Optimal BMI for health and endurance sports performance?

Body Mass Index (BMI) is weight (kg) divided by height (cm) squared. There is a BMI calculator available. 

Normal BMI value range is 18.5-25.

Interestingly, many successful endurance athletes seem to have their BMI around 21:

• Haile Gebrselassie (the current marathon world record holder): 21.1
• Scott Jurek (world's no.1 vegan ultrarunner): 21.2
• Chrissie Wellington (Ironman Hawaii winner 2007 & 2008): 20.8
• Craig Alexander (Ironman Hawaii winner 2008): 21.0

My BMI has been about 21 for years, but due to recent foot injuries and bad performances I've decided to take it lower. My current value is 19.8, which is extremely low for me. I certainly feel lighter and faster, but how low dare I go without risking something?

Paula Radcliffe (the women's marathon world record holder) has an extremely low BMI: 17.7. And if you remember Seb Coe (who used to be world's best middle distance runner), he had an amazingly low value of 17.0! 

On the other hand, many ultrarunners with a relatively high BMI seem to be performing pretty well. Dean Karnazes has a BMI of 23.7, and David Goggins (who used to be a powerlifter) 24.5! 

Also many cyclists and swimmers are in the same category. For example, Lance Armstrong's BMI is probably around 23.6.

So who knows what is optimal for both health and endurance sports performance. I plan to get to around 19 for a while and see how it goes.