Limitations of calorie density



While the principle of calorie density is useful, it has various limitations which are discussed here.  What we would actually like to measure is the ratio of a food's tendency to promote satiety to its tendency to promote weight gain or loss.  This would be difficult to measure accurately, so calorie density is merely a proxy for this theoretical principle.


1)  "A calorie is a calorie is a calorie"  -is this true?


While losing weight generally requires a calorie deficit to force your body to metabolize body fat for energy, the above statement is not exactly correct.  Here are some reasons why:


a) Absorption of macronutrients with metabolic potential energy is not 100% efficient.  


About 5% of potential calories from food are lost in stool and urine (11), and the absorption is different for different foods.  Additionally, certain medical conditions can cause malabsorption (i.e. chron's disease,  cystic fibrosis, or having a history of gastrointestinal surgery).


b) The food you eat affects metabolic rate


The processes of digestion and absorption themselves require metabolic energy.  Peristalsis in the gastrointestinal tract, secretion of digestive enzymes, active transport mechanisms in the small intestine, and other aspects of digestion burn calories.


This is known as "thermogenesis" 


​The following describes an estimate of the amount of energy which is lost in digestion for the three classes of macronutrients (57)


fats: 2-3%

carbohydrates: 6-8%

proteins: 25-30%


As you can see protein rich foods tend to require more energy to digest.  Various other sources support this notion (58, 59, 60).


c) The type of food you eat affects satiety, independent of calories and mass.


There is significant evidence that protein is more satiating than fats and carbohydrates (61,62).  Soy and meat protein appear to equally effective (65).  However, whey may be more effective at promoting satiety in comparison to casein, even though they are both proteins found in milk (67).


"Low-fat dairy products, eggs, and legumes enhance satiety. Although energy dense, nuts have some satiety-inducing effect, when included in the diet by isocaloric replacement of usual foods." (58)


Insoluble fiber also may contribute favorably to satiety (71), although some studies dispute this effect (72, 73).


c)  Specific foods may affect appetite and metabolism in complicated ways.


For example fructose and glucose have identical calorie densities (and the same molecular formula), but fructose may cause less satiety (56) and reduces leptin (a hormone which suppresses appetite) (55).


Green plant membrane extract may suppress food urges through the effects of glucagon like peptide 1 (GLP-1) (64).  GLP-1 secretion may be impaired in obese individuals (66)


Even the specific taste and texture of foods may have some influence


One study suggests that our satiety for specific cravings is taste specific (i.e. "salty" and "sweet") (70).  We may even feel a need to satisfy a craving for a particular food texture (i.e "hard" or "soft") (70).


Even the variety of food may have an effect (75) with less variety promoting earlier satiety (perhaps people get tired to eating too much of a single type of food).  


I could give various other examples, but suffice it say that calorie density significantly oversimplifies the complicated sciences of satiety, appetite, and metabolism.


2) Mass does not necessarily reflect volume occupied in the stomach


Many have argued that volume in the stomach is the most important contributor to the feeling of satiety (69), and mass is merely a proxy for stomach volume.  Barbara Rolls proposes the principle of "volumetrics" which is similar to the concept of calorie density but focuses on volume rather than mass.


Lets take popcorn as an example.  When compared to potato chips, popcorn was more satiating (74) even though both are high in calorie density.  The reason for this may be because popcorn has a high air content, and even after mashing it down with your teeth, some of the air is retained in the stomach.


"You can eat an awful lot of popcorn without taking in a lot of calories...it may not weigh much, but it makes your stomach feel full because it takes up so much space" -Holt (the author of source 62)


If we look at the pictures of pretzels and oatmeal below (both pictures show exactly 100 Calories), they fill up about equal volume on the plate, but the oatmeal is much less calorie dense
















How exactly does one measure the volume of airy foods...and the volume of those foods in the human stomach?  How much is the air in those pretzels contributing to satiety?  It is difficult to say, and calorie density ignores air and does not fully account for the effect of volume in some cases.


