Losing weight, specifically fat, is a struggle for most. This article is about what I believe is the most effective cardiovascular method for shedding fat fast and retaining muscle.
People get too caught up in how many calories they burned, how long their cardio session was, and how much they sweated during a workout. Sorry, but that is only half the equation. Much of the recent research regarding mTOR and its anabolic processes as well as the interplay between AMPK and mTOR have become available in the last few years. These two pathways, and the cycle they follow, are key in understanding the way our bodies build muscle and lose fat.
In terms of losing the most fat and retaining muscle, I believe that high intensity training (HIT) is the most effective cardio you can use. This is amplified when done in a fasted state (I’ll explain). I’m not saying that “aerobic steady state” cardio can’t work. It’s just less effective, more time consuming, and more catabolic. HIT is much more intense. While some might not be able to perform it, it’s the preferred choice if fat loss is the goal. Looking strictly at the energy systems helps, but different exercises can produce unique and profound hormonal responses, some transient and some noteworthy. So here are some of the benefits.
HIT causes a severe increase in AMP-activated protein kinase (AMPK), and this is the key response. Fasting also increases AMPK, which helps to explain why it’s so effective in decreasing fat and increasing insulin sensitivity. This is also why I recommend fasted HIT cardio for fast fat loss. AMPK is often referred to as the “master metabolic switch/regulator.” In laymen’s terms, when you’re quickly running out of fuel, AMPK is activated and drives catabolic processes while inhibiting anabolic processes. It sounds bad but bear with me.
This is where some may argue that they want to avoid AMPK because it’s “catabolic” and can inhibit mTOR, an important pathway for anabolism or muscle building. That is true. AMPK is transiently catabolic, but with HIT, we’re looking for AMPK activation for a controlled amount of time, not continuous. AMPK will be inhibited as soon as we eat a meal and the AMP:ATP ratio of available fuel is raised or returned to normal.
Too much of a good thing is bad for either side. A chronic over expression of mTOR, though “anabolic,” can be detrimental. mTOR aids in the inhibition of certain hormones like hormone-sensitive lipase (HSL), which impairs the body’s ability to break down stored triglycerides. So the “perma-bulkers” can now understand why they’re always fat and have a hard time losing this fat. They’re constantly force feeding themselves and are experiencing an over expression of mTOR.
So when we activate AMPK, the body increases fat oxidation instead of glucose, thereby preventing muscle catabolism. It improves insulin sensitivity by increasing glucose uptake and inhibiting glycogen storage. During HIT, the body shifts to using fat for fuel instead of glucose. Whenever muscles contract, energy is provided through ATP. When ATP is used up faster than it can replenish itself, the AMP/ATP ratio increases and AMPK is activated. The body will sense that energy is being used faster than it can supply it, so metabolic changes will occur. If this didn’t happen, we would run out of energy and die. But this is when AMPK is activated and shifts the body from glycolysis to fat oxidation. This continues long after exercise, so it explains why AMPK has long-lasting effects on fat consumption. Low intensity exercises don’t activate AMPK to the same extent and use mainly glycolysis for fuel. Therefore, the results are slower and less effective.
I’m not saying that aerobic exercise doesn’t have its place, but in terms of fat loss, it’s less effective. According to a report by The Journal of Obesity in their study “High Intensity Intermittent Exercise and Fat Loss,” they concluded that “the effect of regular aerobic exercise on body fat is negligible.”
There is a natural rhythm or balance between AMPK and mTOR. Think of the interaction like a metronome or a pendulum. We constantly shift between the two. During times of glucose deprivation, fasting, or HIT cardio, AMPK is activated and puts us in a prime position to burn fat by using our stored energy as fuel. On the other end of the spectrum, when mTOR is activated by, say, overeating or lifting, we’re primed for cell growth and protein synthesis. This is why fasting is such a useful tool in dieting because it plays off this interaction.
Another important aspect in the HIT versus low intensity steady state (LISS) is HIT’s ability to generate a profound increase in catecholamine production. In a study observing individuals who ran twenty-second wind gate sprints, the catecholamine response was shown to be significantly elevated in both the men and women after exercise. The catecholamines, epinephrine and norepinephrine, were notably raised and are effective in fat burning. Epinephrine has been shown to drive lipolysis, the breakdown of fat for energy, and plays a role in fat release from subcutaneous and intramuscular fat stores.
So the net result of HIT is faster fat loss and increased muscle, as AMPK increases glucose uptake in the liver and muscle tissue, increases fatty acid oxidation, and reduces glucose and fatty acid synthesis. The problem is that it’s slightly harder to perform than slow steady pace cardio and it’s very stressful to the central nervous system.
It may seem like you can’t lose fat and gain muscle technically at the exact same time, but that doesn’t mean that you can’t over the course of a diet. It is possible. I’ve had clients lose body fat and increase lean tissue over the course of a diet. It’s not possible if you’re looking for an excuse to stuff your fat ass with Krispy Kremes hoping to gain twenty pounds and magically develop abdominals at the same time by adding cardio. You need to maximize fat loss during times where AMPK is elevated. This is why fasted HIT cardio is effective. You should also try to maximize anabolism during times of mTOR expression. This is why I like to focus certain days on either fat loss or muscle gain, sometimes even during certain periods of the day. If you’re careful and detailed in your approach, you can achieve your body composition goals at the same time.
Here’s a sample. During AMPK stimulation, a fat loss day might start like this:
- Water and black coffee plus leucine (your choice here)
- HIT cardio (8–10 sprints of 20 seconds max effort or 10–15 minutes where the first 40 seconds are moderate and the last 20 seconds are maximal sprints)
- You can add more leucine here or not; it does generate an insulin response, but it’s your choice.
- Fast until 12:00 p.m. or 1:00 p.m. (depending on how you feel as well as on how much you ate the night before)
- Light pre-workout meal with protein, leucine, and a higher GI carb source
- Intra-workout (I prefer a drink with leucine, glycine, beta-alanine, and citrulline malate.)
- Post-workout (This is when you shift from fat loss to muscle gain. From here out, I have 2–3 quite large protein and low to moderate GI meals, though you can get away with some higher GI carbs in the first meal post-workout as well. If maximum fat loss is your goal, start with a post-workout shake with leucine plus whey and go zero carb for an extra 1–2 hours.)
In closing, stop holding back your own progress by thinking that you need to exclusively bulk or cut. Find a diet and exercise program that works for you and use a well thought out plan to achieve your goal. Throw in some fasts, use some HIT cardio, lift with intensity, and let the rest play out.
Here are some studies that echo this information. Check it out and then make the decision for yourself:
- “High intensity intermittent exercise and fat loss”
- “Just HIT it! A time-efficient exercise strategy to improve muscle insulin sensitivity”
- “Metabolic adaptations to short-term, high-intensity interval training: A little pain for a lot of gain?”
- Bolster DR (2002) AMP-activated protein kinase suppresses protein synthesis in rat skeletal muscle through down-regulated mammalian target of rapomyacin (mTOR) signaling. J Biol Chem 277:23977–980.
- Minokoshi Y, et. al. (2004) AMP-kinase regulates food intake by responding to hormonal and nutrient signals in the hypothalamus. Nature 428:569–74.
- Saks VA (2007) Molecular System Bioenergetics: Energy for Life. Weinheim: Wiley-VCH.