Those of us in wet countries have a higher percentage of obesity, and we bulk up our food with a higher percentage of a very abundant energy source.
The problem with slow-release energy is that you can't burn it. Eat a plate of spaghetti, go out for a two-hour ride, pedal your knees off. Get home, crash out on the sofa, and that spaghetti is still there, making you fat.
If you wanna burn off and get lean, you've got to run out of fuel. That means motivational energy that burns quick. You use the wind in your sails to get going, and you go the extra mile to start using your own energy reserves to keep going.
This works in other ways, too. It takes a lot of energy to digest meat. If you don't consume a slow-release energy source with it, it will require reserve energy. That's why the most fattening part of a cheeseburger is the bread roll...
In the case of the average person, reducing bread/pasta before exercise, and focusing on fruit smoothies for energy instead, can make their exercise go a hell of a lot further.
I should add i'm not taking any special circumstances into account, such as thyroid issues. And zero-carb diets such as Atkins are extreme, and extremes can be hazardous. Just use your sense, keep slow burners for the morning, get variety.
Incidentally, i'm also 45yo, nearly 6ft tall, and 143 lbs. I eat well and don't punish myself on exercise.