Dear Professor Bjorn,

It certainly is a good question, yet an unexplored field in the last few years. From theory, both processes are somewhat opposed. While fat storage (glucose conversion) aim to provide energy for future needs of the fish, muscle growth (anabolism) is an energy consuming process and provide the fish with the muscular capacity to escape, obtain food, migrate, mate and so on. Fat storage is energetically cheap, since aims to conserve energy for future needs. Conversely, muscle growth is one of the most energy consuming processes in fish, involving high rates of protein synthesis. The raw materials for both processes are also different, fat storage require glucose (glycogen), while muscle growth require protein. Then, most of the protein is allocated to muscle growth and carbohydrates and lipids cover most of the energetic needs. If after fueling all fish energy needs still there are carbohydrates left, they are converted and stored as fat. Diet composition and exercise can also be modified in order to obtain fish growth in terms of fat or muscle (as desired). Considering that feed conversion efficiency is calculated based on weight gained, a good feed conversion efficiency can also be obtained by just storing lipids. What the market wants, is a different question, and muscle is much more valued than fat. I might be able to provide a more numerical and accurate comparison (ATP) from theory if you were interested, just let me know.

All the best,

Mauricio U.

Dear Mauricio,

Thank you for your answer, which certainly has a number of valid points. However, I feel there are a number of issues which need to considered further.

You're correct in the fact that feed conversion efficiency is based on weight gained. However, in pointing out the cost of producing fat vs muscle, I think you're ignoring the fact that muscle is about 80% water, while pure fat tissue has very low water content. Thus, gaining one gram in weight through growth of adipocytes demands more or less a deposition of one gram of fat, while gaining one gram in weight by growth of myocytes demands 0.2 grams of protein synthesis. Thus, I still don't believe your statement that a good feed conversion efficiency can be gained by just storing lipids, but you're free to prove me wrong ;-). Also, I don't think it's possible to generalize that in the market, "muscle is much more valued than fat". Fish fat is healthy to eat and conveys taste, and my guess is that a salmon containing 15% muscle fat will be found tastier that one with 5% fat. Also, a certain amount of fat is essential for added-value production such as smoking and curing.

Cheers for now, Thrandur

Dear Denes, Thanks for a valuable comment. The same likely applies for such aquacultured fish species as salmonids, as their feed consists almost entirely of fat and protein and no carbohydrates.

However, what I've been trying to get at, and I see that my original question was not that well formulated, is not really the cost of deposition, but rather how fat vs muscle deposition affects feed conversion efficiency. To exemplify, consider that you have two stains of fish which differ in their genetic tendency for adiposity, i.e. a "lean" and a "fat" strain. If you then feed them the same amount of feed (same energy intake), which of this strains will gain most weight per unit of feed consumed? In other words, which of the strains will have the better food conversion efficiency, and most importantly, why?

Dear Bjorn,

I think that the ambiguity of your question lies within the term "muscle". Rather than muscle deposition, I would speak about protein, water and lipid (fat) deposition within the muscle.

To your last question (strain that has the best FCE), my hypothesis is: it mainly depends on temperature, photoperiod and feed. More especially, I think it depends on the ratio N/E: the optimum N/E to get a maximum FCE is probably different between the two strains. But to my opinion, FCE is a tricky indicator as it increases with water retention...

Hi Neil,

What I'm asking is the cost of growing in weight through the growth of lean muscle (which contains mostly protein and water) or growing through the deposition of fat, within the muscle or elsewhere. Basically, it's a question of the cost of producing a lean or fat salmon steak, to take an example.

There's nothing tricky about FCE, it's a straightforward indicator of how many kilos of fish the fish farmer gets for a given amount of feed, which is the main cost of any aquaculture operation. However, your point that FCE increases with water retention is spot on and is really the basis of my question, as a lean salmon steak will contain more water than a fat one.

No wonder that the poultry industry is increasing its yield by injecting water into the chicken fillets ;-)

Dear Bjorn,

Very interesting issue. Just to state that fat in muscles is good, especially for smoking and other post production. The problem in i.e. sea bass is other fat deposits, such as visceral fat, which is in case of filleting just waste. So, the discussion should go in few directions.

