The antonyms for "putting weight on" are losing weight or shedding weight. These two phrases refer to the process of reducing the amount of body mass, such as fat, muscle, or bone. Losing weight can be achieved by practicing healthy habits such as exercising regularly, eating a balanced diet, and maintaining a healthy lifestyle. Shedding weight, on the other hand, often refers to the process of intentionally getting rid of unwanted weight, such as during a cleanse or diet. Regardless of the method chosen, both losing weight and shedding weight require discipline, motivation, and commitment to achieve desired results.