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Bulletin #60 – The Energetics of Nutrient Metabolism

One of the fundamental problemsin bodybuilding is how to gain musclewithout getting fat. In order to pack onmore muscle we have to be eating enoughto supply all of the nutrients and energywe need to grow, but if you’re not verycareful about how you do this you’ll endup gaining as much fat as muscle. Thismonth I want to talk about the correctway to structure your diet to gain muscleand not fat. To do this we first need toreview some basic science about nutrientand energy metabolism.Over the last ten years or so quitea bit has been learned about how diet af-fects body composition. The old way ofthinking was that all that really matteredwas calories — if you eat more caloriesyou’ll gain weight and if you eat fewercalories you’ll lose weight. Thisconcept is referred to as “en-ergy balance.” The number ofcalories contained in a food isa measure of the energy con-tent of the food — calories area unit of energy. Energy balanceis the number of calories you eatminus the number of calories youexpend (burn) as energy. If youconsume more calories than youburn this results in a “positive en-ergy balance” and the excess calo-ries you consume are retained as bodyweight (either muscle, fat, or glyco-gen).

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If you consume fewer calories thanyou burn this is a “negative energy bal-ance” and you lose weight. If you con-sume the same number of calories thatyou burn this is called “zero energy bal-ance” (or more often simply “energy bal-ance”) and your weight will remain con-stant.No one doubts the importance ofenergy balance in determining bodyweight. It’s just that we understand nowthat there’s a lot more to it. Let’s say we’retrying to gain weight, so we’re intention-ally eating excess calories. It turns out thatwhat kind of food we choose to supplythose excess calories can have a big ef-fect on whether the weight we gain willbe muscle or fat. Furthermore, scientificdata demonstrates that it is possible to losefat without cutting calories. The issue wewant to explore is if diet composition canaffect body composition. Body composi-tion is simply your percent body fat, de-termined by your pounds of fat and poundsof lean mass. You should monitor yourbody composition about once a monthwith the BodyStat Kit to make sure you’regaining muscle and not fat. By “diet com-position” we mean the percentage of calo-ries coming from protein, carbs, and fat.While overall body weightseems to be determined mostly by energybalance (and thus the total number ofcalories you eat) my theory is that bodycomposition is determined more by dietcomposition.

So what I’m really going totalk about here is how to use nutrition tohelp you gain muscle and lower your per-cent body fat. Interested now? First I willreview some basics about energy metabo-lism, then discuss some nutrient balancestudies done in humans, and finally wrapup with some recommendations on howto use this information to lower your per-cent body fat.Like I mentioned earlier, the oldway of thinking was that calories were allthat mattered, and what kind of food youate wasn’t considered that important. Tounderstand how diet composition has thepotential to affect body composition weneed to review the thermodynamics ofnutrient metabolism. All food we eat canbe used as fuel. If you throw food into afire it will burn and release heat. Chemi-cally, this process is called “oxidation”because when something burns in a fireits molecules are combined with oxygenfrom the atmosphere. (This is why firescan’t burn without oxygen.) Inside ourbodies food molecules are oxidized in asimilar process except it occurs moreslowly. As the food moleculesare oxidized energy is re-leased, just like heat energyis released by a fire. Thisenergy is used to build a spe-cial molecule called adenos-ine triphosphate, better knownas ATP.

ATP is called a “highenergy” molecule because itcontains a lot of energy (duh).It contains three phosphategroups which all have strongnegative charges! As you remem-ber from high school, like chargesrepel — negative repels negative andpositive repels positive. So the phosphategroups are repelling each other and thismakes the molecule want to break apart.When the phosphate groups do break apartthis releases the energy which was con-tained in the ATP molecule. This energyreleased by ATP breakdown is the imme-diate source of energy used to powermuscular contractions and all of the otherwork done by the body. After the ATPsplits apart some more energy released byfood oxidation is used to put it back to-gether again.So the energy used to fuel our bodies comes from food, but before wecan use this energy it has to be convertedinto ATP. The thing is that protein, carbs,and fat all follow different metabolic path-ways and the energy released from theiroxidation is captured as ATP with differ-ent efficiencies. Some of the energy gen-erated by oxidation of food molecules islost as body heat and is not captured byATP formation. This is referred to as the“thermic effect of feeding” (or TEF) anddescribes the percentage of energy lostas heat when a particular food substrateis metabolized.

