Basal metabolic rate

We looked at metabolism at the support group meeting last weekend and I found it really interesting - and helpful for sorting out exactly how much I need to eat (or not!) each day to lose weight. Basically, the metabolic rate is the rate at which a person burns energy. Contrary to popular belief, the heavier you are, the higher your metabolic rate is. How many of us have come across a skinny person who eats loads - and heard others say that that person must, therefore, have a high metabolism?! Well it's just not true. Imagine a normal weight person is a car. The car does absolutely nothing all day except idle with the engine switched on (this is the equivalent of this mythical normal person breathing, circulating their blood and beating their heart). At the end of the day, even though that car has gone nowhere, petrol (i.e. energy) has been used up. It's the same for human beings. But now imagine that the person is obese - and they are a lorry (bear with me!). The lorry also sits idling all day and at the end of that day, even though it hasn't gone anywhere, much more petrol (or diesel) has been used up than it was in the car. This is because the lorry is bigger, has a large engine and needs more energy input to keep it going - even though it is doing exactly the same as the car.

So it is with thin people and fat people. For the same activity, fat people burn more energy than thin people. Hence, the metabolic rate of a fat person is greater than that for a thin person. The energy we are talking about here, is the plain and simple calorie. That's what a calorie is - a unit of energy. So, if I did absolutely nothing all day except sit on my bum, I would need a certain amount of calories (energy input) simply to maintain my weight where it is now. That number of calories is my basal metabolic rate (BMR) and can be determined by (a) weighing yourself on one of those clever bathroom scales (or at your clinic or hospital) that will tell you what it is, or there are all sorts of web sites where you can look up your BMR from a table, based on your gender, height and age. I got mine from the clinic at the weekend and it's currently 1960. This value will go down as I lose weight.

In reality, most of us do not sit on our bum all day - we do stuff, and if we are taking any notice of the advice given to us by everybody, we should also be getting lots of exercise. Exercise burns energy so we have to take this into account too. There are two different ways of doing this:

1. If you have a pedometer that records the amount of calories you have burned throughout the day by walking, then you can simply add this number of calories onto your BMR.

2. If you don't have that then you can estimate your level of exercise and apply a multiplier. There is a standard table of four activity levels which can be found on lots of web sites which goes something like this:

Level 1: Essentially sedentary, sit down a lot, don't set aside any time for regular physical activity. Multiplier = 1.2
Level 2: Light activity, do some walking during the day and take part in formal activity / sports 1-3 times per week. Multiplier = 1.38
Level 3: Moderately active, job needs physical activity, you take part in formal activity / sports 3-5 times per week. Multiplier = 1.55
Level 4: Very active, very physical job, you take part in activity and sports every day. Multiplier = 1.73.

So, choose which one of these applies to you and mutliply your BMR by the value shown.

Whichever method you choose, you should now have a figure that represents your AMR (activity modified metabolic rate). In my case, if I choose a multiplier of 1.38 x 1960 gives an AMR of 2705. This means that for that level of activity, your body needs that many calories each day to maintain weight. If you then subtract from that figure what you are actually consuming each day (about 1000 calories in my case), you then have the number of calories of deficit. In my case this would be 2705 - 1000 = 1705. So, if this was the pattern every day, this means that I would be consuming 1705 calories a day less than I need.

Over the course of one week this would add up to 1705 x 7 = 11935. This is how many calories you will have effectively 'saved'. Now, 3500 calories is equivalent to 1lb weight. So if you divide the 11935 by 3500 you get 3.4lbs. This is how much weight you will lose in one week if you do level 2 amount of activity and eat 1000 calories per day (and your BMR is 1960 like mine)!!

It's important to remember two things here: one, that your weight determines the BMR, not the other way around, and two, that you need to adjust your own BMR periodically as you lose weight because it will go down. There's just one more thing. For a person of given weight and age, gender etc, your BMR will be higher if you have more muscle mass. So while I wouldn't recommend starting on body building (!), exercise that will add some muscle tone (e.g. swimming, yoga, pilates etc) will raise your BMR. This will mean that if you don't increase your calorie intake, your calorie deficit each day will be greater - and weight loss greater!! Simple!!