intensity exercise. Glycogen is the type of stored energy you use if you run a short to middle distance race at full pace - it’s the source of energy for what’s called “anaerobic exercise”.
The lower the exercise intensity, the higher the percentage of lipids we use to fuel the exercise. Relatively easy but sustained workouts will use fat as a primary source of energy. Body fats provide almost unlimited energy for weeks or even months. The best way to lose fat accumulated around the body is to engage in frequent, sustained and low intensity exercise. This type of exercise is called “aerobic exercise”.
Regardless of how much fat we store and use for energy, the number of fat cells (also referred to as adipocytes) in our body remains stable. Greater fat storage simply increases the size of each fat cell. When you lose weight, each fat cell shrinks.
Similarly, when we build muscle bulk by lifting weights, we simply increase the size of each skeletal muscle cell.
A lifetime approach
Although getting rid of fat is a long process, the process of gaining it is also relatively slow. The science of how we use stored energy means that if you want to sustainably lose weight, there are no short-cuts. A lifestyle change in which you commit to engaging in exercise over the long-term is the best approach.
So how do some diets promise to lose fat in barely days or a few weeks? It’s a misconception - what you are losing in most cases is water through dehydration, and in some cases muscle mass, but rarely fat. In most of these cases, lost weight is regained rapidly.
It is metabolically impossible to lose a high amount of fat in a very short period of time, unless you exercise four to six hours every day.
To maintain an appropriate weight after losing any excess and reaching your optimal body mass, you should balance energy intake with energy output. It is that simple: you need to use up the energy equivalent of what you are eating.
The good news is that any type of physical activity is useful to keep this balance in check: circuit-classes, gym work, team sports, yoga, running, golf, gardening, cycling, walking and more. The main goal is to engage in some form of activity and to maintain a relatively healthy and appropriate amount of energy intake.
Burning, melting and and shredding are weight loss marketing terms that don’t accurately describe how our bodies respond to exercise over the short and long term.
A focus on ingesting macro and micronutrients in the same amounts as you convert them into energy for bodily functions and daily routines will help you avoid storing nutrients as excess body fat.
About The Author
Naroa Etxebarria, Assistant Professor Sport and Exercise Science, University of Canberra