Updated 04 July 2013

The kilojoule conundrum

Dr Ross Tucker, Health24’s FitnessDoc, takes a deeper look at the importance of exercise as far as weight-loss is concerned in the second article in this series.


The kilojoule conundrum: In vs. out

Dr Ross Tucker, Health24’s FitnessDoc takes a deeper look at the importance of exercise as far as weight-loss is concerned in the second article in this series.

I realised overnight what an enormous challenge this topic presents - there is simply too much content and too many angles to cover, and so I have no doubt that I will fail to do justice to some of the sub-plots in this fascinating area. 

However, as I stressed before, my approach here is to look at weight loss from a general point of view, not an elite one, because weight loss transcends performance boundaries.  Of course, it will feature, and I know many of you work in that performance realm - we're all about high performance here, so it will come up, but I hope to tap into a different area of exercise physiology in this series.

Perhaps one day, I can team up with a dietician and some researchers in this field and produce the book that covers ALL the angles! 

Thinking back to the previous post, where I mentioned that there are already 84 000 books on weight loss, I think a good title for a book would be "The devolution of weight loss.  Back to basics", because it occurred to me that the evolution of the obesity pandemic has coincided with the explosion in "knowledge", when all along, the answers are pretty basic.  Tough to implement, make no mistake, but basic in theory.

And that, the basic approach, is what I will try to unpack in this series, starting today with the basics of energy balance.

Kilojoules in vs kilojoules out:  Simple principle, with complex implications

I have no doubt that all of you know the basic premise behind the "scale" of weight loss, which says that your body mass is a function of the balance between the energy intake and the energy output on a daily, weekly, monthly or yearly basis.

On the left hand side of the scale, you have energy intake, in the form of food.  Each has a caloric 'implication' - 1 g of carbohydrate and protein adds 4 kCal, whereas 1 gram of fat contains 9 kCal (alcohol, incidentally, is 7 kCal/g).

On the right is energy output, and that is made up of resting energy expenditure (a combination of sleeping and awake), the thermic effect of food, because when we eat we actually increase energy use for a short period and different foods produce a different response. 

And then finally, there is the energy expenditure from activity, which is where exercise fits in, and which is the most "malleable" of the different components - it can be zero, or it can be enormous, if you exercise like a Tour de France cyclist and burn 5000 kCal per day in training or racing!  This is shown in the diagram below.

The basic premise of weight loss is this:  weight loss requires that your energy output exceed your energy intake for a prolonged period.  Much like financial management, weight management is simply a balance between spending and saving - if you wish to save money, spend less than you earn.  If you want to lose weight, eat less than you burn.

In any event, this principle is behind pretty much every strategy ever devised to lose weight.  Hours of exercise to add to the right hand side of the scale, or starvation diets or any one of the millions of other diets to reduce the left. 

Or weight training to try to increase resting metabolic rate, and even eating specific foods like chillies to try to increase the thermic effect of food.  Some of these are more effective than others, as you can imagine, and it's the intelligent reduction in energy intake combined with the increase in energy expenditure that will ultimately lead to weight loss (and note that the operative word is "intelligent").

So, in Time magazine's controversial article, exercise took a knock because of what was called the compensation effect.  An hour of exercise could burn anything between 300 and 1500 kCal, depending what you do - run like a Kenyan and you'll be up at 1500 kCal, walk slowly and you're at the bottom end (this huge range illustrates one of the biggest problems with this kilojoule "counting" approach, as we'll see shortly). 

The problem is that if you do this time, but don't also manage your diet, then it's possible to replace all that energy, and then some, with just one meal or snack.  The end result is that the scale tilts in the wrong direction and you gain weight, not because of exercise, but because of the dietary choices you make in association with exercise.

For example, compare the following two ways to spend a Sunday afternoon:


So, on the left, the exercise option can be all but cancelled out by diet, in this case a can of Coke.  In that regard, you'd be better off watching television for 30 minutes and eating nothing. 