TheTEF for dietary fat isabout 5%, whichmeans that 5% of theenergy supplied bythe fat is lost as bodyheat instead of beingconverted into ATP.The TEF for carbo-hydrate is around 8-10% and for proteinis about 20-30% (1).After we account forall of the energy costsof the various meta-bolic pathways wefind that 90% of the energy supplied byfat is available for ATP production, 75%of the energy from carbs is used to makeATP, and 45% of the energy from proteinoxidation is retained as ATP (1).Therefore it seems obvious thatthe relative proportions of protein, carbs,and fat in the diet can have a big impacton the amount of dietary energy which isavailable to perform work or to be retainedas body weight. For example let’s con-sider a diet that provides 2000 calories andis made of 20% protein, 30% carbs, and50% fat. If you do the calculations you’llsee that the thermic effect (TEF) of thismeal is around 200 calories (or 10% ofenergy content overall) which means thatof that of the 2000 calories you ate, 200of them are lost as heat. Now let’s com-pare that with a 2000 calorie diet com-posed of 30% protein, 60% carbs, and10% fat. The overall thermic effect of thisdiet works out to be 270 calories (or 14%of energy content). Notice that althoughboth diets contain 2000 calories, the lowfat diet provides 70 fewer calories whichare available to be stored, because theywere lost as body heat during metabolism.So what does this mean for youas a bodybuilder?

Look at it this way: thediet higher in fat provided 70 more calo-ries which can be used as energy or re-tained as body weight. These 70 “extra”calories are in the form of fat. Fat cannotbe converted to carbohydrate or protein,therefore these 70 calories could not bestored as glycogen or muscle. They couldonly be stored as fat.Therefore the high fatdiet has the potentialto result in 70 caloriesworth of body fatstorage more than thelow fat diet. This is farfrom the whole storyhowever. These con-siderations are justmeant to demonstratethat two different di-ets which provide thesame number of calo-ries can result in a dif-ferent effective en-ergy balance. This concept is known as“food efficiency” and has been around inthe livestock industry for years. It has longbeen recognized that feeding different di-ets to growing animals (say young pigsor cattle being raised for meat produc-tion) will result in different growth rates.A diet with a higher food efficiency willresult in more retention of energy as bodyweight (i.e., more growth) for a givennumber of calories. In practice, however,diets with a high food efficiency result inmore body weight gain because the ani-mal retains more fat.If all the food energy we eat isconverted into ATP then how do we gainbody weight?

Well, in the situation of posi-tive energy balance we are eating morecalories than we burn. In that case not allof the food is burned and instead the por-tion that is not used as fuel is retained asbody weight — either muscle or fat orglycogen. If the “excess” calories are sup-plied as carbohydrate they will be storedas glycogen until the glycogen stores arecompletely full. After that any more carbscan be converted to fat, but as we willsee later this almost never actually hap-pens. If the excess calories are suppliedas fat they will be stored as fat, plain andsimple. Unfortunately, the body cannotconvert dietary fat into carbohydrate orprotein, so if you consume excess calo-ries in the form of fat they will be retainedas adipose. I’d like to say excess caloriessupplied as protein will be retained as pro-tein in muscle, but it doesn’t work thatway. Gaining muscle is not as simple asconsuming excess protein energy. Extracalories you eat as protein can be retainedas protein or else converted into glyco-gen (via gluconeogenesis) or fat. Whenwe get to the discussion of the nutrientbalance studies later in this article, I’llexplain some factors that determinewhether excess calories are retained asglycogen, fat, or muscle.Let’s talk about how we can usethese concepts. Let’s say we want to losefat and lower our percent body fat. We allknow that the fastest way to lose weightis to cut calories. There are several prob-lems with this approach however.

First,somewhere between 25-50% of theweight lost during energy restricted dietsis muscle, depending on the severity ofthe energy deficit. This is unacceptablefor bodybuilders. Second, energy depri-vation will eventually slow down yourmetabolic rate so you will be burningfewer calories. Weight loss will eventu-ally plateau, until you cut calories evenfurther. Third, energy restricted dietscause an increase in the enzymes thatmake and store fat, so that when you re-move the energy restriction (start eatingnormally again) the first thing that hap-pens is you gain back the fat you lost.These adaptations are known as the “star-vation response” and represent yourbody’s attempt to defend it’s fat stores inthe face of energy deprivation. Your bodyperceives the energy deficit created by thelow calorie diet as a famine and tries tohang onto it’s fat stores as long as pos-sible so it can survive. These are the rea-sons low calorie diets ultimately result in failure about 90% of the time. But what ifinstead of cutting calories we shift to alower efficiency diet. We still consumethe same number of calories as usual, butnow a higher percentage of the caloriesare lost as body heat during metabolism.This will result in weight loss (an effec-tive negative energy balance) without re-ducing calorie intake and setting into playthe starvation re-sponse.

