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One study noted that one standard deviation of variance for resting metabolic rate (how many calories are burnt by living) was 5-8%; meaning 1 standard deviation of the population (68%) was within 6-8% of the average metabolic rate. Extending this, 2 standard deviations of the population (96%) was within 10-16% of the population average.
Extending this into practical terms and assuming an average expenditure of 2000kcal a day, 68% of the population falls into the range of 1840-2160kcal daily while 96% of the population is in the range of 1680-2320kcal daily. Comparing somebody at or below the 5th percentile with somebody at or above the 95th percentile would yield a difference of possibly 600kcal daily, and the chance of this occurring (comparing the self to a friend) is 0.50%, assuming two completely random persons.
To give a sense of calories, 200kcal (the difference in metabolic rate in approximately half the population) is approximately equivalent to 2 tablespoons of peanut butter, a single poptart (a package of two is 400kcal) or half of a large slice of pizza. An oreo is about 70kcal, and a chocolate bar in the range of 150-270kcal depending on brand.
Metabolic rate does vary, and technically there could be large variance. However, statistically speaking it is unlikely the variance would apply to you. The majority of the population exists in a range of 200-300kcal from each other and do not possess hugely different metabolic rates.
Exercise and physical activity reliably increase metabolic rate, and the aforementioned study notes that 1 standard deviation of variance in regards to this is 1-2%. Assuming somebody exercises 300kcal (on average), the true value of inherent caloric expenditure can vary from 294-306kcal (68% of the population) or 288-312kcal (96%). Although energy expenditure overall appears to be the strongest correlate predicting fat loss from exercise, it should be noted that alterations in lean mass (muscle) can further influence metabolic rate. Metabolic rate is highly correlated with lean mass (one increases when the other does) and the process of hypertrophying skeletal muscle (growing the muscles you can contract) has a high caloric cost to it.
This 'low' correlation seems to apply to aerobic exercise (cardio exercise that can be maintained at a constant speed for a prolonged period of time). Resistance training (weight lifting) or high intensity intermittent cardio exercise (sprinting, or any exercise which cannot be maintained for long and thus must be done intermittently) has something called EPOC, Post-Exercise Oxygen Consumption, which is an enhancement of the metabolic rate beyond working out.
Although a standard resistance training workout (in which participants were not forced to exhaustion) possesses a relatively low EPOC value, such as 19 extra calories the hour after exercise, higher intensity exercise such as the 'Tabata' protocol (170% VO2 max for 20 seconds, with 10 seconds break; repeat 8 times and lasting 4 minutes in total) are associated with greater EPOC and oxygen consumption. In these anaerobic exercises, the correlation noted above tends to break down and the 1-2% variance likely does not apply.
Exercise will induce increases in metabolic rate in all persons by the inherent nature of physical movement. There is a lower degree of variance for basic, aerobic movements like walking or jogging. The low variance may not hold for high-intensity exercises
A novel study investigating 11 Japanese men under living conditions, but in a metabolic chamber to measure metabolic rate via heat, found that after measuring all participants across 'low' intensity days, 'moderate' intensity and 'high' intensity, that the same persons who averaged a 2228+/-143kcal metabolic rate during low activity increased daily expenditure to 2816+/-197kcal (average increase of 588kcal) during moderate physical activity, and to 2813+/-163kcal during high activity. The increase in metabolic rate via moderately vigorous active living (basically, working 'hard' but barely breaking a sweat and not losing your breath) can negate the difference in metabolic rate found in the majority (68%+) of the population (using numbers from the first study cited).
Moderately active living may be sufficient to negate the inherent differences in metabolism for a good deal of persons not on the extremes of the metabolic bell curve (so, those that are unlikely to be diagnosed with hypothyroidism; a low metabolic rate)