Training & Nutrition - Calories and weight of fat burned during exercise?

OK, so I’ve just acquired my first HRM and have also just completed my first century. Otherwise, I’m 185 pounds and 6’3 and trying to get rid of some stomach bulge…

During the century, the HRM politely informed me I used 4,600 calories (over 6 and a half hours riding time). Now from reading other threads, this seems a little high, so I‘m thinking probably 3,500 to 4,000 is probably about right.

My question is how much weight in fat would my body have used? To answer this I need to know firstly how many calories are in a pound of body fat, and secondly, the percentage of energy my body is getting from carbs (stored and eaten/drunk during the ride) and how much from fat stores. Does anyone have any idea? My average heart rate during the ride was 150bpm.

Sorry, very maths based question, but interested to know know other people’s views on actually body fat used during exercise. I appreciate that it massively depends on a number of factors, including how much you eat during the ride, but any general help much appreciated!

3500 calories in a pound. But you didn't burn all fat.

The general recommendation appears to be to try not to lose more than 500 calories a day. About a pound a week. The reason? If you try to be too aggressive in weight loss your body will think that you are literally starving and will try to conserve body fat!

I have lost about a pound a week for the last 20 odd weeks. I cycle more than I eat. Even have to cycle the beer off sometimes.

and secondly, the percentage of energy my body is getting from carbs (stored and eaten/drunk during the ride) and how much from fat stores.

The fat/glycogen utilization ratio depends on your conditioning (one of the improvements you get from training is being able to utilize more fat at a given level of effort) and on the intensity you rode at. It's possible to get testing done that will show your fat/glycogen usage at different work loads.

Even if you had that info, a single heart rate average is useless for measuring intensity. For example, if your max HR was 200, you could average 150 by riding at 75% of max, which is fairly easy for most people. Or you could loaf at 100 bpm for half the ride and ride the last 3.25 hours at your max HR. That would be impossible of course, but if you could do it, it would be much more stressful (and burn a lot more glycogen) than a very steady 150 bpm.

Then there's cardiac drift... when you stay at the same output (power) over a period of time your HR goes up. So your HR is higher but the amount of calories you are using is the same. These kinds of problems are why people use power meters instead of HRMs.

I recommend that you not worry about how much fat vs carbs you burned and just look at calories. Your body can turn fat into glycogen (not fast enough to keep you from bonking, but for replenishing glycogen stores after the ride) and can turn carbs into fat. So even if you knew exactly how many calories of fat and carbohydrate you burned, that number would change for a while after the ride as part of the recovery process.

To answer you question you would need your age, then calculate the heart rate zone you are in (controversial, as noted by ericm979). Still it could be a decent estimator. You were probably in high zone3 / low zone4 for a lot of your ride at average HR of 150. This would mean burning about 5% of your calories from protein (which remains fairly constant, percentagewise across the zones), 35% of your calories from fat, and the rest from carbs. This phenomenon illustrates why treadmills and stationary bikes in gyms, often show a zone 2 or 3 for "optimal" fat burning. However the absolute amount of fat burned per hour increases across the zones according to the table I consulted, in "The Heart Rate Monitor Book for Cyclists," page 102. it's just the percentage relative to the carbs, that drops.

Also, I assume that on the century you stopped and ate along the way. The composition of your snacks partially determines what fuel your body utilized during the ride. Most people I've observed primarily snack on carbs on these rides.

Assuming that you burned about 4,000 calories (probably was a bit more actually), 35% of that is 1400. Each pound of fat is 3500 calories, so you burned a little less than 1/2 lb of fat. Realize too, that you burn calories even when you're not exercising, so for a 24 hour period you would have burned another several hundred calories of fat.

Thanks all for your very helpful answers. FYI I'm a 29 year old so my max heart rate is about 191 depending on which method used, so working about 78% of max, which seems a little high but suspect it may well be skewed by the cardiac drift you mention (and which I noticed on the ride as well in terms of heart rate against speed on equally flat sections).

Carbs definitely the predominant food during the ride (energy gels and gatoride) but also some mixed protein & carb energy bars, plus tuna sandwich for lunch.