Alone, neither of them is sufficient.
Some people are able to shed pounds just by cutting calorie intake, but the results are unlikely to last without some physical activity. Conversely, you can spend hours in the gym, but if you’re stuffing your mouth with high-caloric and unhealthy food you won’t go far.
To put it differently, both factors are necessary to achieve optimal weight loss.
Exercising without a healthy diet is futile, but an healthy diet is per se not enough.
However, it’s not always apparent how much these two factors plays in the weight loss equation.
In other words, how much should you exercise? How much you should restrict calories?
According to it, 80% of weight loss is achieved by diet, and 20% is achieved by exercise.
That is to say, a big part of losing weight is having a correct eating regimen so as to burn more calories than those eaten (i.e., caloric deficit).
This should be paired with some form of exercise. Crucial, not only to lose weight, but for general well-being and physical fitness.
There’s not a lot of scientific literature specifically on the 80-20 rule as of today, but I like it because of its mild approach that makes it accessible to many.