A primer on weight loss

Permanent weight loss in essense boils down to a negative difference between energy intake and energy expenditure: burn more calories than are getting in. Practically speaking: eat less than you expend. Every pound goes through the mouth (this saying doesn’t work in English, oh well). Having said that, there are some caveats to this. Obviously, weight loss is not the same as fat loss. So it’s important from which source the negative energy difference between intake and expenditure comes from: fat mass or the lean body mass (in particular: muscle mass). Moreover, energy intake and energy expenditure aren’t two isolated factors. They’re dependent on each other. An adjustment of energy intake can affect energy expenditure. There are a bunch of physiological processes which can regulate energy expenditure.

In spite of this, a far too simplistic view of losing weight has emerged among health professionals. For example, many use the ‘7000 kcal per kilogram of weight loss’ rule (or 3500 kcal per pound if you don’t use our superior metric system). This rule prescribes that if you eat 500 kcal less per day than what you expend, you’ll lose half a kilogram of weight each week. In practice this rule is implemented by estimating energy expenditure with a formula (e.g. the Harris Benedict formula) and then subtracting 500 kcal from it. Every week the person who sticks to this should lose half a kilogram of bodyweight. If this does not happen, then often a simple explanation is provided: the formula used to estimate energy expenditure was wrong or the person who attempted to lose weight did not stick to the diet. This ignores the fact that weight loss does not develop in a linear fashion and is simply not well described by this rule.

For that reason, I’ll discuss some stuff in this article which affects weight loss and helps explain why the ‘7000 kcal rule’, as used in practice, is inadequate and why weight loss always is slower and more difficult than thought. Weight loss is pre-eminently a dynamic process and cannot be described well by static methods such as the ‘7000 kcal rule’.

Also, it’s easier said than done to adjust energy intake. Your body resists and increases your appetite. For one person it’s easier to eat less than for someone else and in particular this seems an important issue with those folks who are (or were) overweight. After all, else they might have not gained too much weight to begin with.

One kilo of body weight is not the other

I wrote in an earlier article that you’d need to burn roughly 9000 kcal to lose 1 kilogram of fat mass. That is hard work. However, if you work out on a hot day, you’ll quickly drop a kilo of body weight too. That won’t be fat of course, but mostly a lot of fluid you lost by sweating. In such a situation it is very obvious that it is not fat mass. Nevertheless, it perfectly illustrates the point that I would like to make, namely that one kilo of body weight is not the other. There happen to be a lot of other situations in which the scale ‘deceives’.

Your body weight is the sum of a lot of things. One of those things is the storage of substances from which your body can extract energy: carbohydrates (stored as glycogen), fats (mainly triglyercides stored in adipose tissue), but also protein. Of the latter, most of the energy is extracted from muscle proteins during weight loss. In addition to these three energy-containing substances, a large part of your body weight is also due to it containing a whole lot of water and some minerals, in particular the calcium and phosphate of the skeleton.

During weight loss, you really only want to lose fat mass and a minimal amount of the other stuff (the fat free mass). The fat free mass is in essense the sum of the aforementioned; the weight of the skeleton, fluid, protein and glycogen. The skeletal mass is quite stable and can even slightly increase in weight when weight loss is combined with physical exercise, in particular resistance exercise and exercises which increase bone mineral density. However, the skeletal mass can decrease up to 2% per 10% weight loss of the body mass, especially in postmenopausal women [1]. Nevertheless, due to the small impact on weight, we can ignore weight changes of the skeletal mass when we step on the scale.

Something which cannot be ignored however, besides fat, is the impact of changes in protein, glycogen and fluid mass on the scale. The body stores roughly half a kilogram of carbohydrates in the muscles and liver as glycogen. The storage of glycogen goes hand in hand with that of water. For each gram of glycogen stored, approximately 2.7 gram water is stored in concert [2]. The storage of half a kilogram of glycogen therefore goes hand in hand with the storage of about 1.35 kilogram of water. That makes a total of 0.5 + 1.35 = 1.85 kilogram. The amount of energy stored in 1 gram of glycogen is 4.19 kcal [3]. If you take into account the storage of water that is associated with the storage of glycogen, you’ll end up with an energy density of 1132 kcal per kilogram hydrated glycogen. This is in stark contrast to fat, which has an energy density of 9440 kcal per kilogram [3].

