If I eat too little will I fail to lose weight?

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Ever since I qualified as a trainer this has been a topic that has come up time and time again.

Will eating too little cause weight loss to plateau?  Is it possible to gain weight if calories are too low?

I wanted to present the evidence and establish the facts of what I see to be a pretty fundamental misunderstanding from both the general public and exercise professionals.

First things first, let’s talk about calories and weight loss.  If you are not losing weight it means you are eating too many calories. This is an unpopular viewpoint, but it is the truth. Your body just like everyone else’s adheres to the laws of thermodynamics: energy can’t be created or destroyed.  If you’re eating more calories that you need, your body will likely store the excess, making you gain weight.  If you’re eating fewer calories than your body needs, your body raids its energy stores, making you lose weight.

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So, what is a calorie?

Calories are units of energy.  A small calorie (cal) is about the amount of energy needed to heat 1ml of water by 1°C. A large calorie (kcal) is equal to 1000 small calories.  When people talk about calories in food, they are almost always talking about large calories, and just to confuse things, they are commonly abbreviated to cals.

Our knowledge of calories in food dates to the mid-nineteenth century and owes a lot to German scientist Max Rubner.  He very carefully measured the heat generated when burning the three major macronutrients in food: carbohydrates, protein and fat.  His results are still used today to calculate the calorie content of our food.

We can measure the energy our body burns in calories.  Our bodies are complex systems, and there are several factors which influence total daily energy expenditure:

  1. BMR – Base Metabolic Rate: the energy your body needs to stay alive
  2. NEAT – Non-Exercise Activity Thermogenesis: the energy you burn through activity but not deliberate exercise, e.g. wandering around, fidgeting, shopping
  3. EAT – Exercise Activity Thermogenesis: the energy you burn through deliberate exercise e.g. running, cycling, swimming
  4. TEF – Thermic Effect of Food: the energy required to digest, absorb and store the food you eat

These four factors add up to total energy expenditure.

If your calorie expenditure is greater than calorie intake, then you use stored energy to make up the difference, meaning you will lose weight.

If your calorie expenditure is less than your calorie intake then your body will store the excess energy as glycogen and fat, meaning you gain weight.

Do I have a slow metabolism? Is that why I’m not losing weight?

It is common for people to blame a slow metabolism for a failure to lose weight.  However, while some people do have faster metabolisms than other people, the difference is small.  If you compare a person with a very slow metabolism (fewer than 1 in 100 people) to one with a very fast metabolism (fewer than 1 in 100), the difference between the BMR of these individuals could be as much as 400cals, or about a large slice of pizza.  However, most people have a BMR within about 50cals of the average, the equivalent of less than a teaspoon of peanut butter, or an apple.

It is commonly said that if you eat too few calories, your metabolism will slow down to a point where you no longer lose weight. Part of this is true: as you lose weight your energy expenditure does drop.  However, consuming too few calories cannot and will not cause you to gain weight.  This is simply impossible.  If your body is getting too few calories to operate, it will use your energy stores and you will lose weight.

What happens to my metabolism when I lose weight?

Losing weight is accompanied by metabolic changes:

  1. As you eat less, your body produces less leptin.  This hormone, also known as the satiety hormone, makes you feel full and encourages the burning of fat.  If you eat less food, your body produces less of it, which can make losing weight harder.
  2. As you lose weight, BMR drops.  This is simply because as you get lighter, your body requires less energy to run:
    1. Every kg of fat requires about 30cals a day
    2. Every kg of muscle requires about 45cals a day

So, as you lose weight, you need fewer calories.  It’s not a massive effect, but it’s significant.

  1. When you exercise, the less you weigh, the less work your body needs to do.  It’s no surprise that the best marathon runners in the world are skinny.  That’s because it requires less energy to move a light body.  The same applies to you – as you lose weight, you burn fewer calories when you exercise.