Meta-bolic rate will re-main high and wewon’t losemuscle. So theseconsiderationsabout the ener-getic efficiencyof nutrient me-tabolism allow usto design a dietwhich will resultin fat loss with-out cutting calo-ries, slowingmetabolic rate, orlosing muscle.Pretty slick, huh?Next, Iwant to discusssome experi-ments done withhumans to moni-tor what happens to the nutrients we eatunder different dietary conditions (2-5).In the last few years it has become clearthat overall energy balance really has threeseparate components. The nutrient energysupplied to the body comes in the formof protein, carbohydrate, and fat. Theseare called “macronutrients” because weconsume them in large amounts, in con-trast to the “micronutrients” like vitaminsand minerals which we consume in smallamounts. Protein, carbs, and fat followdifferent metabolic pathways. It turns outthat the protein compartment of the body,the carbohydrate compartment, and thefat compartment are all regulated sepa-rately, although what happens in one com-partment will obviously affect the others.So we have a protein balance (usuallycalled “nitrogen balance” since most ofthe body’s nitrogen is contained in aminoacids) which describes the balance be-tween protein intake and protein utiliza-tion. A positive nitrogen balance meanswe’re gaining protein mass, which effec-tively means we’re gaining muscle mass.Similarly there is a fat balance which de-scribes the amount of fat intake versusthe amount of fat burned for energy. Anegative fatbalancemeanswe’re losingbody fat.

And last isthe carbo-hydrate bal-ance, whichis theamount ofcarbs con-sumed ver-sus theamountused as fuel.A positivecarbohy-drate bal-ance meanswe’re stor-ing glycogenand a nega-tive carbbalance means we’re losing glycogen. Gotit?Since the forms of energy con-sumed and stored by the body are carbo-hydrate, protein, and fat then it makessense that overall energy balance is deter-mined by the sum of protein balance, car-bohydrate balance, and fat balance. Andalthough the protein, carb, and fat bal-ances can affect each other, they are regu-lated separately. As simple as it soundsthis has only been understood for aboutthe last 10 years, and is still somewhatcontroversial. This has dramatic implica-tions for bodybuilders and, for that mat-ter, anybody wanting to lower their per-cent body fat.

As you can see, if our goalis to lose fat we must achieve a negativefat balance, and this is not necessarily thesame thing as a negative energy balance.To lose fat what we have to do is to burnmore fat than we eat — in this situationwe will be in negative fat balance and thefat which is burned in excess over whatwe consume must be derived from adi-pose depots. To lose fat it doesn’t reallymatter what your overall energy balanceis, just so you have a negative fat balance.If you think about it, it’s really the proteinbalance that determines how muchmuscle mass we have and the fat balancethat determines how much body fat we’recarrying. So these nutrient balances (andnot energy balance per se) are what de-termine our body composition. And nu-trient balance is largely determined by diet,since the diet establishes the input side ofthe balance equation. The studies I wantto discuss have looked at how the fat, car-bohydrate, and protein balances are regu-lated, how this relates to overall energybalance, and how all of this is affected bychanges in diet. Afterwards it will becomeclear how you can make changes in dietcomposition to influence changes in yourbody composition.The first paper I want to reviewexamined what happens if we feed peoplean excess of calories supplied as carbo-hydrates (2). For this experiment sixhealthy young men were fed a test mealcontaining 480 grams of carbohydrate,which is about twice as much carbohy-drate as an average person would eat in awhole day. Overall the meal consisted of93% of energy (calories) as carbs, 5%protein, and 2% fat.