Just as an illustration, an extreme example. Suppose you expend 1132 kcal and that energy is completely derived from hydrated glycogen, then you drop 1 kilogram on the scale. However, if exactly the same amount of energy is solely derived from fat, you will only lose 120 grams on the scale. The scale can therefore be extremely misleading for diets that deplete the glycogen. These are diets that have a very low amount of carbohydrates. They exhaust the glycogen, resulting in a rapid loss of weight. You also lose some extra fluid due to the low insulin concentration that comes with it; Insulin is responsible for the retention of sodium (and therefore also of fluid) [4].

And of course, not to forget, the impact of the expenditure of protein. The body does not really has a storage of protein in the same way it does for fat (triglycerides stored in adipose tissue) and carbohydrates (glycogen stored in liver and muscles). All the protein in the body has a certain function. When body protein is used to meet the body with its energy needs, functional protein is broken down. During an energy-restricted diet, these are mainly muscle proteins. Protein is also present hydrated in the body, just like glycogen. The estimate of hydration varies reasonably, but 1.6 grams of water per gram of protein seems to be a good estimate [5]. You could do the same calculation for protein as we did before for glycogen. The amount of energy per gram of protein is very similar to that of glycogen with 4.7 kcal per gram [3]. Hydrated protein has an energy density of 1808 kcal per kilogram, based on the 1.6 grams of water per gram of protein. The same amount of energy stored in 1 kilogram of hydrated protein is again stored in just 192 grams of fat.

So summarized the following:

  • 1 kilogram hydrated glycogen = 1132 kcal
  • 1 kilogram hydrated protein = 1808 kcal
  • 1 kilogram fat = 9440 kcal

If your body expends energy, it is important in what proportion glycogen, protein and fat have contributed to this. The scale does not distinguish between weight loss from fat or glycogen, protein and fluid.

When choosing an energy-restricted diet with a decent amount of carbohydrates, the net contribution of glycogen to weight loss is fairly modest. What then remains is the question how much of the energy is extracted from lean body mass (in particular muscle mass) and how much from fat mass. The ratio between the two is also called the energy partition ratio, or in short the P ratio. The P ratio can take a value from 0 to 1, where 0 means that all energy is derived from fat and 1 means that all energy is derived from lean body mass (protein). Without resorting to a lot of formulas, it is sufficient to know that the P ratio is dependent on the fat mass. The more fat you have, the lower the ratio will be. When an obese person starts to lose weight, the energy will then mainly be derived from his fat mass. However, the more fat mass this person loses, the greater the contribution of protein to the energy expenditure will be. This is illustrated in the image below.

The P ratio illustrated based on the Forbes hypothesis. The P ratio follows a non-linear function of the fat mass. Image taken and adapted from [6].
Note: there are considerable interindividual differences in this that have a genetic basis. John has crappy genetics and loses, for example, with a given fat mass a lot more lean body mass than Arnold who has good genes, while both maintaining the same energy deficit. Talent (your genes) is important. The P ratio also depends on modifiable factors, including the amount of protein in the diet and the physical exercise a person performs. Someone who regularly hits the gym to play with weights and follows a high protein diet will, given equal circumstances, have a lower P ratio than someone who does not. A high protein diet and reistance exercise spare muscle mass.

Weight loss and energy intake influence energy expenditure: adaptation

Without giving it any real thought, people often apply the rule that you need an energy deficit of 7000 kcal for the loss of 1 kilogram. In addition to the fact that this depends on the proportions from which energy sources this is derived from, as discussed in the previous section (if it is derived from fat you would have to create a deficit of 9440 kcal), this rule assumes that weight loss is static. This means that if you create an energy deficit of 500 kcal per day, you should lose half a kilogram every week. This is not in line with reality. This has several reasons, including the fact that weight loss and changes in energy intake also have an impact on your energy expenditure. You would need to adjust your calorie intake continuously to realize the linear weight loss of the ‘7000 kcal rule’. The best is to completely ignore this rule, it just does not work well in practice.

In order to understand how weight loss and energy intake affect energy expenditure, it is useful to know exactly which stuff forms the total energy expenditure. The total energy expenditure is usually split into four components:

  1. resting metabolic rate (RMR)
  2. thermic effect of food (TEF)
  3. thermic effect of activity (TEA)
  4. non-exercise activity thermogenesis (NEAT). Together with TEA this is also refered to as activity energy expenditure (AEE)

The contributions of TEA, NEAT, TEF and RMR to the total energy expenditure of an average person. RMR generally accounts for more than half of the total energy expenditure and TEF about 10%. NEAT and TEA fill the remainder and in most people NEAT’s contribution is greater than that of TEA. There are, of course, large differences in this between individuals. A top athlete who trains hours a day will have a very high energy expenditure with a huge contribution by TEA. As a result, the contribution of the other three decreases in terms of percentage. Someone who is literally in bed all day has a very large contribution from RMR, and a small contribution from the other three. Note: there are some other names that are used for the four components I mention here, but usually they boil down to (approximately) the same thing.

