How to work out how much food you should eat

Daily kilojoule requirements are based on many variables, and no two people are the same across all of these parameters. Photo / 123RF

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?

I'm going to stick with kilojoules in this article because kilojoules - not calories - are the metric unit for measuring energy, just as kilograms - not pounds - are the metric unit for measuring 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. These include your sex, weight, age and activity levels. Some calculators also take height into account.

Such calculators are based on prediction equations that estimate true energy requirements - as measured in a laboratory - from readily available parameters.

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.

For this you will need a comprehensive kilojoule counter (I recommend CalorieKing, which has extensive data bases for Australia and the United States) and a way to track your food and beverage intake throughout the day (MyFitnessPal comes highly recommended). Numerous other programs and apps are available for both.

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.

Using trial and error to get there

Regardless of how you estimate the amount you need to eat, the current obesogenic environment prevailing in most countries, which pushes us to gain weight, means you will need to use some trial and error to determine exactly how much to eat.

If you are gaining weight, or you find your clothes are getting tighter, then you're consuming more kilojoules than your body needs, period.

To prevent further weight gain, you need either to cut your energy intake or to increase the number of kilojoules you burn via physical activity - or both. Do this by becoming mindful of the kilojoule content of the foods and drinks you consume. Drinks are particularly important, especially alcohol, because they sneak into your day and may not be accounted for.

Or you need to keep a written hunger-and-satiety diary to ensure you're eating only when hungry and only until you feel satisfied. And that your food choices are as close to natural as possible.

In today's obesogenic world, attaining or maintaining a healthy body weight doesn't just happen naturally for most people. It usually requires conscious attentiveness to how much and what you're eating. The good news is that you can do this using external signals such as kilojoule counting, or via your body's own internal signals.

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

This article was originally published on The Conversation.

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