I have been asked to do a post about energy expenditure and how this can be measured. So here goes! This should complement the ‘All things energy in’ article published last month (click here). Straight away it is important to emphasise that this is not an exact-science and outside of a research lab, energy expenditure is an estimation. However, good equipment and gadgets can make an estimation more accurate (close to the true value).
Key question: How can I estimate my energy expenditure?
If you read Article #2, you’ll know that your energy expenditure consists of three components, resting metabolic rate (basic energy needs), dietary induced thermogenesis (energy needed to break food down) and physical activity (energy needed to move the body, including exercise and also non-exercise activities). We can measure these components in a lab environment by measuring samples of your breath (to look at how much oxygen and carbon dioxide you are breathing in and out) and applying this info to some equations. This is known as indirect calorimetry. There are many techniques that can be used by scientists to measure energy expenditure very accurately, but this is not the focus of this blog, because unless you are taking part in a research study those facilities might not be accessible. That being said, taking part in research is REALLY fun and you can get great info about your body!
The second best option for understanding your energy expenditure is to estimate. This can be done by using equations that are developed by measuring lots of peoples energy expenditure using the best techniques and then looking at patterns/predictors. Let’s take each component of energy expenditure and work things out:
Resting metabolic rate
For women it is likely to be in the region of 1,400 calories and men around 1,600 calories, but this will vary depending on how big you are and your body composition. To find out your resting metabolic rate (in calories per day) it is possible to use formulas such as the Schofield equation (click here):
1) Men (≤30 years) = 15.057 × body weight in kg + 692.2
2) Men (30-60 years) = 11.472 × body weight in kg + 873.1
3) Women (≤30 years) = 14.818 × body weight in kg + 486.6
4) Women (30-60 years) = 8.126 × body weight in kg + 845.6
Another widely used equation is the Harris and Benedict equation (click here):
1) Men = 88.362 + (13.397 × body weight in kg) + (4.799 × height in cm) - (5.677 × age)
2) Women = 47.593 + (9.247 × body weight in kg) + (3.098 × height in cm) - (4.330 × age)
When I did the equations for myself the difference between the two was less than 5 calories per day. The important thing to remember is that these equations are estimates based on big groups of people and will give an indication of your resting metabolic rate, but that’s all. If you have a higher amount of muscle than most people your resting metabolic rate could be higher. When you know your calories/day you can divide this by 24 to get calories/hour.
Diet-induced thermogenesis (DIT)
The second component is diet-induced thermogenesis (DIT) and is roughly 10 % of energy intake if you have a mixed-diet with carbohydrates, fats and protein. Yo work this out you can multiply your total daily calories by 0.1. You might be able to refine that estimate if you know how much carbohydrate, fat, protein and alcohol you’ve consumed (click here). If you write down your diet over a ‘normal day’ and calculate how many calories you got from carbohydrates, fats, protein and alcohol you can work out DIT for each macronutrient. This can be done by multiplying the calories by 0.01.5 for fats, 0.075 for carbohydrates, 0.25 for proteins and 0.15 for alcohol. For example, 1000 calories from carbohydrate would require (1000 x 0.075) ~75 calories to process. However, this would require recording your diet very carefully over at least a few days, and the percentages above are also based on estimations. So the 10 % of your total calories is a good start. For example, if you eat and drink 2500 calories, this would be 2500 x 0.1 (10 %) = ~ 250 calories.
Physical activity energy expenditure
Resting conditions get a metabolic equivalent (MET) score of 1, so we can compare other physical activities to this score depending on a number of factors, including the intensity and type of activity you are doing. An activity with a MET score of 3 means you would be burning 3 times as much energy as rest. To work out energy expenditure (in calories) for an activity you can use an equation: MET x your body weight in kg x the duration in hours (click here).
So how do I know my MET’s? You can click here for that. The activities are in alphabetical order and also look out for the intensity. Walking for pleasure gets a MET score of 3.5. So if you walk for one hour per day and you weigh 75 kg your energy expenditure from walking would be 3.5 x 75 x 1 = 260 calories from that activity. Some people think exercise burns more calories than it does, which can lead to confusion. This doesn’t mean you shouldn’t exercise, it is just something to remember as I talked about in this previous blog post.
A complete example
You are a 60 kg women, aged 60, who eats 2200 calories each day, walks for 2 hours each day and is at their work desk for 10 hours each day.
Resting metabolic rate = 8.126 x 60 (body weight) + 845.6 = 1333 calories (55 per hour).
Diet induced thermogenesis = 2200 x 0.1 (10 %) = 220 calories
Physical activity energy expenditure = walking 3.5 (METS) x 60 (kg) x 2 (hours) = 420 calories
= desk work 1.5 (METS) x 60 (kg) x 10 (hours) = 810 calories
So there are 12 hours of activities above rest (10 hours of desk work and 2 hours of walking) which would equal 420 + 810 calories = 1230 calories. This would be added to 12 hours of resting metabolic rate (e.g. sleeping and sitting still) 12 x 55 calories = 660 calories.
Total calories in a day = 1230 calories (activity) + 660 calories (complete rest) + 220 calories (dietary induced thermogenesis) = ~2100 calories.
The take-home message: Knowing how much energy your burn each day can help you understand your energy needs and plan your diet so that you can manage your body weight. However, these values are estimates so if you want to know more, taking part in research studies can be a great way! Have a go at estimating your energy expenditure!
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