The idea of training in the "fat burning zone" (FBZ) is an outdated idea, that doesn't hold with current ideas. However, using the 70% of heart rate reserve (HRR) that Maarten suggests is *likely* to be *higher* intensity than traditional FBZ. Furthermore, with HRR training zones, they paradoxically increase (i.e., go up) if you are ill or not recovered.<br /><br />To loose weight (fat mass) you need to expend more energy than you consume, i.e., have a negative energy balance. It doesn't matter whether the substrate used is carbohydrate, fat or most likely a mix.<br /><br />Estimating HRmax from 220-age may or may not result in completely inappropriate zones. Whilst, that equation is good, it's only a regression equation based on a large population. The standard deviation is in the order of ± 15 b/min. This could result in widely inaccurate zones. If you are going to use HR to train with (and I'm not suggesting that you are), it would be more appropriate to ascertain correct zones from some sort of test (e.g., incremental test to exhaustion).<br /><br />During a 45-minute high intensity session, very little fat may be "burnt". However, it would be impossible to expend the same amount of energy as a 3 or 4-hour ride. It's the total amount of energy expended, irrespective of substrate utilisation, that's important for weight loss.<br /><br />In the aforementioned 5-hour (race) ride, expending 4000 kcal, this would equate to a power output of ~ 222 W, for a trained cyclist at a 'normal' cadence. A high intensity 45-minute session might be completed at ~ 300 W, thus expending ~ 810 kcal, which would equate to 20% of the energy expended in a 5-hour ride.<br /><br />Blimp, specifically asked about weight loss and increasing hill climb ability, with a view to racing next season. Although, weight loss may be warranted (impossible to know without ascertaining body fat %), it is also imperative to understand that the defining factor in performance is to increase power output, which needs to be done with higher intensity efforts. Ballpark figures, would suggest that climbing a 10% grade at ~16 km/hr (for Blimp at current weight) the power requirement is ~ 484 W. If Blimp lost 4 kg, that would increase speed by 0.57 km/hr or decrease time to climb a 2 km grade by ~ 16-secs. On the other hand, a 10% increase in power output with no weight change would result in an increase of speed of 1.47 km/hr, or a saving of 37-secs.<br /><br />Accordingly, my recommendation would be to increase power output, via both endurance rides (e.g., 2 - 4 hours) and also by completing fairly intensive, moderate duration intervals (e.g., 5 - 30 minutes).<br /><br />Ric<br />