Had you recovered from your previous workouts? Did you take the same/any food with you? Was your pre ride meal similar? How much of the ride forced you to go anaerobic. Temperature change? It is more likely that your muscle glycogen stores were not topped up before you went than your low BF%. Elites are around your range and lower. Some marathoners are <5%. The importance with less BF% is making sure to top up your glycogen stores after each workout.

Many that I have been around, who have a low body fat content, actually like to go into IM's or Ultras a "Tad" bit heavy. Maybe a pound or two. The tank well topped off with carbs before their big day.
I think RJ is right on this to nutrition has a BIG effect of downstream performance..you can ruin an "A" race by not eating well and becoming too lean in the days prior. You have some time to play with this...find what works for you...

Bonking is caused by a lack of carbohydrates in your system to continue fueling the workout. You are never burning pure fat.

When you exercise, you take in carbs to delay the depletion of your glycogen, and this trains your body to access the circulating blood glucose preferentially to the stored glycogen. Even so, most of us can only hope to replace about 50% of the calories (and most of it is sugar) we are burning; therefore the glycogen is being tapped into right away. The theory is that when your brain senses you are close to exhausting your stored glycogen (remember your brain pretty much runs on it), it makes the muscles stop working in order to protect itself. If you somehow override this command, you can become disoriented, dizzy, etc. This is a bonk.

Glycogen is stored in your liver (whose size remains about the same no matter what you weigh) and in your muscles. How much a person can store depends more on how they train, not on how fat or lean they are, and it's around 2,000 calories. The reason the term "hitting the wall" in a marathon is associated with 20 miles is from assuming that running 1 mile is about 100 calories. Obviously, through training and taking in nutrition during an event, you can delay the exhaustion of the glycogen stores by much longer than running 20 miles, or else how would people complete an Ironman? :D

When you train long on successive days, if you aren't careful to replenish your stored glycogen daily, you can begin a workout with a partly empty tank. Even if you stuff your face with carbs during the workout you can bonk.

How do you keep glycogen reserves topped off? By religiously following sports nutrition guidelines for post-workout feedings and ensuring that you are taking in adequate carbs daily based on your weight and training volume.

I love the term SWAG that made me laugh out loud!!!!! Awesome

TriTalk just this week did a segment on this very topic! It's a podcast available on iTunes.

I don't know where the 7% and 16% figures came from. They are high from information sources I've checked, most notable being Wilmore and Costill's Physiology of Sport and Exercise
which says 3-5% for men and 11-13% for women, respectively, for essential fat stores.

ACE (American Council on Fitness) goes even lower: 2-4% for men and 10-12% for women.

Essential fat is really all you "need," but different sports will favor different body types.

Women shouldn't get so lean as to cause disruption to their menstrual cycles. Men or women who notice their skin, hair or nails losing luster or not growing normally or problems recovering from exercise or becoming ill often may have compromised their immune systems if their body fat is too low.

In my N=1 experiment, I am 10-11% body fat, and I feel great at it. It sucks for my swimming (have to work more on technique now as I have very little natural flotation), but is awesome for biking and running. Even though people tell me I have no fat on me, believe me, there's still plenty there! I'm getting a blood workup done soon to check (did this 2 years ago) that everything is A-OK. My physician (who is plump herself) has never told me I'm too thin--in fact she remarks at my overall health.

I'm with the group that says (up to a point) the thinner the better for your longevity. It's bad enough how much I have to eat to train for Ironman distance--I'm happy to keep it as little as possible, though. Also saves me a few pennies!

According to my amateur calculations, I am somewhere near 14.5-15% at 130lbs. So why do I still have chub? To get rid of it, I have to drop 3 lbs. But if I lose more than 2-3 pounds (putting me at about 13%), I lose power. I have not been able to maintain a weight/fat balance at exactly the right point, about 128. I don't lose anything or lose too much.

