The calories in/calories out model does work as long as you properly count the calories going in. The number shouldn't be what's on the nutrition label since that assumes an ideal digestive system that can fully extract all the energy from the food that a person eats.
This number doesn't include factors such as the health of the person. If the person has digestive problems such an a malfunctioning intestine and can't digest and absorb the energy from the food, then amount of calories extracted from the food will be lower than what's on the label.
If the person has metabolic problems such diabetes or certain other hormonal imbalances, then the energy will be extracted from the food, but it will be stored as fat rather than being used as useful energy. The person will feel tired even though they are consuming enough calories.
Reducing food intake or increasing energy consumption will help a person lose weight if all other factors stay the same. However, as a person consumes less food, they might end up feeling tired and crappy due to the body trying to compensate. Alternatively, changing a person's hormonal balance (e.g. by consuming low glycemic load foods to reduce insulin spikes and allow better utilization of energy in the food) will also cause a person to lose weight. This is the basis of the good calories/bad calories model of dieting, the Atkin's diet, and Tim Ferriss's Slow Carb Diet.
This number doesn't include factors such as the health of the person. If the person has digestive problems such an a malfunctioning intestine and can't digest and absorb the energy from the food, then amount of calories extracted from the food will be lower than what's on the label.
If the person has metabolic problems such diabetes or certain other hormonal imbalances, then the energy will be extracted from the food, but it will be stored as fat rather than being used as useful energy. The person will feel tired even though they are consuming enough calories.
Reducing food intake or increasing energy consumption will help a person lose weight if all other factors stay the same. However, as a person consumes less food, they might end up feeling tired and crappy due to the body trying to compensate. Alternatively, changing a person's hormonal balance (e.g. by consuming low glycemic load foods to reduce insulin spikes and allow better utilization of energy in the food) will also cause a person to lose weight. This is the basis of the good calories/bad calories model of dieting, the Atkin's diet, and Tim Ferriss's Slow Carb Diet.