The resting metabolic rate is the energy expenditure of your body in a state of rest and on an empty stomach. That basically means the energy required to keep you alive. So it is the sum of the energy expenditure of all metabolic processes that take place in your body at rest. This includes the metabolic processes that take place in the muscle and fat tissue, which account for 13 and 4.5 kcal per kilogram per day, respectively [7]. Also stuff such as energy expenditure for protein, fat and glycogen turnover contribute to the resting metabolic rate. When you lose fat and muscle mass due to an energy deficit, there is also a so-called adaptive component that affects the resting metabolic rate. This adaptive component is also referred to simply as a slowing of metabolism. The RMR deduction of this is obviously smaller than the energy deficit created and will therefore never be able to compensate for a created energy deficit. (This is sometimes mistakenly thought so that it is advised to eat more in order to lose weight so to speak.)

In the famous Minnesota Starvation Experiment subjects were put on an energy-restrictive diet of ~44% of their basal energy maintenance (~1585 kcal/d) for 24 weeks. There was a significant decrease in total energy expenditure. Part of this decline was due to the reduced body mass and reduced physical activity. However, about 11% of the decrease in total energy expenditure (~175 kcal) appeared to be due to adaptation of resting metabolic rate [8]. Moreover, the subjects moved less and thus less energy was expended by the muscles. In addition, the subjects ate less, which reduced the contribution of TEF. All in all, the total energy expenditure was halved after the 24 weeks on the energy-restrictive diet, so they were back in energy balance more or less. If the 7000-kcal rule was correct, the subjects would have lost 45 kilograms. In real life they ‘only’ lost 16.8 kilograms of body weight. In other words, that rule was wrong by about 270%.

Anyways, here’s a summary and some remarks per component of the total energy expenditure.

  • The resting metabolic rate (RMR) changes during weight loss. On the one hand because the body mass decreases: fat mass and muscle tissue is lost, and both consume a small amount of energy per kilogram. Some organs, in particular the liver and kidneys, also decrease in size, which also contributes to reduced energy expenditure [9]. On the other hand, your metabolism ‘slows down’ a little, reducing energy expenditure per kilogram of tissue. This is called the adaptive thermogenesis of the resting metabolic rate. For example, a reduced protein turnover and a decrease in sodium, potassium and calcium ion leakage along membranes, as a result of which the ion pumps need to waste less energy on this [10]. Your body, so to speak, starts to be a little more economical with energy. This only works in one direction though. Your body will not waste more energy if you eat more than you need, so you do not get an ‘acceleration’ of your metabolism when eating in excess [11].
  • The thermic effect of food (TEF) is formed by the amount of energy needed for the digestion, absorption, transport and metabolism of the carbohydrates, proteins and fats that you eat. The TEF amounts to 5-10% of the energy content of eaten carbohydrates, 0-3% of lipids and 20-30% of proteins [12]. On average, TEF comprises about 10% of the total energy expenditure. When you eat less, TEF also decreases, especially when you eat less protein. Given that most diets restrict carbohydrate or fat intake, the TEF decrease is often small. In fact, athletes often also start to eat more protein when they wish to lose fat.
  • The thermic effect of planned activity (TEA) is formed by the energy required for sports activities. So lifting your weights in the gym, running through a park, struggling on the treadmill, playing football with your friends (if you have friends), etc. In people who exercise a lot, this can contribute a great deal to the total energy expenditure. During weight loss, especially muscle loss, there is ultimately a lower capacity to burn energy. Moreover, the work efficiency of the muscles increases in experiments where weight loss has been achieved [13]. In other words, less energy is used for the same movements. (And not just because you weigh less.)
  • The thermic effect of ‘spontaneous’ activity (NEAT) is quite variable from person to person. Research also shows that if someone increases their TEA during an energy-restrictive diet, NEAT will drop [8]. So what sports you plan extra for your day during dieting can – as a rule – be at the expense of your energy expenditure of physical activity outside of exercising.