A calorie deficit will fail to work when it is no longer a calorie deficit:

Let’s say you decide to cut your calories from 2000cals a day to 1700cals.  You stick to it for 4 months and lose an impressive 6kg.  But then your stop losing weight.

This is not starvation mode.  You’ve made a modest cut to your calorie input and lost a steady 300g a week.  What’s happened is your body now requires less energy to function.  1700cals is no longer a calorie deficit for you.  If you want to keep losing weight, it’s time to drop your calorie intake again, or increase your exercise.

What happens if I eat far too few calories?

If you are on a 20% calorie deficit, you should lose weight at a steady and safe rate.  If you set your target too low, say a 50% deficit, you will lose weight, but there will be some other side-effects:

  1. You’ll get very hungry.  This will make it very easy to fail and abandon the diet entirely.
  2. You’ll be unhappy.  An extreme calorie deficit will likely make you miserable and irritable.  A healthy calorie deficit on the other hand can be enjoyable if done correctly.
  3. You will be very low on energy.  This will likely make you less active, reducing your calorie expenditure, working against your weight loss plans.

What about the difference between carbohydrates, protein and fat?

There is good evidence that eating a low-carbohydrate diet encourages more rapid weight loss.  However, if you are in a calorie deficit, you will lose weight regardless of how many of your calories come from carbs, protein and fat.

Other pitfalls.

Let’s say you calculate your maintenance calories accurately, based on your age, current weight, gender and height.  You keep track of your calorie intake and use an online tool to calculate how many calories you’re burning on your weekend bike ride and Wednesday afternoon jog.  You work out that you should eat 2200cals a day to maintain weight, so you try to eat 1800cals, a 400cal deficit.  You keep this up for 1 month but you’re not losing weight.  What’s happening?

It’s easy to blame a slow metabolism or starvation mode, but there’s a lot of pitfalls that could be causing the problem.

Are you sure you’re not losing weight?

Your body weight changes by as much as 2-3kg a day.  You’re unlikely to have lost more than 1.5kg in a month with a 400cal daily deficit, so it might be hard to detect this change.  Try weighing yourself every day and taking a weekly average.  It can be bad for your health to lose weight too quickly, so try to be sure you’re not losing weight before changing your diet.  And give it time.  You shouldn’t expect to see any obvious results from a diet until about a month in.

How many calories is your exercise routine really burning?

Most tools to calculate exercise overestimate calories burned, sometimes by as much as 100%.  Extremely high intensity exercise can burn an additional 600cals an hour for an average person.  Moderate intensity exercise may be around 400cals.  So, if you go for a 2-hour gentle cycle and your heart-rate monitor watch tells you you’ve just burned 2000cals, this is almost certainly wrong.

Do you really know how many calories you are eating?

Most people underestimate their calorie intake even when keeping a food diary.  This mistake is commonly as much as 20-30%, which in this case could account for a 500cal error.  What about that drizzle of olive oil over your salad, or the mayonnaise with your potatoes?  Did you really have a teaspoon of peanut butter on your toast, or was it a heaped teaspoon?  Also, don’t rely on the calories quoted by restaurants and takeaways.  These are often underestimates.

Here’s some sample calories:

  • Peanut butter, 1 heaped tsp: 100cals (1 level tsp: 50cals)
  • Olive oil, 1 level tsp: 45cals
  • Dark chocolate, 1 piece: 50cals
  • Almonds, 10: 80cals
  • Dates, 2 medjool dates: 140cals

In summary:

If you are not losing weight, you are probably eating too much.  If you think you are in a calorie deficit but are not losing weight, it is probably because:

  • You calculated maintenance calories incorrectly.
  • If you recently lost weight, you haven’t adjusted your diet to account for your new maintenance calories.
  • Your food intake may be higher than you think.  Be vigilant and precise if you’re keeping a food diary.
  • Your energy out in exercise may be less than you think.  An average person will not burn more than 600cals an hour. 

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