It makes sense.
Think of it this way: no immediate fuel supply from a prolonged fast (12 hours+) and no pre-workout meal means your body has to find alternate sources of energy besides glucose. That source? Fat.
It would be logical to think that you’d burn the fat from your midsection if you were to train abs hard in a fasted state, right?
It’s plausible, but it’s a little more complicated than merely being able to target your abs and get the tight, toned core you’ve always wanted by training abs in a fasted state.
Luckily, we’re here to help you. By the time you’re done reading, you’ll understand the importance of fasting and what happens during a prolonged fast, why training in a fasted state may be effective for fat burning, and how you can maximize your ab works to achieve the results you want.
Why You Should Fast
The entire premise behind training in a fasted state is that you can ramp up fat loss because you force the body to adapt to different fuel substrates.
In a fed state, your cells are in “growth” mode. Insulin signaling and mTOR pathways are in top gear, telling your cells to grow, divide, and synthesize proteins.
In a well-fed state, your body has no concern over things like cellular clean-up and recycling (autophagy); it’s much too preoccupied growing and dividing.
When you are fed, your body turns on the genes for growth and proliferation, but it turns other genes off - genes related to fat metabolism, stress resistance, and damage repair.
However, during starvation, or prolonged periods of fasting, things are quite a bit different.
The body sees fasting as a type of environmental stress (i.e. low food availability) and reacts by changing gene expression to protect you from the so-called “stress”.
This stress mode kicks your cells into a completely different and unique mode when glucose and sugar aren’t around.
The 5 Phases Of Fasting
According to experts, there are five stages to intermittent (or prolonged) fasting. Here’s what happens 1:
* Keep in mind that these 5 phases are further reaching than what a typical 16 hour intermittent fast is, but it gives you an idea of what switches are turned on in the body during prolonged periods without food.
- By 12 hours—the body has depleted its glycogen stores and has entered into the metabolic state of ketosis, whereby fat is being broken down and burned to supply energy. Some of that fat is used by the liver to produce ketone bodies, which serve as an alternate energy source for the heart, brain, and yes, even skeletal muscle. Ketones burn very clean (there are fewer by-products produced during the metabolism of ketones than glucose), but they can also give a kick to the production of brain-derived neurotrophic factor (BDNF).
- By 18 hours—your body is in fat-burning mode, and you’re producing high levels of ketones. As blood ketone levels rise, they can act as signaling molecules that tell your body to switch the stress-busting pathways into top gear that curb inflammation and repair cellular DNA.
- By 24 hours—you’ve started into autophagy, the process of recycling old cell components and breaking down dysfunctional proteins to replace them with new ones. Without the process of autophagy, the risk of developing health conditions like neurodegenerative diseases increases 2. Fasting also activates the AMPK signaling pathway, which inhibits mTOR activity stimulating autophagy. Exercise, together with fasting, can increase autophagy in humans 3.
- By 48 hours—your body is running on limited calories and growth hormone levels have spiked up to 5x higher than when you started your fast 4. This is partly because ketones and ghrelin, your hunger hormone, both promote GH secretion 5. But interestingly, ghrelin also helps preserve lean muscle mass and decrease fat tissue so lack of food isn’t compromising your muscle mass and strength.
- By 54 to 72 hours—insulin levels are at their lowest point and insulin-sensitivity is increasing 6. Low insulin levels have a wide range of health benefits, but it also halts the insulin and mTOR signaling pathways, which keeps autophagy going. Prolonged periods of fasting decreases levels of IGF-1 and PKA activity 7; PKA promotes cell survival and growth, as well as activates the mTOR pathway. Suppressing both increases stress resistance and stimulates cell regeneration.
There’s a lot that goes on in your body when you stop food intake. Aside from helping your body burn fat stores, you’re actually helping your body run more effectively and efficiently, which, in the long run will do your body a lot of good, not just in terms of fat loss, but pretty much every body function.