I wasn't advising anyone to get down to just their essential fat; what I was pointing out is that people throw the terms around without explaining what they mean. Essential fat is just that--it's considered the bare minimum a body needs to do what you said--manufacture hormones, etc. I doubt any physician would recommend to an athlete to reduce their BF% to essential fat levels.

However, the numbers are guidelines, and there are individual variations, which is why there is a range and also why you read about some people who measure lower than the essential levels. Bodybuilders, in particular. I've looked at women's "figure" competitors, and many of them state they are at 7-8%. They look absolutely beautiful! Is that healthy? Who am I to say? But most of them can't maintain that level year-round. It is just too stressful and probably not too healthy.

For SundayND--unless you lose weight extremely slowly, it's likely you will lose muscle, which explains a loss of power. I lost a good 10 lbs. of fat (but only about 2 lbs. of muscle) over a period of 2 years. Think about that--that's 1/2 lb. PER MONTH! When you try and speed things up, it's more likely (and I say this anecdotally, not with scientific backup) that you will lose a higher % of muscle.

So if you are really just trying to reduce your bodyfat, AND not lose power, AND not lose much muscle mass, the way to go is very slowly. How? Reduce your daily intake by 100 calories per day. This is difficult for many people to do unless they spend a few months eating purposely with a pattern--as you know, it's easy for your calorie intake to be all over the place when you are trying to eat a variety of foods, and you are going through different training cycles. What worked for me was to have my BMR measured, and then I had my exercise burn rates calibrated from there, so I had a pretty good idea of what I actually needed to maintain weight. And then I just cut out a few calories here and there. My original intent was to get to 15%, but I guess what happened is that the level I train at is no longer conducive to me hanging on to much more than 12% fat, and I am OK with that.

But what I've found is that if you get yourself on a regular eating SCHEDULE (5-6 smaller meals per day), then you will get your stomach used to a certain intake of calories that you can deal with and then it's easier to just limit yourself by a little bit.

The other thing to remember is that our bodies are very adept at modulating our energy intake/output. So as you lose ANY weight, you automatically need less intake and are burning less when you exercise. Many people don't get this--they think "OK, I reduce what I'm eating by a lot, I lose weight and then I can eat almost as much as I was before." Not true.

Many of the calculators on the Internet are not going to work for you, either. They are based on some sort of averages, and who knows what type of athlete they are using to figure out the burn rates? I've found some of the numbers to be extremely high compared to what I know about myself, and others possibly low. So knowledge is power; if you are dead set on dropping body fat, you need to know about YOU and what you burn; not what some Internet site thinks. Also be wary of the cardio machines in the gym--they are usually calibrated to something like a 150-lb. male, which is not what most women are, so the readouts will be wrong. For me, the bike readouts match about what I'm actually burning, but the treadmills are way high.

This is why the best way to lose fat is the S-L-O-W way. It gives you a chance to physically and psychologically get accustomed to overall eating less without feeling deprived or having less energy.

Many overweight folks took a few years to put on their extra 10 or whatever lbs., and so to expect your body and mind to adapt to trying to lose them in just a few months is flat out silly. But we all like the quick fix!

I don't weigh myself that often (maybe once a week, if that), because I can tell how things are by how my clothes fit and how cut I look while lifting weights (yes, there's a vanity component). But if I notice 2 extra lbs. on the scale that I can't otherwise explain (always weigh yourself at the same time of the day under the same hydration circumstances, as water weight can make your weight fluctuate by 2+ lbs. depending on your sweat rate and when you last exercised), it's time to clean things up a bit.

As always, YMMV.

Just to clarify, I am not trying to get down as low as I can because I think that's good. I've concluded through experience and with correct training that the best racing weight for me is 128, where I can still maintain power and feel good, but I naturally gravitate to 130, making it hard to keep even those 2 lbs off. Especially post race....

I think I'm mostly just frustrated with genetics. I work my butt off and still have these little pockets of visible fat. Doh.