Energy requirements are calculated by combining resting metabolic rate (RMR) together with an activity factor which measures how active you are. The more active a person is, the more energy your body requires on top of the RMR. Even if you are fairly sedentary, your body will still require additional energy to walk around, sit, stand, digest food and many other things.
Resting metabolic rate (RMR) is the minimum amount of energy required to keep your heart pumping, lungs breathing, brain firing, cells reproducing and healing… basically to keep your body functioning on all levels.
Usually in practice, RMR will be estimated using an equation. There are a variety of different equations with various assumptions or minimum requirements. Some require height, weight and age, others use a measure of lean muscle mass and some are also gender specific. However it’s really important to recognise that these are estimates, and can vary quite a lot when applied to an individual person. The most common equations used are Harris-Benedict, Cunningham or Mifflin-St Jeor.
The most accurate way to establish RMR is to measure it. However this requires expensive kit which is usually only available at universities or research labs. Measuring RMR requires you to lie completely still under a clear hood for at least 20 minutes first thing in the morning in a fasted state. Special machines measure the oxygen you consume as well as the carbon dioxide you produce to determine whether you are burning fats or carbohydrates at rest, the rate of which establishes your measured resting metabolic rate.
I’ve been doing some RMR testing in the labs this week which has been a good eye opener into the differences between measured and estimated RMR. An athlete I tested earlier this week had a RMR that was 200kcal higher than predicted by equations, while my own RMR was 100kcal lower than estimated. There are many factors that can influence these sorts of differences, for example habitual diets, level of training, medications and amount of lean muscle tissue to name a few. While they only seem like small numbers, it becomes important on an individual level because in certain circumstances those tiny differences can result in either weight gain or weight loss depending on if more or less food is consumed.
On the whole I’m not an advocate of calorie counting, but it is definitely a useful tool to increase awareness of the foods that you are eating. It also demonstrate how easily little extra’s throughout the day can add up to make a big difference on the scales over time.
As part of the research trial I am currently a participant for, I have been keeping a food diary on myfitnesspal for the past month or so. It’s been interesting to see my weekly eating patterns and how it’s influenced by my training loads. Based on my age and weight, myfitnesspal uses an equation to set me a generic calorie target for the day – a low, unrealistic ‘goal’ which I typically need twice as much of to meet my training needs. If I were to eat the recommended energy intake that myfitnesspal suggested, I would be considerably under-eating. On the other hand, if I overestimated my activity levels with an equation I might end up eating too much. It’s a fine balance.
Next time you see a diet plan recommending that you eat 1800kcal for example, remember that these sorts of numbers are guides. Not hard-and-fast numbers to stand by. As a dietitian, I don’t usually have machines on hand to measure RMR, so would use equations to estimate energy requirements – as a rough starting point. Rather than being a definitive number that doesn’t change, this would be tweaked and either increased or decreased over time depending on individual goals, activity levels and how each individual responds to establish a more true value.
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