Benefits of Fasted Training For Your Abs
One of the biggest benefits of training your abs in a fasted state is accelerated fat loss.
While there is no sure-fire way for you to only reduce the fat in your abdominal region (because there really is no such thing as “spot reduction”), boosting overall body fat loss will have a positive effect on making your abs pop.
But keep in mind that a few training sessions going hard on abs won’t give you an immediate six-pack. It takes hard work and consistency, along with a clean diet, to reduce overall fat mass and increase lean muscle.
In the interest of why fasted training is good for fat loss, this is what you need to know.
Training In a Fasted State Increases Fat Oxidation
As we said earlier, by 12 hours of fasting, your body has gone through a significant chunk of your glycogen stores, meaning it has to look for alternative fuel sources to supply organs other than the brain.
Training in a fasted state forces the body to be more efficient with energy because there isn’t a ready supply of glucose on hand to use. This means that you become metabolically flexible (i.e. you can use glucose or fat for fuel) without hurting your energy levels.
But there’s another thing that happens when you train in a fasted state—you activate the sympathetic nervous system, or the branch responsible for the fight-or-flight (stress) response.
Activation of the SNS prepares the body for a fight by increasing heart rate and respiration rate, decreasing digestive function, and increasing muscle tension.
But activation of the SNS also increases lipolysis, the mobilization of fat stores to supply the body with energy 8. Mobilization of fat leads to increased fat oxidation and a higher rate of fat burn.
How You Can Get The Core You Want
The one thing you have to keep in mind is that during fasted training, you’re not counteracting what you’re trying to achieve.
Cortisol levels tend to rise during as fasted state, especially when training, which can negatively affect weight and fat loss long term.
If you’re training super hard in a fasted state and achieving chronically high cortisol levels as a side effect, it’s signalling to your body to store fat, not burn it.
Getting a rock-solid core comes down to 3 factors:
- Training smarter, not harder
- Eating clean
- Staying consistent
With those three factors in mind, you’ll get yourself to every fitness goal you want to achieve.
- SD Anton, K Moehl, WT Donahoo, et al. Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting. Obesity (Silver Spring). 2018; 26(2): 254-268.
- PB Jarreau. Time-Restricted Feeding Clears Mutant Huntingtin Protein From the Brains of Mice. Medium. https://medium.com/lifeomic/time-restricted-feeding-clears-mutant-huntingtin-protein-from-the-brains-of-mice-c02fc92c2141. Published July 11, 2018.
- KA Escobar, NH Cole, CM Mermier, TA VanDusseldorp. Autophagy and aging: Maintaining the proteome through exercise and caloric restriction. Aging Cell. 2019; 18(1): e12876.
- ML Hartman, JD Veldhuis, ML Johnson, et al. Augmented growth hormone (GH) secretory burst frequency and amplitude mediate enhanced GH secretion during a two-day fast in normal men. J Clin Endocrinol Metab. 1992; 74(4): 757-765.
- N Amole, S Unniappan. Fasting induces preproghrelin mRNA expression in the brain and gut of zebrafish, Danio rerio. Gen Comp Endocrinol. 2009; 161(1): 133-137.
- S Klein, Y Sakurai, JA Romijn, RM Carroll. Progressive alterations in lipid and glucose metabolism during short-term fasting in young adult men. Am J Physiol. 1993; 265(5 Pt 1): E801-E806.
- CW Cheng, GB Adams, L Perin, et al. Prolonged fasting reduces IGF-1/PKA to promote hematopoietic-stem-cell-based regeneration and reverse immunosuppression [published correction appears in Cell Stem Cell. 2016 Feb 4; 18(2): 291-2]. Cell Stem Cell. 2014; 14(6): 810-823.
- J Achten, AE Jeukendrup. Optimizing fat oxidation through exercise and diet. Nutrition. 2004; 20(7-8): 716-727.