Cardio is a good fat burner. However, cardio can be a great fat burner if we look for ways to maximize the amount of body fat burned during cardio sessions. One of the easiest ways to encourage maximal fat burning is to do your cardio in the morning on a completely empty stomach, soon after rising and before eating. The reason is simple, in a “fasting” state (Ex. not having eaten for probably close to 10 hours), glucose – the basic energy component found on carbohydrates – circulates within the bloodstream at its lowest levels of the day. When this occurs, the body is more likely to turn to storedfuel – body fat – as energy. Upon the introduction of cardio, the body begins to look for fuel. If there are no significant amounts of glucose, the body responds by immediately digging into body fat stores for energy. Bingo.
The body will not utilize deep fat stores for energy during cardio if you have eaten before performing cardio. Instead, it will first take a look at what has recently been ingested – what you have eaten – and try to use that first before starting to utilize stores of body fat. Even a small meal, or a small carbohydrate snack, can inhibit fat burning by slowing the rate at which your body begins its search for stored body fat. Carbohydrates quickly digest into glucose, which alters the speed at which the body burns fat. In essence, food consumed before cardio exercise acts as a slowing agent, altering the body’s ability to burn body fat. On the other hand, with no food, there is no slowing effect and the body can quickly start burning body fat. Glucose, the digested form of carbohydrates, plays a big role in determining your fat burning status. With low levels, fat burning increases. With high levels of glucose, fat burning dramatically slows. To ensure you are burning as much body fat as possible, schedule your aerobic sessions in the morning, without food, to keep glucose levels flat.
The second reason to avoid eating before cardio exercise has to do with our pesky little friends, hormones. With a lack of food in the body, the prevailing hormones that kick into play immediately upon aerobic exercise are catecholamines and glucagon (try saying that five times fast). Catecholamines are tiny messengers released from the adrenal glands that are dumped into the blood with exercise. Their job is to target fat cells, helping them to open up, allowing fat to be liberated and burned as fuel. However, when you eat food, specifically carbohydrates, catecholamines become less effective at doing what you want them to do – help breakdown body fat. Glucagon is the second fat burning stimulus that increases upon exercise, especially when there is a lack of food in the body. Glucagon is a major player in fat burning as it helps kick start the entire fat burning process. Glucagon helps tear down fat cells and upgrade the enzymes that use body fat as energy. Specifically, glucagon supports the production of hormone sensitive lipase, (HSL) a gatekeeper on fat cells that helps determine the flow of fat. When HSL is active, it allows more fatty acids from body fat to flow into the blood where they can be burned as fuel. On the other hand, when HSL falls, the flow of fatty acids out of fat cells becomes greatly impaired. In addition, glucagon suppresses the activity of Malonyl CoA and supports Carnitine Palmityl Transferase 1 (CPT-1). Malonyl CoA influences the liver’s ability to start the fat building or fat storing process. As Malonyl CoA activity rises, fat storage increases and as it drops – by avoiding food before cardio- fat burning increases. CPT-1 is another enzyme that supports the burning of body fat by dragging fatty acids –from body fat – into muscle cells, where they are burned. When you avoid eating before cardio and allow the maximum production of glucagon, you’ll experience greater CPT-1 activity which facilitates the burning of body fat. On the other hand, when you eat before cardio, all these mechanisms are compromised; less potent catecholamine activity with negligible glucagon production translating into less active fat burning.
Another point on eating before cardio is that food, especially carbohydrates, increase insulin levels. Generally, insulin is known as a fat storing hormone. It makes catecholamines less effective at triggering the breakdown of body fat and it also suppresses the release of glucagon. Remember, our little friend glucagon is the chief hormone that sets in place fat burning and support (Glucagon sounds like a Transformer). Surprisingly, it does not take a lot of food to increase insulin levels and to block the maximal fat burning effects of aerobic exercise. Even a small piece of toast is enough in to trigger sufficient insulin production to set in motion a hormonal change to mildly interrupt or slow the fat burning effects of a hard 30-40 minute cardio session. In the world of maximizing fat loss, it’s just not a good idea to eat before cardio.
A lot of bodybuilders argue adding cardio to the end of a weight training session will exert the same effect as doing cardio in the morning on an empty stomach. The idea: weight training depletes the body of sugar in the blood, (glucose) which is akin to a fasting state. In other words, the hard training empties fuel from the bloodstream and when this occurs, it’s a great time to do cardio. They’re half right. Hardcore training does burn through glucose in the blood and it even lowers muscle glycogen stores. Both can favor the burning of body fat. If you jump on the treadmill after training, you might expect catecholamines, already floating about as a result of training, to target fat cells. In addition, with low glucose levels, you might also expect glucagon levels to be elevated. These two points can be considered true. However, the problem is duration. When training sessions become too long – be it cardio or weight training, catabolism (muscle breakdown!) sets in. Cortisol – a hormone that causes muscle breakdown – can rise too high as a result. When cardio is done after a weight training session, cortisol – one of the chief muscle wasting hormones – rises to unreasonable levels. Studies have shown cortisol to lower testosterone levels and trigger a rapid spike in free radicals. Free radicals ignite muscle inflammation, thereby knocking out the immune system and paralyzing muscle recovery. When you follow up your weight training sessions with cardio, you push the body over the edge into a quasi overtraining state. In this state, hormones like testosterone and other anabolic hormones in the body decline. As a result, it becomes very difficult to retain muscle mass.
Testosterone levels. They are the other reason that I am a huge “cardio in the morning advocate. Some studies indicate testosterone levels are higher in the morning and decline from there, only to experience a late afternoon surge. Doing cardio in the morning makes sense as it is scheduled at a time where one of the most anabolic hormones – testosterone – is at its peak. The higher testosterone level can act as an anti-catabolic, preserving muscle mass. By definition, aerobics is a potentially catabolic event. It does not “build the body up” but rather “tears it down.” We always hope that we are tearing down body fat to be used as fuel. Still, with a poor nutrition strategy or if you are borderline overtrained, aerobics can become truly catabolic. It can lower testosterone levels causing a loss of metabolic supporting muscle mass! Having a hormonal profile that is higher in testosterone levels will help spare the body from eating away at its own muscle. Crushing your cardio in the morning is the best way to ensure you can burn fat without sacrificing muscle mass.