Sloths are known for taking it easy and just hanging around, presumably with a relatively low metabolic rate in light of their relative inactivity. A lower metabolic rate would probably only be found in a hibernating bear. On the other hand, hummingbirds are known for their high metabolic rate as they flap their wings at incredible speeds in order to stay aloft. In humans, our metabolic rates can vary dramatically depending upon whether we're active or not and whether we're hypothyroid, euthyroid or hyperthyroid. In fact, our caloric intake can affect our metabolism as noted by those who recommend an ultralow caloric intake which appears to improve markers of lifespan.
In a study to be published next month, the authors followed 652 healthy, non-diabetic Pima Indian volunteers for over a decade after measuring resting metabolic rate (RMR) or 24 hour energy expenditure (24EE). Their conclusion? Higher metabolic rates, whether measured by RMR or 24EE, was associated with greater mortality. The authors questioned how a faster energy turnover could accelerate aging as determined by mortality? Perhaps from increasing oxidative stress?
Clearly this study opens up a can of worms since all other studies have demonstrated decreased mortality in patients w/more muscular body composition consistent w/increased RMR & 24EE, whereas obesity, and thus decreased RMR & 24EE, has been associated w/increased mortality.
The take home point from this study is that we don't have all the answers, just more questions. But for now, I would consider this result a fluke rather than a trend.
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