Breaststroke 2 kilometers a day can help you lose weight, because swimming is an aerobic exercise, which can consume calories and fat in your body. Moreover, breaststroke can also exercise the muscles of shoulders and upper limbs, which helps to enhance the strength and endurance of upper limbs.

However, in order to lose weight, you need to pay attention to the following points:

1. Diet control: Aerobic exercise such as swimming can consume calories and fat in the body, but if the calorie intake exceeds the energy required by the body, it will lead to weight gain. Therefore, while doing breaststroke, we need to pay attention to diet control, arrange diet reasonably, and control the intake of calories and fat.

2. Keep exercising: 2 km breaststroke every day is a relatively large amount of exercise, and you need to keep exercising to achieve the effect of losing weight. If you only swim occasionally or don't insist on swimming every day, the weight loss effect may be greatly reduced.

3. Arrange other exercises reasonably: In addition to breaststroke, you can also combine other aerobic exercises to help you lose weight, such as jogging, cycling and skipping rope. This can diversify the exercise and improve the metabolic rate of the body, thus achieving better weight loss effect.

As for the problem of shoulder widening, breaststroke mainly exercises the upper limb muscles, and swimming has less burden on the shoulders, which will not lead to shoulder widening. However, it should be noted that breaststroke may cause injury or pain to the shoulders if the action is incorrect or overtraining, so pay attention to the correct posture and proper rest.