Originally written for www.stuff.co.nz, published 19 June 2018

The idea of counting calories in/calories out as a tool for weight loss has been around forever. The general rule states 0.5kg of fat is around 3500 calories so if you reduce calorie intake by 500 calories a day for 7 days, you’ll lose 0.5kg. Even better, do 500 calories of extra exercise for those 7 days and you’ll lose a whole kilo. If you’ve ever tried this, you probably know that’s not entirely how it works, well, not in the long term anyway.

It’s true that if you consume fewer calories than you expend, weight loss happens. But the actual process of calorie counting is surprisingly inaccurate and totally flawed.

Calories were first defined in the 1800s and a kilocalorie (or Calorie, with a capital C) is the amount of energy needed to heat 1kg of water 1 degree Celsius. Defining the calorie content of food involved drying and burning it to see how much energy was produced.

There’s a range of things that influence your carefully constructed calorie-controlled meal and daily weight loss calculations:

  • Calorie counts on food labels and databases are averages and the number stated can vary 20% to 50% from the reality.
  • Our unique gut microbiome influences how many calories we absorb from our food. If you have more Firmicutes bacteria in your gut, you’ll absorb more calories from food than someone with more Bacteroidetes.
  • How we prepare food (cooking, chopping, blending etc) also changes total calories available for absorption from our meal.

Your carefully planned 2000 calorie a day diet can have quite a large margin of error.

From the calorie expenditure perspective:

  • Calorie burn trackers can be off by as much as 30%, meaning a 500-calorie workout could have burnt anything from 350 to 650 calories.
  • And just as we all absorb calories differently, we also all burn them differently. Genes and epigenetics (the way your genes are expressed) can make you more or less efficient at burning calories and sleep deprivation or poor quality sleep, even for one night, can decrease your calorie burn.
  • Your hormones also play a part, especially for women, as your resting metabolic rate (the energy used to regulate all the things you don’t think about – blood pressure, breathing, heart rate etc) varies through the month and therefore so does your total calorie expenditure.
  • What you eat, the quantity, and your weight history also affects metabolism. Some people’s metabolism adapts better and quicker in response to an increase in food than others, restricting weight gain. However, if you’ve been overweight or obese in the past, its likely your metabolic rate will be lower than ‘normal’.

Finally, we all often underestimate the calories in our portions as well as the portion sizes themselves and overestimate our calorie expenditure.

So the science of calorie counting may not the best way to weight loss or health.

Food is more than the sum of its calories – the number of calories in food do not necessarily co-relate to its health benefits or nutritional value and shouldn’t be the only factor in determining what or how much we eat.