Dear Branco,

You're absolutely right that this discussion could/should go in several directions, and your point is well taken that sometimes, fat deposition is a good trait (e.g. fat in salmonid muscle), but sometimes it is wasteful such as large visceral fat deposits seen e.g. in rainbow trout. It's interesting to note that different species have very different strategies for fat deposition. Lean fish as Atlantic cod deposit fat primarily in the liver, and Icelandic cod farmers observed that the aquaculture cod had very large livers. They worried about this being wasteful, until they found out that there's a very good connoisseur marked for canned cod liver and now they're exporting this "waste product" to e.g. France as "Foie de morue" :-)

Fat enthalpy of combustion is about 2.25-fold greater than protein's. On top of that, whereas addipose tissue contains very little water, muscle tissue is about 2/3 water (in cattle). Thus addipose tissue ends up being 6-7-fold energetically more expensive to produce than muscle tissue.

Hope this helps.

Dear Professor Bjorn,

Your question is very interesting and you have made your question clear through the discussion. The different amount of water in fat vs muscle, is a very good point. However, I am still reluctant to believe that fat production and storage is more expensive for a fish that muscle growth. If this were the case, the an obvious question emerge: Why animals store energy as fat if it is so expensive to do?

@Emilio, you have raised a good point regarding the amount of energy stored in fat vs muscle protein. As you stated, fat metabolism ended up generating far more ATP than burning proteins. This is the energy content. However, we must not confound the energy content in fat and muscle protein, with the amount of energy invested for the fish to generate or produce them. I think there are extensive literature pointing out protein synthesis and muscular growth as one of the most energy consuming processes in fish.

Returning to the original question, i still do think that a good FCE can be obtained by fat growth. Yet, as pointed our before, several factors might affect this such as exercise, temperature, fat/prot ratio of the food, and of course fish species, metabolism, age, selection and so on. I'll be happy to collaborate in such kind of research.

All the best,

Dear Bjorn,

Thanks for your answer. I've worked a little on fish growth but I'm not a fish nutritionnist. Therefore, I won't try to go too far in energetics matters. However I'm interested in everything when it comes to fish and especially aquaculture. Are there practical implications lying behind your question, or is your question purely on a fondamental point of view ?

Dear Neil,

As a fish physiologist, working the the endocrine control of growth, metabolism and appetite I have a fundamental interest in the topic, but at the same time, I think it's of great applied interest in aquaculture.

In modern, intensive aquaculture, feed cost is by far the greatest operation cost. The growth you get out of your fish per ton feed (FCE) is really the economic bottom line for most producers.

So far, most breeding selection in fish has been focused on growth rate, as this is a strongly heritable trait, but less focus has been put on the quality of the product, e.g. the fat content of the fillet.

Thus, fundamental understanding of the energetic costs of growing through increasing lean muscle or adipose tissue, as well as the regulation of these processes, may help us optimize aquaculture production further.

This discussion has been very fruitful so far. We understand from it that it's more costly to build muscle protein in myocytes than to deposit fat in adipocytes. However, as muscle contains water and fat contains more energy than protein, the enthalpy of combustion, i.e. the energy obtained from burning one gram of adipose tissue is 6-7 fold greater than burning of one gram of lean muscle.

One of the hormones I study is growth hormone (GH). This hormone has many functions, but in terms of energy metabolism, it stimulates protein synthesis and at the same time stimulates lipid mobilization. Many studies, including my own (see attachment) indicate that not only does GH stimulate growth through increased appetite and food intake, but also through improved FCE. However, nobody has really explained why FCE should be improved by GH - and this is really the basis for my question and ensuing discussion.

Please note that I'm not an advocate of using hormones in animal husbandry or aquaculture production, I use GH treatments only to better understand the mechanisms behind its effects

Article The role of growth hormone in growth, lipid homeostasis, ene...

This is a very interesting discussion.

