Why can’t I lose weight?

A common perception, which is confirmed by clinical and experimental data, is that some people lose weight more easily than others, even on the same diet.  The large variation in response to caloric restriction (CR; “dieting”) can lead to confusing results in experimental settings and can result in participants being labeled as noncompliant in clinical settings.  Reasons for this variability may include physiologic parameters existing prior to the CR and changes in physiologic parameters that may occur to compensate for the CR.

A study published in the February, 2012, issue of Obesity explored the role of preexisting parameters and compensatory responses on the actual weight loss following CR in mice.

The mice were continuously monitored for body temperature and activity.  Resting metabolic rate (RMR) and body composition were evaluated at baseline and during or after CR.  Body composition was expressed as fat mass (FM) and fat free mass (FFM).  The baseline period lasted four weeks during which the mice had ad libitum food and water.  The CR period lasted 28 days during which the mice were restricted to 70% of their baseline food intake, calculated individually for each mouse.

Most mice lost both FM and FFM.  Gender and baseline body mass (BM), FM, RMR, food intake, and general activity accounted for approximately forty-eight percent of the variation in BM loss.  “BM and FM loss were greater with lower baseline food intake and increased baseline physical activity, or increased RMR.”

CR resulted in a significant change in all variables with similar changes for both female and male mice.  RMR decreased by approximately one fourth in both sexes.  Most animals increased their activity level especially in the final three hours before feeding time.  Average body temperature was slightly decreased by CR.  Changes in activity (more) and body temperature (less) accounted for approximately fifty-five percent of the variation in BM loss.

 Combining these factors into an overall statistical model, baseline food intake and BM and compensatory changes in activity were the most important predictors of weight loss.  On average, mice that lost more weight increased their activity level while mice that lost less weight decreased their activity level.  Baseline RMR and activity were also strong predictors but change in RMR was not.  The authors conclude that, “Individual differences in food intake and activity were the most important factors in determining weight loss in both sexes.”

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