That said, it should also be noted that the above adaptations of your body are not binary. So they do not follow an ‘all-or-nothing’ principle. It must be seen as a gradient; The greater the energy deficit, the greater the adaptations will be. With a small energy deficit, the adaptations will therefore also be small. And of course, if you start to eat even less, you will lose weight even more.

To conclude this section, here is an interesting study from the Netherlands [14]. In the early 1990s, Klaas Westerterp et al. investigated what the effect would be of physical activity on the energy balance and body composition. They recruited untrained men and women to prepare them for a half marathon in 44 weeks. At the start of this process, about 25 kilometers were ran weekly by the participants, divided over four training sessions. This was continually increased to 50 kilometers per week. What you would expect was that the energy expenditure (which was also measured) would rise further and further because of this continual increase in running distance. However, the figure below shows what was actually measured:

Runners suck ok.

On the left you see the weekly distance ran by the test subjects versus the time. On the right you see the daily activity energy expenditure (that is, both NEAT and TEA) of the test subjects plotted against time. What is noticeable is that an increase takes place initially. The test subjects get rid of their lazy ass and start doing something: the energy expenditure increases. Then they start doing more and more (read: more running), but the increase in energy expenditure does not continue. This can be explained by a few things that I have discussed above. For example, the work efficiency of your body will improve as you run more often. A trained person will therefore expend less energy to run a certain distance than an untrained person (this is also due to the fact that the participants lost a small amount of weight). You also expect, especially in case of a caloric deficit (in the study there was a small caloric deficit, although the energy intake was ad libitum ), a decrease in NEAT. This further offsets the increased energy expenditure by running.

Finally, the researchers observed that in the men the sleep metabolic rate (the resting metabolic rate, but measured during sleep. In practice this is a few percent lower than when you measure it when someone is awake) decreased a tiny bit. Besides that this effect was very small, you would not see that in the graph either, because the graph shows the activity energy expenditure. But nevertheless worth mentioning.

Problems in regard to changing energy intake and maintaining lost weight

In practice you have to deal with two major problems when adjusting the energy intake. The first problem boils down to the fact that counting calories is not an exact science. If you buy a bread from the bakery, you do not know exactly how many calories it contains. If you get fries in a cafeteria you don’t know it either. But even if you buy a piece of meat at the supermarket it can be somewhat problematic. The nutritional value on such a packaging is pretty accurate, it is not an exact number, but it is good enough for all practical purposes . However, if you then bake that piece of meat in butter, the question is how much you of that butter will end up in your mouth and how much will stay in the pan. Two grams, five grams, ten grams, …? And so there are plenty of examples where you have to make an estimate of the number of calories. Consequently, it may well be that you think you can eat an number of calories on paper, while in practice you might be off by a few hundred calories. Also if you change your diet considerably in terms of products, but not in terms of calories (on paper at least), you might introduce some notable differences.

And then you also have those folks who want to count their calories, but refuse to use a scale for it. It doesn’t take a rocket scientist to figure out that estimates of the amount of food by sight vary considerably. People, especially when they’ve never neurotically measured their foods for months or years on end, simply are bad at estimating quantities. Period. Oh well, at least this is something which is easy to fix. Walk in the store and buy a damned kitchen scale. Problem solved.

Then another problem with adjusting the energy intake, especially when you want to lose weight … Your body ‘resists’. Simply put it is quite difficult to control yourself and to ‘starve yourself’. Your body has various systems in place to ensure that you want to eat more the moment your energy intake is lower than your energy expenditure. I refer the interested reader to the following two references from the scientific literature of Stephan Guyenet & Michael Schwartz [15] and van der Klaauw & Farooqi [16].

Most people actually manage to initially lose some weight, but maintaining this turns out to be very difficult for many in practice. Therefore, most people do not succeed in achieving permanent weight loss. After losing weight it simply bounces back. Hi belly. In part this may be explained by the fact that someone who has lost weight seems to have a lower resting energy consumption than would be expected based on the new body weight and composition [17]. However, the contribution of this is at best small and to a certain extent controversial [18]. A more important factor responsible for the lower energy expenditure seems to be the reduced contribution of physical activity. Particularly due to the increased work efficiency of the muscles, which seems to increase by 20% when maintaining a lower weight [19].

