While the ultimate goal of weight loss is to lose fat, unintentional loss of lean body mass generally occurs concurrently. A new study published in the American Journal of Clinical Nutrition investigated whether skeletal muscle, along with other tissues and organs, decreases in mass following weight loss.
Obesity is a chronic condition characterized by excessive body fat which can lead to secondary conditions including diabetes, high blood pressure, and stroke. In 2014, at least 20% of Canadians were considered obese, and this number is steadily rising. Not surprisingly, the usual suggestion for those who are obese is to lose weight, either through increased exercise or decreased calorie intake. However, while the overall goal of losing weight is to lose fat, an unintentional loss of lean tissue also occurs. The amount of lean tissue loss often depends on degree of caloric restriction, activity level, adiposity, age, sex, and ethnicity.
An increased amount of skeletal muscle (SM) is required to support an obese individual compared to the same individual following weight loss. However, excessive SM loss may negatively affect physical function. This is particularly concerning in the aging population who are already experiencing a loss of muscle tissue due to natural aging processes, as preserving physical mobility is important for independence.
Another concern is the effect of weight loss on other human tissues and organs such as heart, liver, kidney, lungs, and brain. Importantly, organs with a high metabolic rate such as brain, liver, and heart may experience a disproportionate loss in size compared to those with low metabolic rates such as muscle and skin. A limited number of studies in animals have indicated that caloric restriction decreases the size of the heart, liver, and kidney but not the brain.
A new study published in the American Journal of Clinical Nutrition investigated the loss of SM and organ mass during a weight-loss intervention in type 2 diabetes patients. Magnetic Resonance Imaging (MRI) measurements of SM, heart, liver, kidney, spleen and pancreas were acquired over a two-year period in 53 women and 39 men separated into an intensive lifestyle intervention (ILI) group and a diabetes support and education group (DSE).
This study delivered several key findings. First, SM and kidney mass decreased significantly in both study groups. In the ILI group, liver mass decreased in Caucasians but not in African Americans, indicating an ethnic difference in the loss of liver mass during weight loss. Spleen mass also decreased, whereas heart, pancreas, and kidney mass did not. Lastly, there was no evidence of a disproportionate loss of organs with a high metabolic rate (heart, liver, kidney and spleen).
Limitations of this study include that the sample consisted of older, obese type 2 diabetes patients and the results cannot be extended to other younger and healthier populations. Additionally, the conversion of organ volumes imaged by MRI to mass measurements required an assumption of density which does not take into account ethnic differences or individual changes in lipid content. Further studies would be beneficial to determine if organ mass decreases following weight loss in otherwise healthy individuals, as well as to study the implications of this.
Written By: Neeti Vashi, BSc