The Effect of Different Weight Loss Strategies on Body Composition

When it comes to weight loss, quicker results are often favoured over slower results, even if slower is more sustainable. By looking at different weight loss studies, we can see that the weight loss method influences body composition outcomes and maintenance. This is an important factor to consider, as it can also significantly impact health. Simply observing a reduction in body weight does not guarantee that an entirely desirable outcome is being achieved.

A popular method for determining a healthy weight is the BMI scale. This has proven to be an inferior tool, with the elderly, people of certain ethnic groups and those with high muscle mass tending to be inaccurately classified. Instead, looking at body composition, with the use of various tools, such as skinfold callipers and DEXA scans, can give us a much better idea of the body’s state of health.

This article will look at different weight loss methods, how they affect body composition and the reasons for these differences.

Effect of Weight Loss Methods on Body Composition

When losing weight, it is ideal if fat is lost and muscle is maintained or gained. Excess adiposity promotes metabolic dysfunction, while muscle promotes healthy metabolism and strength. If a significant proportion of the body weight that is lost is muscle, this puts a person at risk of injury and lowers their metabolism.

Although there are many drugs, supplements and lifestyle changes that can promote weight loss, we will more closely examine three popular weight loss methods: diet changes, increased exercise and GLP-1RAs.

Diet

Changing to a healthier diet, such as the Mediterranean diet, has been found to produce reductions in body fat percentage and an increase in lean mass¹ as well as confer a host of other health benefits². Although some studies have found the Mediterranean diet to reduce weight, this is not always the case. Additional strategies, when combined with this diet, produce more reliable weight loss, as you will discover, further on³.

Calorie restriction is another dietary strategy adopted by some to quickly lose weight. This can lead to significant and rapid reductions in adiposity, but can also result in a greater loss in lean mass when compared to eating ad libitum⁴. Crash diets typically involve going on calorie-restrictive diets to lose weight quickly. They come with long-term risks, including deficiencies, muscle wastage and an increased risk of osteoporosis.

Although they do bring about significant reductions in weight, crash diets are difficult to maintain. “Yo-yo” dieting is more likely, where a person loses weight quickly, then adopts their previous eating habits and regains the weight, repeating this cycle of weight loss and weight gain. This weight regain is made more difficult to prevent due to epigenetic changes induced by the initial obesity⁵.

When permanently switching to a healthier diet, such as the Mediterranean diet, although body weight is not lost as quickly, and may still be difficult for some to adhere to, it does not come with the risks associated with calorie restriction.

Exercise

Exercise, particularly resistance training, is one of the most effective ways to increase lean mass and also results in reductions in fat mass⁶.

When the Mediterranean diet is combined with exercise, some studies have reported significant reductions in weight being achieved³. The weight loss results from the combination of a healthy diet and exercise may not always appear to be as impressive as other weight loss methods, and this is thought to be due to the concomitant gain of muscle mass while burning fat.

When exercise is combined with calorie restriction, it significantly reduces body weight and causes minor reductions in lean mass. In one study, calorie restriction and exercise produced a reduction of 2.6% and 0.4% in fat mass and lean mass, respectively, after 12 months, while exercise alone produced a 0.8% reduction in fat mass and a 0.3% increase in lean mass over the same time period⁷.

GLP-1RAs

Semaglutide has been on the market for several years now, so we now have some reliable data on its weight loss effects. One 2025 study, which followed people for 12 months, found that fat mass decreased by 18% by month 12, while lean mass decreased by an average of 3kg by month 7, but stabilised after this initial drop. Despite this reduction in lean mass, 22% of patients with sarcopenic obesity no longer had sarcopenic obesity by month 12, while 5% developed this diagnosis⁸.

Although retatrutide is a very new drug which is still undergoing clinical trials, researchers have gathered some valuable information on the effects of this drug on body composition already. Retatrutide is known to cause a rapid reduction in body weight, with one study reporting that participants lost an average of 26.1% of their total fat mass over 36 weeks after being administered a dose of 8mg retatrutide, once weekly⁹. Another study reported a 24% reduction in body weight after 48 weeks on 12mg of retatrutide weekly¹⁰. Regarding lean mass, studies on rodents have found a similar trend in lean mass loss as with semaglutide, where muscle loss stabilises after an initial drop¹¹.

