Food and Behaviour Research

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02 February 2015 - The Conversation - Health Check: how to work out how much food you should eat

Amanda Salis, NHMRC Senior Research Fellow in the Boden Insitute of Obesity, Nutrition, Exercise & Eating Disorders at University of Sydney

Dietary guidelines broadly recommend a daily intake of 10,000 kilojoules (2,400 calories) for men and 8,000 kilojoules (1,900 calories) for women. But what do these figures mean in the context of the number of kilojoules or calories you personally need to consume to attain and maintain a healthy body weight?

Daily kilojoule requirements are based on many variables, and no two people are the same across all of these parameters. It makes sense then that if we all followed the same prescriptions for kilojoule intake, some of us would gain weight while others might lose some. That’s because any excess or deficit between the number of kilojoules you consume and the number your body uses results in weight gain or loss.

So how can you know what you need to maintain your energy balance? There are two options: the mathematical approach, which requires kilojoule counting, and the instinctive approach, which involves “listening” to your body (my personal preference).

Both approaches take some trial and error.

The mathematical approach

One way to estimate how many kilojoules you need is to use an online calculator that takes into account major factors regulating energy requirements.

While some prediction equations may be slightly better than others, none of them can tell you exactly how many kilojoules you need to consume to maintain your weight. That’s because no prediction equation can take your genetic make-up, which may impact how fast or slow you churn through kilojoules, into account.

Nor can they account for things like whether you have more or less lean mass or fat on your body than the average adult. Or that you may have spent the last few months on a weight-reducing diet that has dropped the amount of energy you need to maintain your weight by a few thousand kilojoules per day.

Despite these limitations, once you have a ballpark figure of your energy requirements, you can roughly work out how much you need to eat and drink based on the kilojoule content of the foods and beverages you consume.

The instinctive approach

We also have the ability to maintain a healthy body weight without any regard for kilojoule counting; the human body is endowed with a remarkable system that enables us to attain and maintain an optimum weight instinctively.

This system works by controlling your appetite. When your body needs more kilojoules, you may feel hungrier more often. And you may need to eat and drink a greater number of kilojoules in order to feel satisfied.

When your body needs fewer kilojoules, you may not feel hungry enough to eat all the meals and snacks you normally eat in a day. When you do eat, it may take much less than usual to satisfy your hunger.

Provided you heed your body’s appetite signals by eating when you are hungry – and only when you are hungry – and eating enough to feel satisfied but not too full, then you will be able to attain and maintain an optimum body weight.

Sounds simple, doesn’t it? It is when you eat a diet of foods that are close to the form they appear in nature – that is, food that’s minimally processed.

Processed foods, which have many added ingredients such as fat and sugar to make them palatable, make it harder for the body’s automatic weight-management system to do its job. That’s because diets high in fat, or high in fat and sugar, change the balance of natural chemicals in the brain that control this system.