Also in regard to energy intake, your body seems to be doing its best to return to its previous body weight. Indeed, a recent publication seems to argue that weight loss, and the maintenance of weight loss, is especially hampered by the influence your body exerts on energy intake [20]. Or in short: you will get more appetite because of weight loss. With an elegant experiment, researchers have tried to quantify the increase in appetite expressed in kcal per day per kg of lost body weight. The researchers found an increase in appetite of about 100 kcal per day per kg of body weight lost. It is quite straightforward that this is a huge hurdle to overcome to achieve permanent weight loss.


Losing weight and achieving permanent weight loss is very difficult. If it were easy, about half of the Dutch adult population would not have been overweight [21]. Although it boils down to having to expend more energy than you consume, it is too easy to say that you ‘just’ have to eat a number of calories under energy maintenance. Your energy expenditure adapts to your energy intake. If you lower your energy intake in such a way that you lose weight, your energy expenditure will also decrease. Although the drop in energy expenditure is less than the drop in your energy intake, it makes things more difficult. It also seems that a little sports / exercise does help to increase your energy expenditure, but that more sports do little for this. This is because your body will compensate for this. Of course, your body can not continue to compensate this till infinity, but you will have to exercise excessively to drive that energy expenditure further up. It is wise to add some resistance exercise to your regime: this saves the muscle mass. In fact, muscle gain is not a strange phenomenon. It slows the weight loss a bit (see the section ‘one kilo of body weight is not the other’), but the ultimate goal should be fat loss. In order to achieve weight loss, you will have to adjust your diet and to some extent your physical activity.

Moreover, it is also no easy task to lower your energy intake below that of your energy expenditure. Your body ramps up your appetite for food when you start to eat less. Research also shows that your appetite increases further with weight loss. This also makes it difficult to maintain the weight loss you have already achieved.


  1. Shapses, Sue A., and Deeptha Sukumar. “Bone metabolism in obesity and weight loss.” Annual review of nutrition 32 (2012): 287-309.
  2. McBride, J. J., M. Mason Guest, and E. L. Scott. “The storage of the major liver components; emphasizing the relationship of glycogen to water in the liver and the hydration of glycogen.” Journal of Biological Chemistry 139 (1941): 943-952.
  3. Livesey, Geoffrey, and Marines Elia. “Estimation of energy expenditure, net carbohydrate utilization, and net fat oxidation and synthesis by indirect calorimetry: evaluation of errors with special reference to the detailed composition of fuels.” The American journal of clinical nutrition 47.4 (1988): 608-628.
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  9. Müller, Manfred James, et al. “Metabolic adaptation to caloric restriction and subsequent refeeding: the Minnesota Starvation Experiment revisited.” The American journal of clinical nutrition 102.4 (2015): 807-819.
  10. Müller, M. J., and A. Bosy‐Westphal. “Adaptive thermogenesis with weight loss in humans.” Obesity 21.2 (2013): 218-228.
  11. Hall, Kevin D. “Predicting metabolic adaptation, body weight change, and energy intake in humans.” American Journal of Physiology-Endocrinology and Metabolism 298.3 (2010): E449-E466.
  12. Tappy, L. “Thermic effect of food and sympathetic nervous system activity in humans.” Reproduction Nutrition Development 36.4 (1996): 391-397.
  13. Rosenbaum, Michael, et al. “Effects of experimental weight perturbation on skeletal muscle work efficiency in human subjects.” American Journal of Physiology-Regulatory, Integrative and Comparative Physiology 285.1 (2003): R183-R192.
  14. Westerterp, Klaas R., et al. “Long-term effect of physical activity on energy balance and body composition.” British Journal of Nutrition 68.01 (1992): 21-30.
  15. Guyenet, Stephan J., and Michael W. Schwartz. “Regulation of food intake, energy balance, and body fat mass: implications for the pathogenesis and treatment of obesity.” The Journal of Clinical Endocrinology & Metabolism 97.3 (2012): 745-755.
  16. van der Klaauw, Agatha A., and I. Sadaf Farooqi. “The hunger genes: pathways to obesity.” Cell 161.1 (2015): 119-132.
  17. Rosenbaum, Michael, et al. “Long-term persistence of adaptive thermogenesis in subjects who have maintained a reduced body weight.” The American journal of clinical nutrition 88.4 (2008): 906-912.
  18. Schwartz, Alexander, et al. “Greater than predicted decrease in resting energy expenditure and weight loss: results from a systematic review.” Obesity 20.11 (2012): 2307-2310.
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  21. Link: https://www.volksgezondheidenzorg.info/onderwerp/overgewicht/cijfers-context/huidige-situatie#node-overgewicht-volwassenen. Geraadpleegd op 17 mei 2017.

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