3) Empirically measured satiety is not perfectly predicted by the mass of food



Various attempts have been made to estimate the satiety which a particular food produces more accurately


For instance, nutrition data has developed a "fullness factor" (FF) which is calculated as follows (76):


FF=MAX(0.5, MIN(5.0, 41.7/CAL^0.7  + 0.05*PR + 6.17E-4*DF^3 - 7.25E-6*TF^3 + 0.617))

where CAL is total Calories per 100g (30 minimum),
PR is grams Protein per 100g (30 maximum),
DF is grams Dietary Fiber per 100g (12 maximum), and
TF is grams total Fat per 100g (50 maximum).


They produce the following list of foods:


(This picture is taken directly from their website here: http://nutritiondata.self.com/topics/fullness-factor)






































As you can see, satiety is presumed to be positively correlated with calories, fiber, and protein...and inversely related with fat.


In one major study, reported satiety was recorded in 38 subjects who were fed 240 Calorie servings (62), and a "satiety index" was developed, comparing everything to white bread which was giving a ranking of 100%.  Foods with satiety indexes <100% were less satiating than white bread per calorie, and foods >100% were more satiating per calorie


The chart below is copied from the mendosa website as I could not obtain the full original article.


The website is linked here: http://www.mendosa.com/satiety.htm


Bakery ProductsCarbohydrate-Rich Foods
Croissant47%White bread100%
Cake65%French fries116%
Doughnuts68%White pasta119%
Cookies120%Brown Rice132%
Crackers127%White rice138%
Snacks and ConfectionaryGrain bread154%
Mars candy bar70%Wholemeal bread157%
Peanuts84%Brown pasta188%
Yogurt88%Potatoes, boiled323%
Crisps91%Protein-Rich Foods
Ice cream96%Lentils133%
Jellybeans118%Cheese146%
Popcorn154%Eggs150%
Breakfast Cereals with MilkBaked beans168%
Muesli100%Beef176%
Sustain112%Ling fish225%
Special K116%Fruits
Cornflakes118%Bananas118%
Honeysmacks132%Grapes162%
All-Bran151%Apples197%
Porridge/Oatmeal209%Oranges202%


Of note, the author does note that this is a reflection of short term satiety over only two hours which may overstate the benefit of foods which are digested quickly such as fruit


Combining the above data with calorie density form my list page, I created the following "Calories per satiety" list where grams are adjusted for satiety index


Food..................................................................................."Calories per satiety" (calories per gram)


oranges.........................................................................................0.23 (0.46)

apple..............................................................................................0.26 (0.52)

potatoes........................................................................................0.30 (0.97)

grapes...........................................................................................0.41 (0.67)

bananas........................................................................................0.73 (0.86)

brown pasta..................................................................................0.75 (1.4)

lentil...............................................................................................0.80 (1.5)

brown rice.....................................................................................0.83 (1.1)

pinto beans...................................................................................1.01 (1.7)

light beef........................................................................................1.14 (2.0)

egg.................................................................................................1.33 (2.0)

french fries.....................................................................................2.24 (2.6)

popcorn..........................................................................................2.51 (3.87)

cheese...........................................................................................2.74 (4.0)

white bread....................................................................................3.0 (3.0)

cork flakes.....................................................................................3.05 (3.6)

cookie, butter................................................................................3.89 (4.67)

crackers.........................................................................................3.42 (4.4)

cake................................................................................................5.71 (3.56)

doughnuts......................................................................................6.15 (4.18)

peanuts..........................................................................................7.14 (6.0)

croissant........................................................................................10.15 (4.06)


Certainly, this is somewhat different from what the list would like like if we looked at calorie density alone, and it is arguably more accurate. 


4) Eating speed is another factor, and some foods force us to eat slowly


Generally speaking, the sensation of satiety is somewhat delayed, so eating very quickly may promote weight gain by overriding normal satiety mechanisms.


