I'm trying to figure out what the optimum heart rate should be for cardio exercise to burn fat. According to this website apparently between 60% and 70% is best for fat burning.
Is this true?
That is partially true. Zone training is a common way of improving fitness. However, the zones adapt to your level of fitness, so the percentages you see are SWAGs (Seriously, Wild-A## Guesses). A better measurement is the breath/talk test.
Now, studies have shown that the lower your heart rate, the larger portion of the energy you expend is from fat. The higher your heart rate, the larger portion comes from glycogen stores--or stored sugar. However, what people forget about percentages is that it is in relation to the total energy burned.
Your body doesn't simply switch over from burning fat to burning sugar. It's merely that the amount of glycogen your body burns is much larger compared to the amount of fat it is burning. The truth of the matter is that your body is burning just as much fat in the red-line zone as it is in the aerobic zone. The difference is in the "after burn" effect. When your body has to replenish its glycogen stores, that energy has to come from somewhere. It will come from a portion of dietary carbohydrates, and pulling the stored energy out of your fat.
This is precisely why shorter HIIT (High Intensity Interval Training) has better fat burning effects than long same-intensity sessions of cardio. However, there is a trade-off. If you are aiming to run a marathon you would be better off training for that marathon. If all you care about is fat, HIIT is effective, as is resistance training (lifting weights). Combine the two, and you can increase your muscle mass, which in turn burns more calories throughout the day.
I do not know optimals, since I keep hearing different theories and numbers changing over the years. The good news is that detecting 65% or higher is pretty simple. When you can start hearing yourself breathe, your heart rate is over 65%.
When you can no longer hold a conversation with someone you are above 85%, so basically stay in a range where you are breathing hard, but can talk, and you are pretty close to being within that fat burning range, and no HRM or widgets required.