Mauricio, I think that animals store energy as fat because of two reasons:

1) Because it is energy-dense, it takes much less space. An animal would need at least twice as much water-free carbohydrates or protein to store the same amount of energy. And much more if there was water in those tissues. Animals store some energy as glycogen for shorter term use, as it can be mobilized faster than fat and is a source of glucose, unlike fat, but glycogen won't allow them to survive for long periods. For the animal, storing energy as fat is not inefficient, because, whereas it is energetically costly to deposit fat, it at the same time releases proportionally more energy when utilized compared to carbohydrates or protein.

2) Fat is insulating and decreases heat loss to the environment. This is important in animals that keep a constant body temperature; not sure that this would be a factor in case of fish.

Energy invested in producing a product from the same reactants is proportional (and greater according to the second principle of thermodynamics) to the energy contained in the product, given that the ratio ΔGproduct/ΔGATP doesn't vary much provided that the same reactants are used.

The one situation in which I can perhaps see fat deposition being more efficient if when we talk about dietary fat deposited as addipose tissue, compared to muscle tissue being deposited using carbohydrates or protein as energy source. In this case there is no cost of synthesizing the fat (perhaps some minor cost of saturation or elongation?). But this comparison assumes different diets, so it wouldn't be an energetically fair comparison (although it can be more relevant for fish production, which I don't know).

Emilio - you say at the end of point 1) that " it is energetically costly to deposit fat", while Mauricio maintains that "protein synthesis and muscular growth as one of the most energy consuming processes in fish". So the question remains, which is costlier?

By "costlier" we must need to consider two factors: 1) the amount of ingested energy which is actually deposited in the tissue and 2) the amount of energy spent in the process of creating the tissue. As Emilio has stated, adipose tissue contains 6-7 times more energy than muscle tissue, thus, 6-7 times more of the ingested feed energy goes into creating it (indicating a lower FCE). So unless it's more than 7 times more costly to synthesize the protein for 1 g of muscle than it costs to deposit fat in 1g of adipose tissue, muscle growth must give better FCE than adipose growth. Or?

I agree that there an obvious reason to store energy as fat rather than protein, as adipose tissue will be much more energy-dense than muscle tissue.

If you like the discussion, don't forget to "vote up" the question (and good comments) in order for this discussion to be noted by others.

Dear all,

It has been a really good discussion. I think Emilio just made a key point at the end of his comment. Where is the fat coming in a fish?. In fish most of the lipids or fat came from dietary source (as far as I know), and if fat is absent in the diet, fish may not synthesize it nor store it. Therefore, I must recognize that I been assuming that dietary fat is one of the main sources of fat in fish, and I think it is. However, if you were comparing both prot and fat synthesis from scratch, as Emilio suggest (being fair), I would not have an answer from the top of my head. But, I will agree with Emilio's comment.

In a more realistic context, fish farming for example, where fat comes mainly from the diet (as far as I know), I would still think that muscle protein growth is more energetically expensive for a fish than growing by fat storage.

All the best!,

When we talk about fish diets and where the energy for growth comes from, we can't generalize as there are both carnivorous and herbivorous (most of these are in fact omnivorous) species of fish in the wild and in aquaculture.

However, almost all aquaculture in Europe is based on the carnivorous species: Atlantic salmon, rainbow trout sea bass and sea bream. The feed is essentially made up protein and fat, the level of fat being as high as 25-30% for salmon. Thus fat is the main energy source of the feed, and can thus both be laid down as adipose tissue as well as used as energy for muscular growth.

Agree with the considerations you guys have been making. I think that from a purist point of view fat deposition is energetically more costly than muscle; however, if the physiology of the fish of interest in this case dictates that their made energy source is going to be fat, then deposition of dietary fat as addipose tissue is much more efficient than if fatty acids had to be synthesized.

if fat in fish is derived from a diet with 25% fat, it is important to know where the dietary fat comes from. Many toxins are fat soluble and get stored in body fat, which is a problem for obese humans losing weight rapidly and simultaneously possibly releasing toxic material into their circulation. I know nothing about fish nutrition, but it seems to me that salmon grown on a high fat diet would be a potentially dangerous food source for humans. Elaine Newman, microbiologist

In an previous communication, we had such type of discussion even if it was not closely related to energy cost of fat retention. Comparing warm and cold water species, we conclude that the more effective species are those with the lower capacity of energy retention and then leaner fish.

