A calorie deficit is the energy shortfall that results from using more energy than you absorb from food. It’s the key to weight loss.
When you create a caloric deficit your body is forced to meet this energy shortfall using stored energy from body fat (stored fat), muscle protein (stored protein) or glycogen (stored carbs). So creating and maintaining a calorie deficit is the key challenge of weight loss.
In this post we are going to run though what a calorie deficit is, how you can create one and why your body will adapt to protect itself from a prolonged deficit. Let’s begin!!
What is a Calorie Deficit?
A calorie deficit is the energy gap that results when we expend more energy than we absorb from food over a set period (we typically talk about a day). Creating a caloric deficit forces the body meet this energy shortfall from its energy stores.
These energy stores are glycogen (stored carbs), muscle tissue (stored protein) and body fat (stored fat). With glycogen in limited supply the aim during weight loss is to maximise the energy dragged from fat cells by maintaining as much muscle mass as possible.
Here’s a simple visual explanation:
In this graph the caloric deficit is the section in the middle. This is the energy the body has to draw from its energy stores to maintain energy balance. On the left hand side is the energy absorbed from all the food and drink that is eaten in the day, and on the right hand side is the energy expended.
Bottom line: In a deficit the body draws energy from its stores
The Simplistic Caloric Deficit
Creating and sustaining a caloric deficit is the primary challenge of weight loss. But the way most popular way it is described is far too simplistic. It’s often called the ‘3,500 calorie rule’ and a common version goes like this:
cut 500 calories a day and you’ll lose a pound of fat a week
Here’s the way I most commonly see it applied for men and women.
Now if you happen to be a woman that expends about 2,000 calories a day, or a man that burns 2,500, this type of deficit will be a great start for you. But this simplistic thinking has two major problems.
- Energy needs vary enormously between people
- Energy needs adapt over time in response to diet
Let’s tackle these two issues one at a time..
Bottom line: Energy needs are varied and change over time
Personalising the Caloric Deficit
The idea that men need 2,500 calories a day and women need 2,000 calories a day is a massive oversimplification. Let me show you the true energy needs of a sample of 645 people adults aged 20-70.
You can see energy needs are all over the place. Here’s what average actually looks like:
- 65% of women need 2,000-2,800 calories
- 60% of men need 2,600-3,800 calories
These two ranges sit in the middle of a bell curve distribution for energy needs. Think of this like shoe sizes, height or weight. Although some people need lots of calories and some very few, most people sit in the middle.
To give you a clearer idea of what this means for setting calorie targets to create a caloric deficit I’ve applied a 25% deficit to this entire distribution.
Bottom line: Energy needs are personal
A 25% Calorie Deficit for Women
Here’s the female distribution after a 25% deficit has been applied.
Let’s look at the key points in terms of different needs.
- low energy needs: target range 1,200-1,500 calories
- average energy needs: target range 1,500-2,100 calories
- high energy needs: target range +2,100 calories
The calorie target for an average woman trying to create a 25% falls into a 1,500-2,100 calorie target range. Women with low energy needs will be forced to eat 1,200-1,500 to achieve this deficit. While high energy need women can lose well at +2,100.
I’ve shaded all the figures below 1,400 calories in orange because because these targets deserve some caution. In this sample 1,400 calorie figure does not meet the resting metabolism needs of 40% of this sample. Although it is well above the NIH safety floor of 1,200 calories for women.
Bottom line: Most women need 1,500-2,100 calories for a 25% deficit
A 25% Calorie Deficit for Men
Here’s our male distribution after the 25% deficit has been applied.
Here’s what we can see for men:
- low energy needs: target range 1,600-2,000 calories
- average energy needs: target range 2,000-2,700 calories
- high energy needs: target range +2,700 calories
The bulk of our men create a 25% deficit on 2,000-2,700 calories. Some lucky guys will do well at +2,700 while those with low needs will need to jump straight in below 2,000.
I’ve shaded all the figures below 1,800 calories in orange because this figure doesn’t meet the resting metabolic needs of 43% of this sample. Although once again this is well above the NIH safety floor of 1,500 calories for men.
Now that we’ve dispelled the myth that our energy needs a similar, we also need to remember that energy needs change.
Bottom line: Most men need 2,000-2,700 calories for a 25% deficit
Why Your Caloric Deficit Shrinks
One of the reasons it so hard to lose weight and then keep it off is that your body actively fights being in a deficit, by reducing energy expenditure and increasing hunger. That’s right, there’s a part of your brain (the hypothalamus) that actively tries to defend your fat mass so you can remain in reproductive health.
This is a wonderful adaptation in times of food shortage, but not great when you live in a world full of junk food and just want to shift a few pounds.
To show you how this works I grabbed some data from a great free living study that looked how people adapt to caloric restriction. I’ve used data from 12 people that were placed on a 25% caloric deficit for three months. Here’s what their average adaptation looked like:
Over the three months on a 25% calorie restriction the individuals in the group lost an average of 13 lbs (6kg).
The really interesting thing thought is that at the end of the three months on the diet the original 712 calorie deficit had been reduced to just 258 calories. Average energy expenditure fell by 454 calories, mostly drops in activity and to a lesser degree resting metabolism. This adaptation was 371 calories greater than what was expected from the change in muscle and fat mass alone, something people in the field call ‘adaptive thermogenesis‘.
Losing weight not only means we need less energy for moving a lighter body, it also triggers your body to become more efficient in maintaining cells, organs and tissues, as well as reducing our subconscious activity in order to conserve energy. This makes regaining weight particularly easy to do after significant weight loss.
Anyone trying to lose weight needs to realise that some level of adaptation is inevitable. So you need to be patient and do what your can to make you maintain a healthy metabolism. This can involve diet breaks, refeeds, heavy lifting and sufficient protein intake.
Bottom line: We adapt to weight loss by using less energy
How to Keep Your Deficit Going
Even if you do all the ‘right things’ like lift heavy, eat adequate protein, choose mostly whole foods and add strategic cardio, your body is still going to adapt as you lose weight. So to keep weight loss progressing you’ll need to adjust what you eat, or how much exercise you do.
Here is a rough attempt to explain this visually.
The image above is a stylized version of what a six month fat loss program could look like for a man going from 180 lbs (82 kg) down to around 160 lbs (73 kg). Our area of focus is the caloric deficit, the section coloured with grey stripes above the energy consumption but below energy expenditure.
This graph may look a little intimidating but the process is actually quite simple. Here’s the basic three step formula that has served bodybuilders well for generations:
- Find a way to eat a consistent amount
- Track what this does to your body
- Adjust gradually to keep progressing
Bottom line: We need to adjust to maintain the deficit
The bottom, bottom line
Creating a caloric deficit is the first priority for weight loss. You don’t necessarily need to count calories to do this, but you do need to find your own way of controlling what you eat.
Check out Five Simple Strategies for tips.