To monitor nutrientmetabolism the respiratory quotient (RQ)was monitored by indirect calorimetryover the next 10 hours. What does thatmean? Each of the different macronutri-ents (protein, carbs, and fat) require dif-ferent amounts of oxygen to burn. Whena food molecule is completely burned it isconverted into carbon dioxide (CO2) andwater (H2O). A fat molecule contains veryfew oxygen atoms, so when you burn afat molecule it can combine with a lot ofoxygen molecules. So if you are burningfat for fuel you will consume a lot of oxy-gen. A sugar molecule (a carbohydrate)contains some oxygen atoms already builtinto the molecule, so when it is burned it consumes less oxygen. In other words,from a chemical point of view the carbonatoms in a carbohydrate molecule are al-ready partially oxidized, so you can onlyadd so much more oxygen when you burnit. The carbon atoms in a fat molecule arefully reduced (which is the opposite ofoxidized) so when you burn fat more oxy-gen is consumed than when you burncarbohydrate. Protein works out to be inbetween.So what you do is place the per-son in a room or a special chamber whereyou can monitor how much oxygen heconsumes and how much carbon dioxidehe produces, and from this you can cal-culate if he’s burning carbs or fat for en-ergy, and how much of each.

To monitorprotein metabolism urine samples are col-lected and the amount of protein utilizedcan be determined from the amount ofurea nitrogen excreted in the urine. Indi-rect calorimetry is a special techniquewhere we monitor oxygen consumptionand carbon dioxide production and fromthat determine how much protein, carbs,and fat are being burned as fuel. The res-piratory quotient (RQ) is the ratio of car-bon dioxide produced divided by theamount of oxygen consumed, and this(along with urinary nitrogen excretion) iswhat tells us what mixture of protein,carbs, and fat is being used as the fuelsource (the substrate mix, or fuel mix).Some amazing things were dis-covered with this experiment. Mostpeople would think that if you overfeedthat many carbohydrate calories at onesitting you would store the excess energyas fat. But that’s not what happened. Dur-ing the ten hours following administrationof the test meal, 133 g of carbohydrate,17 g of fat, and 29 g of protein were oxi-dized (burned). Disposal of the 480 gramcarbohydrate load was accounted for byoxidation of 133 grams, storage of 346grams as glycogen, and conversion of lessthan 3 grams into fat. Probably most ex-citing was that during this same time 17grams of fat were burned. The diet onlyprovided 8 grams of fat, so the subjectsactually lost fat as a result of the test meal,even though it contained excess calories.

This study demonstrates thatfeeding excess calories in the form of car-bohydrate results in glycogen storage, butnot fat storage. Feeding excess energy ascarbohydrate increases the rate of carbo-hydrate oxidation, so if you eat more carbsyou burn more carbs. This effect mini-mizes any conversion of carbs into fat. Ifyou consume excess calories in the formof carbohydrate they will mostly beburned as energy or else stored as glyco-gen. Subjects in this study actually had anegative fat balance (i.e., lost fat) evenafter carbohydrate overfeeding. But don’tget too carried away with this. This ex-periment was just overfeeding carbohy-drates for one meal. If this was contin-ued over a few days eventually glycogenstores would become saturated and thenI expect we would see net fat storagebegin to occur. The conclusions from thispaper are that acute (short-term) carbo-hydrate overfeeding increases carbohy-drate oxidation, so that if you eat excesscalories in the form of carbs some of theexcess will simply be burned and lost asheat. The remainder will be stored as gly-cogen, but will not be converted to fat.Next month I’ll pick up the discussion withan experiment to test what happens if weconsume excess calories in the form offat. And it ain’t pretty.


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2. Acheson KJ, Flatt JP, and Jequier E.Glycogen synthesis versus lipogenesisafter a 500 gram carbohydrate meal inman. Metabolism 31: 1234-1240 (1982).

3. Schutz Y, Flatt JP, and Jequier E. Fail-ure of dietary fat intake to promote fatoxidation: a factor favoring the develop-ment of obesity. Am. J. Clin. Nutr. 50:307-314 (1989).

4. Hill JO, Peters JC, Reed GW, SchlundtDG, Sharp T, and Greene HL. NutrientThe Energetics of Nutrient Metabolism, Part Ibalance in humans: effects of diet com-position. Am. J. Clin. Nutr. 54: 10-17(1991).

5. Jebb SA, Prentice AM, Goldberg GR,Murgatroyd PR, Black AE, and CowardWA. Changes in macronutrient balanceduring over- and underfeeding assessedby 12 day continuous whole-body calo-rimetry. Am. J. Clin. Nutr. 64: 259-266(1996).

2018-03-13T11:10:33+00:00 June 4th, 2009|Technical Supplement Bulletins|

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