All the best

Hi Elaine, I think what you write is self-evident for any any food production for human consumption - the producer needs to be sure that the ingredients, including animal feed, does not contain toxic chemicals. In this regard, there's no principal difference between production of beef, pork, poultry or fish, although the fat content of the feed may vary from species to species. A responsible producer needs to buy feed or feed ingredients which are certified free of toxicants.

Such control of intake is not possible for wild-caught fish, and unfortunately, we have a situation in the Baltic sea and certainly also in many polluted freshwater areas, which leads to wild-caught actually having much higher toxicant levels than found in aquacultured fish.

Dear Bjorn,

I might add some of my experience in bioenergetics to this discussion:

Overall, the energy expenditure to deposit fat is lower than the energy expenditure to deposit protein –– see below a publication regarding a comparison among three fish species.

But the real question is, which factors would influence FCE ? Total energy and protein requirements of a fish depend on the requirements for maintenance plus requirements for growth. Thus to satisfy those requirements a pre-determined amount of feed should be fed, which is formulated with the proper protein to energy ratio.

The feed conversion efficiency FCE is thus influenced by several factors, which includes the growth potential of the fish, the maintenance requirements and the composition of the weight gain.

As the FCE includes the demands for maintenance and growth it is understandable, that the proportion of energy which is required for maintenance will increase with increasing body weight and/or with decreasing growth rate, influencing the FCE. Faster growing fish will usually display a better FCE, as do juveniles of a certain species.

Let’s take an example, a faster growing salmon compared to a slower growing salmon (assuming the same body composition) would have a better FCE because the maintenance requirement of the faster growing salmon as a proportion of the total daily requirements would be lower and FCE only considers roughly the ratio between input as feed and output as growth.

A leaner salmon compared to a fatter salmon (assuming the same growth) would require less non-protein energy in the feed, thus the dietary protein to lipid ratio will have to be changed, but not necessarily the feed amount, as the protein requirements still have to be satisfied for the fish to grow. Here only a change in feed formulation can change the FCE. But then feed costs could be reduced by a lower lipid content of the feeds.

Ingrid Lupatsch

Dear Ingrid, Your answer goes a long way to answer the question I set out with, although I did formulate it rather awkwardly. Thanks especially for the attached publication, which I've only skimmed through so far, but will certainly study further.

Earlier answers have mostly be concerned with the relative energetic cost of creating adipose vs muscle tissue, as well as the subsequent energy content of these tissues. These are very interesting aspects, but you highlight a different aspect of energetics, which is the energetic cost of maintenance in relation to the energetic cost of growth and give the data to back you up!

To draw on your two clarifying examples, when I experimentally stimulate the lean growth of my rainbow trout through growth hormone treatment (see attachment), their FCE is increased both because the relative amount of energy used for maintenance is less that for the controls, and because lean growth is more energy efficient than a more adipose growth. Or does this hold in an experiment where both groups receive the same feed composition and the same amount of food (presumably)?

Article The role of growth hormone in growth, lipid homeostasis, ene...

Hi Bjorn, regarding your last observation. Perhaps growth hormone could also act through decreasing protein turnover, thus decreasing maintenance energy costs (and probably making N utilization more efficient)? I understand a mechanism of that sort happens in poultry, although I'm not aware of the details

Good point Emilio, one I need to look into. I know that GH increases protein accretion, but I need to look up if GH can actually decrease protein break-down, as that would be the mechanism for how it could decrease protein turn-over.

I should clarify some points regarding the efficiency question : When I say, that the energy expenditure to deposit fat is lower than the energy expenditure to deposit protein –– I actually meant units of energy (cal or kJ) to deposit lipid as energy (cal or kJ) and units of energy (cal or kJ) to deposit protein as energy - however as lipid contains 39 kJ per g and protein contains 24.6 kJ per g - the total energy expenditure to deposit 1 g lipid or 1 g protein would be roughly the same around 45 to 46 kJ per g tissue.

But, protein growth – aka muscle growth – only occurs if sufficient protein is available from the diet, fish cannot grow at all or add more muscle without dietary protein. The question if a leaner fish is more efficient can be answered that he needs less energy per day as he will deposit less lipid, which has to come ultimately from the feed.

