This is a question most people want to know the answer to, so I sat down with a sports nutrition book, my old anatomy and physiology textbook and the text I’m currently teaching from.  The answer is not simple, and it depends on factors such as energy needs of the moment, availability of oxygen and carbohydrate and body composition, to name a few.

Fat needs oxygen (O2) and carbohydrate (CHO) in order to burn as energy.  It is important to know that the metabolic process by which we glean ATP (units of energy we use to move) that fat goes through takes a lot of time but yields ALOT of ATP.  Metabolizing CHO is much faster, but doesn’t yield as much ATP.  It is also important to know that body fat is the preferred source of muscle tissue at rest–the more muscle tissue you have, the more fat you will burn just sitting around doing nothing.  Remember though, that the more time you spend sitting around doing nothing, the less muscle you have, generally speaking…so let’s talk about the fuel we use at different levels of exercise intensity.

If I’m sitting around doing nothing, I’m burning mostly fat.  If I get off the couch and head out for a walk with Otto and I can sing to him, this is the lowest level of intensity, and I’m burning a mixture of fat and CHO, mostly fat.  If I pick up the pace and can talk comfortably to Otto, but can’t really sing as easily, I’m still burning a mix of fat and CHO, but the amount of fat I’m burning is starting to decrease, and the amount of CHO I’m burning is starting to increase.  The faster I go, the higher the demand is for ATP right NOW and my body switches to more CHO and less fat being used as fuel.

Once Otto and I are going so fast that I can’t even talk, and I’m breathing pretty hard, I’ve crossed over into anaerobic respiration.  What that means is that since there isn’t any O2 available, I’m not using fat as fuel, just CHO since that can burn without O2 and I really need energy QUICKLY.  You might be wondering why in the world I would cross over into a form of respiration that doesn’t use fat as a fuel source if my goal is to burn fat???  The more I cross over into anaerobic respiration, the more I increase my O2 carrying capacity, which means that the next time I work out with Otto, I can stay longer in aerobic respiration or “fat burning mode”.

“If you want to lose weight, before you change a thing regarding your exercise or eating habits, focus on getting QUALITY sleep for at least 2 weeks.”  This was one of many attention grabbing phrases I heard on Friday at a fascinating seminar I attended.   I know this is not going to be very “foodie” of me, but I’d love to share a few tidbits from my notes about the importance of sleep in reaching and maintaining a healthy weight.

It turns out there are several diet/health related consequences when you are not sleeping well on a chronic basis.  You are at increased risk for obesity, (MI’s) heart attacks, arrythmias, and higher blood pressure–these problems are directly related to the fact that sleep deprivation compromises thryroid function, increases production of ghrelin and stress hormones adrenaline and cortisol, and decreases levels of growth hormone, insulin and leptin.

What does all that mean in plain english?  Well, ghrelin is a hormone that increases appetite, and leptin is involved with appetite suppression.  If the absence of quality sleep means that ghrelin levels go up and leptin levels go down, we are going to eat more.  Then we have chronically elevated levels of stress hormones like adrenaline and cortisol which promotes storage of fat (especially around the belly area because there are more receptors).  If our levels of growth hormone drop, so does the building of muscle mass on our bodies–so now we have more body fat and less muscle.  That means higher BMI, and a slower, less efficient metabolism which will make shedding extra pounds difficult.

There are 5 stages of sleep in a 90 minute cycle.  The first two stages are more involved with falling asleep, and if you find that you are easily awakened or have sleep that is fragmented all night long, chances are that you don’t spend much time in stages 3-5.  Before you rush off to the nearest sleep clinic to have yourself analyzed, start with some basic good sleep habits including keeping a regular bedtime, limiting caffeine and alchohol and not getting overstimulated by TV or the computer in the evening.