This was basically the take home message in Time's article, which clearly misses a big part of the picture.  If you make the same dietary choices by drinking a Coke in Option B where you remain inactive, then you shift in the other direction, and may gain weight. 

All things being equal with regards to diet, exercise is valuable - how many people exercise as a reason to indulge in foods that they otherwise would not be able to?

Let me emphasise, however, that there are many other reasons why Option A is the better one - for one, you can't simply not eat in order to lose weight, and so Option B is not really feasible in the long term. 

Second, life is not a game to see if you can balance your kilojoules - there's more to it than that.  Third, the health benefits from exercise are not captured in a single number of kilojoules out minus kilojoules in.  These are all crucial aspects and I want to stress that one should not get too hung up on the minutiae, but rather understand the principle.

When details matter

All of this is obvious, and forgive me for oversimplifying the situation, but you'd be surprised at how easy it is to be tripped up by this simple principle. 

If you are embarking on a weight loss plan, and have yet to see significant results despite diligently exercising for 45 minutes a day, then the answer is likely that you still haven't addressed the other side of the scale adequately. 

If you have reached a plateau in weight loss, then the same may be true - it's time to consider how much you eat, when you eat it, and what you are eating, because you may unwittingly be negating your exercise with simple dietary practices. 

And I must stress this - what you read in books and see on TV is generic advice - it can only take you so far.  There will come a point where you need specific advice and that is where consulting a dietician becomes vital, especially for the more complex situations.

Even 'healthy meals' can be damaging

Take for example a more serious athlete, who is looking to lose the final two or three kg to get down to racing shape.  They may train for two hours a day, and burn in excess of 2 000 kCal during those two hours. 

However, even a "healthy meal option" can put that back in one sitting.  Here in SA, we have a chain of health shops called Kauai, and most serious athletes would not think twice about eating there (think organic food, smoothies, rye breads and everything you read is healthy).

Problem is, a typical meal at one of these stores, consisting of a breakfast wrap and a low fat protein smoothie, adds back 1 600 kCal.  The end result is that two hours of exercise goes nowhere, and our cyclist, despite training as hard as his body can tolerate, does not lose that weight. 
 The only solution here is to manage the details - portion sizes, content and even the timing of the meal.

For example, there is evidence that if you delay eating after exercise for about an hour, you burn more fat than if you eat right away. 

This has to do with keeping insulin levels down, and insulin is a hormone that drives carbohydrate use, while "tuning down" fat use.  The problem is, if you delay eating, you may compromise your recovery, which means you can't sustain high quality training day after day. 

You also can't cut carbohydrates out, and you certainly can't under eat - there is compelling evidence that the biggest risk factor for becoming sick during intensive training is an energy deficit.  

When to eat?

You may also have read that if you train before eating, you rely more on fat and this would lead to greater weight loss.  In theory, yes (it has to do with availability and insulin again), but practically, you might battle, because the risk of hypoglycemia is higher, you may not recover well from the metabolic stress, and because the body is often too smart to be tricked in the long term. 

You therefore have a dilemma if you are trying to train hard and struggling with stubborn weight.  Each case would have to be managed on a case by case basis, and I can't stress enough that seeking out expertise to discuss these details is essential - generic advice only takes one so far, after which time books, websites and magazines can't help any longer. 

Dieticians fulfil this role, although I will touch briefly on these issues in future posts, so don't worry, I'm not leaving you hanging completely.

Kilojoule counting: guaranteed weight loss or futile exercise?

Returning to the energy balance scale, everything I have written so far is probably steering you into thinking that if you simply measure what you eat, and measure how much you exercise, you can balance your own scale and lose weight.

And you'd be swimming into dangerous waters by doing this.  Let's look again at the scale - there are two sides that you would have to manage, energy intake and energy output.