I might also answer the question, where does the lipid in the fish come from? Fish are able to use all energy containing components from the feed to store them as lipid, and those are lipids, carbohydrates and also protein. If the protein in the feed is too high, or non-protein sources such as carbohydrates or lipid too low, then the fish will use protein as an energy source which is a waste as it is too energy demanding and protein is expensive. Therefore nutritionists are putting all efforts in defining the proper ratio between dietary protein to energy so protein is only used or ‘spared’ for growth.

Thus for a leaner fish I would formulate a feed with a lower energy content. For a faster growing fish, feed formulation would be accordingly (higher protein content) or the fish has to consume more feed to supply the necessary protein.

Also an interesting aspect is the comparison between carnivorous and herbivorous species, and actually they ‘behave’ in the same way. I am attaching an article that I published in a trade magazine, which might be interesting to add to the discussion. In the end, the most efficient fish is the fastest growing fish, and growth means protein accretion.

Hi Ingrid, Thanks again for your answers. However, when you say "the total energy expenditure to deposit 1 g lipid or 1 g protein would be roughly the same around 45 to 46 kJ per g tissue", I just need to ask if you so that I don't misunderstand, if you're taking the water content of muscle tissue into consideration (you first say g protein, and then g tissue)?

Hi Bjorn,

you are correct to point this out, it is per g protein or per g lipid so "the total energy expenditure to deposit 1 g lipid or 1 g protein would be roughly the same around 45 to 46 kJ per g "

Dear Ingrid, Let me then see if I've got it right:

First you say that it takes less energy to deposit fat than protein gram by gram. I can accept this - the deposition means synthesis and protein synthesis is more expensive than fat synthesis (however, nobody in the forum has given any numbers to this, in absolute or relative terms).

Then you say that a gram of fat contains more energy than a gram of protein. I can also accept this, and earlier in the discussion Emilio Ungerfeld indicated an even greater difference than your numbers indicate: "Fat enthalpy of combustion is about 2.25-fold greater than protein". (Below I'm using 2-fold, just for simplification).

So it's less costly to deposit fat, and it contains more energy, but then you claim that is about equally costly grow by muscle or adipose tissue. This must then be due to muscle water content, right.

Given that protein content of a muscle is 0.25 g per gram tissue (while fat content of of adipose tissue is 1 g per gram tissue) and that the energy content of fat is twice that of protein, it must follow that protein synthesis is about 8 times as energy consuming as fat synthesis during deposition of feed-derived amino- and fatty acids. Interesting! Can anybody confirm that this is correct?

Dear Bjorn, just to verify, I say, that the efficiency to deposit fat energy is higher than to deposit protein energy,

Let me give an example: When feeding fish graded levels of feed from no feed to maximum feed intake, and when plotting this relationship (x =DE intake, y = energy gain) the resulting equation is linear and the efficiency of utilization of energy is defined by the value of the slope.

As total energy gain in fish consists of protein energy as well as lipid energy, a multiple regression can be set up which determines the energy coefficients for protein and lipid deposition simultaneously with the maintenance requirement: kP = efficiency for protein energy deposition and kL = efficiency for lipid energy deposition. DEmaint = Digestible energy for maintenance.

DE requirement (kJ) = DEmaint + 1/kP_protein energy (kJ) + 1/kL_lipid energy (kJ).

In fish, kP is on average 0.52 and kL is on average 0.85, thus to deposit 1g protein, which contains about 23.6 kJ one needs a total of 1/0.52 * 24.6 kJ = 45 kJ or to deposit 1g lipid which contains 39 kJ one needs a total of 1/0.85 * 39 kJ = 46 kJ.

If you want to calculate what is more efficient: daily weight gain of a lean fish or weight gain of a fat fish you have to know the composition of this unit of weight gain, how much protein and how much lipid it contains. Protein content of a fish (whole fish, representing a unit of weight gain) is quite stable and is around 17 to 18%, lipid content of a lean fish would be lower than for a fat fish, and water content would be a slightly higher. Thus in total a lean fish would need less energy to deposit 1 g of weight gain.

I attach a publication which explains this concept in more details