Fasted cardio and very low carb diets have both been consistently touted as the special ingredient for fat loss due to their ability to increase fat burning.
You don’t have to be a veteran of Bodybuilding.com to have heard that skipping breakfast before some morning cardio or eating chicken and broccoli, hold the rice, is the secret to getting ripped.
Both fasted cardio and very low carb diets have gained popularity based on the assumption that if your body does not have carbs to burn for energy, it will burn fat instead.
From a superficial level, it would make sense to assume that burning fat for energy would equal fat loss. But does this claim really check out?
What is fat burning?
‘Fat burning’ or fat oxidation refers to the use of fat as a fuel source by the body.
Fat is stored in our bodies in the form of triglycerides. Triglycerides can be found in the bloodstream, in adipose tissue (fat cells) and to a lesser extent, within muscle mass.
In order to be used as fuel, the triglycerides need to be broken down into fatty acids through a process called lipolysis.
Your body will prioritise utilising the triglycerides in the bloodstream and muscle first. Once these supplies have run low, triglycerides will be mobilised from the fat cells.
This is where the notion of fat burning must equal fat loss comes from. Your body is literally taking fat out of your fat cells to produce energy.
However, if you are not in a calorie deficit, fat burning will not result in actual fat loss. This is due to the fact that energy does not just disappear.
Fat is a storage of energy and calories are just the unit in which we measure that energy.
If you are putting 2000 calories a day into your body and that’s how much energy your body burns in a day. The body can’t magically burn through its energy storages because what you are feeding it is adequate for the energy it requires.
Another way to look at it is, your body will tend to utilise what you are feeding it the most.
If you are feeding your body a lot of carbs, it will be using predominantly carbs as an energy source. Vice versa, if you are feeding your body high amounts of fat and very little carbs, it will burn mostly fat as a fuel source.
This is because that is what is most available in the bloodstream. If you are continuously eating high fat, low carb meals, there will be a continuous source of triglycerides in the bloodstream to oxidise for energy. There will also be a higher rate of fat storage.
So although your body may be mobilising fat from fat tissue, it will also be storing fat at the same time equating to higher fat burning but nil fat loss.
When energy in versus energy out is balanced on a low carb, high fat diet. Fat oxidation and fat storage is also balanced.
The history of low carb diets
Low carb diets have been a staple of diet culture for what seems like forever.
Low carb diets have even been dated back to Ancient Greece where olympic athletes would eat a diet high in animal products and very low in grains and other plant foods.
This was thought to provide some performance benefit for these athletes.
Low carb diets for weight loss and specifically fat loss emerged in the mid 1800s in a variety of places around the world. Furthermore, low carb for weight loss gained most of its current popularity in the early 1970’s with the emergence of The Atkins Diet.
The Atkins diet has received many spin off fad diets since the 70’s including a couple of updates of its own.
The most well-known fads to come from Atkins are the Keto and Paleo diets which both limit carbohydrate intake and promote higher fat consumption.
Low carb diet and fat loss
Since the low carb narrative has persisted for literal decades, you would think that there would be sufficient evidence to show its superiority to other diet approaches.
But alas, there is no such evidence.
Research shows that whilst low carb diets increase fat oxidation, they don’t actually result in superior fat loss in comparison to higher carb diets when calories and protein are matched.
When studied, neither low carb diets or low fat diets when combined with a calorie deficit have been found to be significantly better than the other for fat loss long term.
Fasted training – what is it actually good for?
Fasted cardio is another term with a rich history in both diet culture and the fitness industry.
It carries the same premise that if you do not have carbohydrates available as your body’s fuel source, fat will be mobilised from the fat cells and used as energy instead.
This is true. Fat burning is higher in fasted training versus training after a carb rich meal.
Endurance athletes even strategically use fasted training or training with very minimal carbohydrate intake to enhance their ability to use fat as a fuel source. This practice is called ‘training low’.
However, once again fat burning does not equal fat loss.
Calorie deficits & fat loss
When energy or caloric intake is equal to total energy expenditure from daily activity and exercise, your body will not need to tap into reserved energy stored as body fat.
It will have everything it needs from the food you have consumed that day.
If you want to lose body fat you have to eat fewer calories than you expend, independent of the fuels you use!
For example, let’s compare the same imaginary person on two different diets for weight loss.
Both diets are calorie matched (both 2000 calories). In this example, 2000 calories per day puts this person in maintenance calories because her usual daily energy expenditure is also 2000 calories.
Despite one diet containing 100g of carbs and the other containing 300g of carbs, both diets will result in keeping the person’s weight stable because there is no caloric deficit in either.
Now if we took both of those diets and made them 1600 calories.
Calories in < calories out
Both diets would result in very similar, if not the same weight loss because the body is required to take the same amount from its fat storages to make up the other 400 calories. This would reduce the size of the fat cells over time at a fairly equal rate.
The greater the discrepancy between the calories you consume and the calories you expend, the greater the shift in body weight will be.
The proportion of fats to carbs in your diet is almost irrelevant.
“Fat burning” and “fat loss” are not interchangeable concepts.
The biggest factor to weight or fat loss is the old fashioned calorie deficit. If you are not in a calorie deficit, you will not lose body fat!
Some people may find that significantly reducing carbs over fat or vice versa, may help them to maintain a calorie deficit and therefore make weight loss easier. But going low carb will not inherently be better for fat loss when calories are matched.
Moral of the story: skipping the carbs will increase fat oxidation but will not cause fat loss to occur. Nor will it significantly increase the rate of fat loss whilst in a caloric deficit.
Leah is an accredited practising dietitian from Brisbane. She also competes as an under 75kg powerlifter with Valhalla Strength Brisbane. As both an athlete and dietitian, she spends much of her time developing her knowledge and skills around sports nutrition, specifically for strength-based sports. Although, she works with a range of athletes from triathletes to combat sports and powerlifting.
Leah also follows a plant-based diet and her greatest passion is fuelling vegan/vegetarian athletes and proving that plant-based athletes can be just as competitive as their non-vegan counterparts.