Road Bike Racing - Effect of body fat on power output

Anyone know what the theoretical effect of body fat is on power output? Right now I'm probably at ~13% BF and output XXX watts FTP; assuming I could in theory drop fat to an 'optimum' (no theoretical muscle, etc, loss), would my output change?

In other words, does 'extra' body fat effect absolute (not divided by mass) power output?

None, AFAIK. Less fat = more w/kg (somewhat arguable), and more li/kg for V02, but I don't think you'd see a wattage increase.

The conventional wisdom in bodybuilding is that you're best able to build strength above ~12% body fat. I don't know how applicable that is to cycling though...interesting question.

This study illustrates that fat mass does not effect absolute VO2 max, although it's focused on the obese, not athletes:
http://www.nature.com/ijo/journal/v24/n7/full/0801241a.html

From that you might be able to infer that if obesity doesn't handicap absolute VO2 max, then say, 5-8% body fat wouldn't hurt VO2 max either.

Well I was hoping a few more would at least take a shot at this first, but here is my answer anyways.
Body fat has an independent effect on power output, yet again, according to a central governor mechanism.
Body composition effects the rate of heat transfer and heat build up will shut down muscle fiber activation according to the thermostat regulator.


Heat production and storage are positively correlated with measures of body size/composition and heart rate drift during vigorous running.
http://www.ncbi.nlm.nih.gov/pubmed/16270704?ordinalpos=10&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum
The percentage of produced heat stored was significantly correlated with body mass (r = .427), fat mass (r = .455), and BSA (r = .414).

The rate of heat storage mediates an anticipatory reduction in exercise intensity during cycling at a fixed rating of perceived exertion.
http://www.ncbi.nlm.nih.gov/pubmed/16497719?ordinalpos=6&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum
We conclude that the regulation of exercise intensity is controlled by an initial afferent feedback regarding the rate of heat storage, which is used to regulate exercise intensity and hence the rate of heat storage for the remainder of the anticipated exercise bout. This regulation maintains thermal homeostasis by reducing the exercise work rate and utilizing the subjective RPE specifically to ensure that excessive heat accumulation does not occur and cellular catastrophe is avoided.

Models to explain fatigue during prolonged endurance cycling.
http://www.ncbi.nlm.nih.gov/pubmed/16180946?ordinalpos=7&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum
These include the following models: (i) cardiovascular/anaerobic; (ii) energy supply/energy depletion; (iii) neuromuscular fatigue; (iv) muscle trauma; (v) biomechanical; (vi) thermoregulatory; (vii) psychological/motivational; and (viii) central governor. More recently, however, a complex systems model of fatigue has been proposed, whereby these aforementioned linear models provide afferent feedback that is integrated by a central governor into our unconscious perception of fatigue.

Influence of aerobic fitness and body fatness on tolerance to uncompensable heat stress.
http://www.ncbi.nlm.nih.gov/pubmed/11641344?ordinalpos=14&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum
Exercise times were significantly greater in T(Low) (116 +/- 6.5 min) compared with their matched sedentary (UT(Low); 70 +/- 3.6 min) or fatness (T(High); 82 +/- 3.9 min) counterparts, indicating an advantage for both a high aerobic fitness and low body fatness.

Total body fat does not influence maximal aerobic capacity.
http://www.ncbi.nlm.nih.gov/pubmed/10918530?ordinalpos=9&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum
Fatness and excess body weight do not necessarily imply a reduced ability to maximally consume oxygen, but excess fatness does have a detrimental effect on submaximal aerobic capacity. Thus, fatness and VO2max should be considered independent entities.

I saw some of those same studies, but only posted the last one - as it seems that none of the models addressed athletic performance at what some would deem unusually low body fat percentages.

Lots of elite male athletes float around 8% body fat - the question I think the OP is asking is, are there any studies that have addressed the issue of "can you be too lean?" for endurance sports.