On the energy intake side, you have the following problems:

1. You would have to cook all your own food and measure everything that goes into it, which means you can't eat out and you limit your options to only what you can measure.
2. Not all foods have clear labels, so you would invariably miss certain measurements.

3. On the energy output, the problems are even more numerous:

4. You can't know with certainty how much energy you have burned during exercise - there are reasonably accurate estimations, based on heart rate and many studies, but precision is beyond us.

5. You cannot quantify the other aspects of this side of the scale (see the diagram above), particularly the resting energy expenditure and the thermic effect of feeding, which make up an enormous part of the total.  Get this wrong by even 5% and you're looking at a big error in your calculation.

There is a substantial increase in energy use after exercise - this post-exercise elevation in metabolic rate is not accounted for by tables.  So you might run for 60 minutes and burn 800 kCal, but the actual total thanks to exercise is higher.  Quantifying this is a problem.

Now, the point is that you cannot measure these things accurately, and therefore you cannot micro-manage this. 

Diet awareness your most powerful ally

Here's an illustration of the complexity:  An 80kg man who remains at the same weight for 10 years has, over this period, managed to balance a grand total of about 9.1 million kCal.  In contrast, if he gains 10 kg over this period, it's because of a mismatch of about 70 000 kCal.  In ten years, that works out at 19 kCal per day.

In other words, you can try to measure your kilojoules in and kilojoules out, but if you're systematically inaccurate by 20 kCal per day (the equivalent of a sip of Coke), you gain 10kg.  My point here is that counting kilojoules, unless it's done with absolute precision, is not a good method to manage your weight.

Where kilojoule counting is helpful is in creating awareness about diet, and if there's one thing you remember from this series, it is that awareness is perhaps the most powerful ally you have in trying to lose weight. 

There are studies, for example, that have found that having people write down what they eat every day (volumes, sizes and content) leads to significant weight loss, even though they are not told to do anything else differently. 

Similarly, if you are on an exercise programme, and you write down what you do and what you eat, you lose more weight than if you exercise 'blindly'.  In both instances, it is awareness that helps, because it guides sensible food choices, smart exercise and it is this combination that helps to produce sustainable weight loss.

So knowing what you put into your body, and knowing how much you burn, can help guide you. 

 It may be the catalyst behind making sensible choices to reduce portion sizes, or change the content of your meals slightly, or maybe increasing the time you spend exercising. 

If you have an idea that your daily energy intake is 2 500 kCal (give or take, because as I've said, absolute precision is out of reach), and you're exercising for 30 minutes at a moderate intensity (± 400 kCal, added to a typical day of about 2000 kCal), then you can correctly deduce that you need to change something - either train for 40 minutes, or train slightly harder, or cut down on portion sizes, and you'll start to see results.

Fat burning 101

Forgive me for what may seem a very basic approach - I did say that a book on this topic should be called "Back to basics", because so much complexity has been added to this by all kinds of gimmicks and 'revolutionary' ideas that hopefully it's a refreshing reminder that the actual principle behind weight loss (and the reason we often fail) is pretty straightforward.

I don't mean to trivialise the problem, though.  If it were simple, fewer people would struggle.  But the principle is straight-forward - implementing it, sticking with it, not quite as easy.

What we need to do next is discuss exercise and energy use, particularly around fat burning.  What intensity?  How long?  Do you only start burning fat after an hour?  How do you manipulate it? That's coming up in Part 3


And just so you know, you've probably burned 20 kCal while reading this!  But don't worry about the number!

Dr Ross Tucker, is Health24’s FitnessDoc and has a Ph.D. in Exercise Physiology from the University of Cape Town and a Post-Graduate degree in Sports Management from the UCT's Faculty of Commerce. He is currently employed at the University of Cape Town and Sports Science Institute of South Africa, and works as a consultant to various sporting teams, including South African Sevens, Canoeing, Rowing and Triathlon SA. He also blogs on


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