The weight loss that comes with GLP-1RAs does not require active effort to make permanent lifestyle and dietary changes. The reduced appetite, lack of cravings and altered metabolism are all a result of the action of the drug. These drugs are often dispensed along with dietary and lifestyle advice, which would help maintain good nutrition, reduce muscle loss and enhance fat loss. But based on what we know from long-term studies, once the drug is no longer administered, patients rapidly regain weight¹². If lean mass is lost during the initial period of weight loss, this return to obesity can lead to an even unhealthier body composition. For many people, GLP-1RAs can become more of a chronic treatment, which they are afraid to stop lest they regain weight.

There are certain factors can contribute to the magnitude of change in adiposity and lean mass when adopting these weight loss strategies. We will look at these more closely now.

Body Composition Determinants

Diet

A diet with insufficient caloric value will force the body to burn body fat as fuel instead of carbohydrates. As such, a calorie deficit can bring about a significant loss in fat mass. However, if the diet is not only low in calories but also low in nutrients and protein, the body will not be able to maintain muscle mass and instead break down lean mass to support tissue maintenance and repair¹³.

When taking GLP-1RAs, since appetite is reduced, it is easy to reduce food consumption. But nutrition must be carefully considered in order to ensure a reduced risk of lean mass loss or the development of deficiencies.

Exercise

Exercise can help to promote the burning of fat, reducing fat mass, while also promoting the growth of lean mass. This means that overall body weight may change more slowly, but body composition will be shifting towards a healthier ratio of fat to lean mass.

When taking GLP-1RAs, exercise can lead to greater reductions in visceral fat. Additionally, it signals to the body to maintain and grow muscle tissue. Muscle tissue is beneficial to metabolic health and increases the metabolic activity of the body, improving overall health in several ways. Exercise also improves the maintenance of weight loss results after cessation of GLP-1RA treatment¹⁴.

Age

As people age, lean mass tends to be lost naturally. Weight loss, unless carefully done, could potentially exacerbate frailty. A combination of aerobic exercise, resistance training and diet-induced weight loss was found to be an effective and safe way of improving aging and obesity related diseases¹⁵.

Both older and younger populations experience comparable weight loss results and health benefits from GLP-1RAs. No significant increase in risk of bone fractures or heart failure was seen in older, high-risk patients¹⁶, however, older adults are more likely to discontinue treatment due to adverse events¹⁷.

Fat Distribution

Excess visceral fat is associated with several health problems, including heart disease, Alzheimer’s, cancer, type 2 diabetes, stroke and high cholesterol. Subcutaneous fat tends to make up the majority of the body’s total fat percentage and is also associated with an increased risk of cardiometabolic disease and joint and bone problems¹⁸. Each of these weight loss strategies causes a reduction in both visceral and subcutaneous fat. Typically, a greater mass of subcutaneous fat is lost, but a greater percentage of visceral fat is lost when losing weight^19,20.

Weight Loss Velocity

When weight is lost very rapidly, more lean mass is likely to be lost²¹. Rapid weight loss tends to occur when there is an extreme calorie deficit. The stress of this deficit signals to the body to decrease muscle protein synthesis and instead break down muscle to help support the functioning of the body’s tissues and organs.

The rapid weight loss seen by some patients taking GLP-1RAs is a result of a combination of metabolic changes and reduced appetite. This emphasises the importance of good nutritional advice for patients taking them.

Conclusion

The weight loss strategy adopted, as well as diet and activity levels, will all affect how quickly body weight is lost and what proportion of fat and muscle is lost. When weight is lost quickly from a crash diet or GLP-1RAs, there is an increased risk of muscle loss.

Those who go on crash diets may be putting their health at risk and only gaining temporary benefits. Patients taking GLP-1RAs often feel that they must continue to take the drug otherwise they will regain weight. If behavioural changes are adopted, this increases the changes of weight loss maintenance.

Overall, a reliable weight loss strategy needs to ensure adequate nutrition and exercise to minimise lean mass loss and improve metabolic benefits.

References

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