Understanding what happens to your body when you lose weight can help you set realistic goals and maintain a healthy balance while shedding those extra pounds. Body composition refers to the proportion of fat, muscle, bone, and water in your body. Unlike the number on a scale, it gives a clearer picture of your overall health. 

Let us break down what happens in weight loss. 

Water loss

In the first couple of weeks, most of your weight loss comes from a reduction in the water weight which is up to 70% of the initial loss. The body stores 500-600 g of glycogen, the stored form of glucose that holds 3-4 grams of water and is lost when you reduce carbohydrate intake or begin a calorie deficit. Glycogen stores drop, leading to rapid weight loss. Lower insulin levels promote sodium excretion, further reducing water retention. 

Fat burning 

In the fat-burning phase, the fat cells begin to shrink, but the number of fat cells remains constant in adults. The triglycerides, which are the fat found in blood, break down into fatty acids and glycerol. This breakdown of triglycerides increases the production of hormones like glucagon which signals the liver to break down glycogen. It is important to remember that fat loss is not immediate and often varies depending on factors like genetics, activity level and diet. 

Another hormone released with the breakdown of triglycerides is epinephrine. It triggers the release of fatty acids from the fat tissues and helps mobilise the fat from stubborn areas. It also boosts energy expenditure and enhances muscle strength temporarily. In this phase, you typically lose 1-2 pounds or 0.5 to 0.9 kg. 

Muscle Mass 

Without proper exercise and protein intake, up to 25% of weight loss can be from muscle. This is because your body partially uses the protein and glycogen stored in your muscles to replace some of the energy that comes from food when you cut calories to lose weight. Protein intake of 0.8g – 1.2 g/ kg /day helps maintain muscle mass.

Muscle is a metabolically active tissue that supports your basal metabolic rate, meaning the calories your body burns at rest. Muscle loss can reduce your metabolic rate and make further fat loss harder. 

Organs and Bones

Although there are no significant changes during weight loss there are some changes when you go on extreme weight loss without any weight training exercise; your bone mineral density can be decreased and increase osteoporosis risk. 

Organs like heart, liver, and kidneys may also experience slight reductions in mass as overall metabolic demand falls, but this typically does not impair function unless weight loss is extreme or rapid. 

To avoid this, eating a balanced diet helps with nutrient deficiency.

With weight loss, your body undergoes several metabolic changes and your basal metabolic rate decreases by 5-15%. Hormonal changes like leptin levels decrease and ghrelin levels increase which stimulates appetite. Over time weight loss can slow down and this is called a weight loss plateau. 

Prioritise eating balanced meals rich in whole foods, lean protein, healthy fats and complex carbohydrates. Do regular resistance training and stay hydrated.

Kripa, is a Specialist Dietitian at The London Obesity & Endocrine Clinic. She has helped many patients overcome weight management barriers. ©Simplyweight

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