Maths of Dieting - Understand the Numbers Behind Weight Loss

Updated on January 1, 2018
Susan Hambidge profile image

Susan occasionally writes about health and fitness issues taking the facts from her husband - a fitness professional and personal trainer.

Use the Calorie Deficit to Lose Weight

The Calorie Deficit - What it Means

My husband repeatedly hears people say that they cannot lose weight. It is often the first thing people want to talk about when they hear he is a personal trainer. Dieters complain that they eat sensibly but cannot shift the pounds. Once he explains the tried and trusted 'calorie deficit' approach, they go on to understand what is needed to successfully shed the pounds.

Our bodies needs a certain number of calories to function on a daily basis. Just breathing uses calories. But when we consume more calories than we need, the body stores those calories as fat to use as energy later on. If we continue to eat more calories than needed, the body doesn't get to use this stored fat, instead it just stores more and more and we put on weight.

If we consume roughly the same number of calories as we use on a day to day basis, then no fat is stored and we maintain the weight we have.

But if you eat fewer calories than you use - you have a calorie deficit. The body then starts using the stored fat to keep it going. If you continue to do this day after day, the fat will gradually disappear.

Most people understand this; but what they don't always understand is the numbers involved.

You have to eat 3,500 fewer calories than you use to lose just one pound of fat!!

Yes - that is correct - 3,500 fewer calories for 1lb of weight

This number is often a lot higher than people think - commonly dieters think you only need to burn a few hundred calories to lose weight. This is a common mistake, but it does make a considerable difference.

A 3500 Calorie Deficit Equals 1LB of Fat Loss

Eat 3500 fewer calories than you burn each week to lose 1LB a week
Eat 3500 fewer calories than you burn each week to lose 1LB a week

How to Create Your Calorie Deficit

The real key to doing this is to be 100% realistic. Do not be untruthful to yourself, don't guess the numbers, don't exaggerate, do not fool yourself!

  1. Work out how many calories in an average day you burn. The number will depend on your age, weight and height, and how active you are. The older you are, the fewer calories you naturally use in a day. See useful calculators below.
  2. Count every calorie you consume. There are lots of apps and websites which have details of every food - but be very truthful about your portion size. Actually weigh things, or check the packet carefully. I have personally used 'My Fitness Pal' app on my phone and found it incredibly helpful.
  3. Count accurately the exercise you do; do not guess. It often takes a lot more physical activity than people imagine to burn 100 calories.
  4. Use around 500 calories more than you eat per day. Do this for one week - 7 days - and you will have had a calorie deficit of 3,500.

The Maths Behind Working Out Your Daily Resting Calorie Use

To work out how many calories you normally use in day when you are just resting - do the following mathematics:

For men: 10 x weight in kg + 6.25 x height in cm minus 5 x age in years + 5
For women: 10 x weight kg + 6.25 x height in cm minus 5 x age in years – 161

Ref: Harris/Benedict Formula

But to make things easier, use the following website to calculate accurately

For a rough guide - women use between 1400 and 1800 calories a day at rest, and men use 1800 to 2500 a day. The heavier and taller you are, the more calories you naturally burn.

To this resting calculation - you add the amount of exercise you do a day to see how many calories you usually use a day. If you walk a lot at work, having a step counter will help you calculate this. If you sit all day at work, then you will not usually burn extra calories.

Now you have your starting point - be realistic!

Ways to Create Your Calorie Deficit

  1. To lose 1lb of fat a week, you must consume 3,500 few calories than you burn.
  2. Break this down to 500 calories per day.
  3. Either eat 500 calories less - if you're a woman that could mean only eating 1000 calories a day, everyday, which is not to be recommended. This is too few for good health and for morale so eat 1200 calories a day and move more.
  4. Burn 500 calories more a day - that would mean running on a treadmill for 45 minutes, or walking briskly for 2 hours, or swimming for 50 minutes or an hour's exercise class. Every day.
  5. Or do a combination for both - this is the recommended way to do it, but you can work out your week to suit your lifestyle. The idea is to makes sure over the 7 days or a week, you've hit the 3500 target.
  6. Being generally more active all the time will raise your daily calorie burn, so get up from your chair frequently, walk to the shops, use the stairs etc.

Tips for Using Calorie Deficit to Lose Weight

Understanding the maths helps a lot of people reach their weight goals. These are the top tips for making it work.

  • Losing weight slowly but constantly is the best way to do it. You will tend to make life changes that will stick with your forever this way, rather than carrying out changes that you cannot maintain. Slow weight loss also gives your skin more of a chance to shrink with you.
  • You can realistically aim for 2lbs of weight loss a week - 7,000 calorie deficit by eating less, moving more and exercising regularly.
  • Be accurate, do not 'forget' food and drink you consume and 'exaggerate' the exercise you do.
  • Do things you enjoy - but just in an educated way.
  • Drink water. Thirst is often mistaken for hunger, and being hydrated enables the body to function properly.
  • Snacking must be counted. It is best to plan for snacks and have fruit, yogurt or nuts ready to eat. If you do not plan you will find yourself reaching for the high fat, high sugar and high calorie options that always seem to be around.
  • If you find it difficult to maintain a 500 calorie deficit a day - try a 350 calorie deficit a day. If you do the mathematics, it means you will lose 1LB every 10 days, so it will take a little longer, but if that helps you reach your goals, that is fine. A total of 3 LBS a month is still fantastic if you carry on month after month!

How to Burn 500 Calories

  • Do a Zumba class
  • Walk for 2 hours - broken up into 30 minute bursts
  • Jog for 45 minutes
  • Go to the gym for an hour
  • Swim for an hour
  • Do an Aquasize class
  • Dance for an hour

Mix and match your days to suit you

How to Reduce Your Calories

Most takeaway food is higher in calories than food cooked from scratch, so get in the routine of making your own lunches, dinners and snacks.

Pack extra veg and salad into everything to reduce the calories. Try half potato, half root vegetable mash or chilli with more tomatoes and peppers than beef, or half spaghetti, half courgetti bolognaise.

Portion control is essential. You will get use to eating smaller portions after a while, but it can come as a shock at first. Weigh carefully to be accurate or you could be eating more calories than you think you are.

Do not take it for granted that 'low fat' versions are lower in calories - often more sugar is added to compensate!

Look at what you drink; water flavoured with lemons, oranges or mint have no calories and are much better for you than anything else. Keep hydrated.

Replace regular rice with cauliflower rice.

Eat fruit in place of cookies or muffins.

Do not add cheese to everything if it tastes good without it.

Reduce sugar in your tea or coffee. Don't add the sugary flavourings into coffee. Try your beverage without milk.

This content is accurate and true to the best of the author’s knowledge and does not substitute for diagnosis, prognosis, treatment, prescription, and/or dietary advice from a licensed health professional. Drugs, supplements, and natural remedies may have dangerous side effects. If pregnant or nursing, consult with a qualified provider on an individual basis. Seek immediate help if you are experiencing a medical emergency.

© 2017 Susan Hambidge


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