"One study demonstrated that eating the same meal over 30 minutes instead of 5 minutes leads to higher concentrations of gut peptides and favors earlier satiety." (63)


We have all experienced the feeling of eating very quickly and then feeling uncomfortably full 15 minutes later.


This may be another explanation for why popcorn is more satiating than potato chips (74)-popcorn takes longer to eat.  


Often, processed food allows us to eat more quickly which may cause us to override our mechanisms of satiety.  In this video, I have great difficulty eating nuts quickly (although I still consume 1000 calories of macadamia nuts in just over 4 minutes)


5) Low carbohydrate diets confer specific advantages


​People on carbohydrate restricted diets often report less hunger (77, 78) and have reduced calorie intake (86).  This is likely because ketone bodies (which are the primary source of fuel during carbohydrate restriction) suppress appetite (87).  Also, high carbohydrate meals can cause reactive hypoglycemia (low blood sugar) which stimulates appetite (88).  Avoiding blood sugar variation during low carbohydrate dieting may be an advantage.


In many head to head trials where low carbohydrate diets are compared to low fat diet diets, the subjects in the low carbohydrate group often lose more weight (78, 79, 80, 81, 82) despite no difference in calorie intake (79).  Of note, not all trials show this finding (i.e. 85)


"Greater weight loss with a [very low carbohydrate diet] over a [low fat] diet is consistent with the findings from other studies, and provides further support for the concept of metabolic advantage" (79)


The cause of the "metabolic advantage" is likely due to the high metabolic requirement to digest proteins (57, 58, 59, 60) and the use of proteins to form carbohydrates through gluconeogenesis (83).  Also, some ketones are lost in urine during ketosis (79) which could otherwise be metabolized for energy.


​Some studies even report that fat is preferentially lost over other tissues during low carbohydrate dieting (79, 89), though some of the weight lost is due to glycogen depletion and water loss (90)


5)  Calorie density deals with only one aspect of nutrition


Perhaps most importantly, it cannot be overstated that the principle of calorie density ONLY relates to regulation of calorie intake and weight loss and maintenance (or gain if desired).  There are many other important aspects of nutrition such as the following:


1) Micronutrient nutrition (vitamins and minerals)

2) Dietary balance/diversity of foods

3) Type of fats (saturated fats, polyunsaturated fats, monounsaturated fats, omega 3/omega-6 fatty acids et cetera)

4) Glycemic index-tendency of foods to spike blood sugar and promote hyperinsulinemia

5) Salt content

6) Food allergies/intolerances

7) The effect of food on gut flora

8) Tailoring of the diet to specific medical problems

9) Risk of food borne illness and toxin exposure


Please do not neglect other aspects of nutrition and health while structuring your overall diet, and do not neglect regular exercise.


Conclusion


Despite writing all of the above, I still feel that calorie density is the most pragmatic single measurement to use when adjudicating whether or not a food will promote weight loss or weight gain.  I think that the effect of the above factors is generally small in comparison to the effect of large differences in calorie density between foods.  Also, information on calorie density is readily available and can be calculated instantly for most packaged food or where nutritional facts are available.  Most natural foods can be referenced easily on the list page or from other sources.  Satiety index is an empirical finding, and we simply don't have that information for most available foods, and it appears to vary somewhat from person to person (62).  


With respects to the low carbohydrate diets, one of their key weaknesses is that they are very difficult to maintain in the long run (93).  In a large national dietary registry, almost all of the individuals who successfully maintained weight loss over several years had low-fat, low-calorie diets (94, 95, 96, 97).  Only 10.8% of individuals who had maintained >30 lbs of weight loss for > 1 year had used a low carbohydrate diet (98).


In sweeden, low carbohydrate dieting became popular in around 2004 but did not slow the continuing trend of increasing obesity (91)


Low carbohhydrate diets can also cause bad breath (92) and constipation (93)


In addition to the benefit of weight loss and maintenance, eating a low calorie density diet does tend to have other ancillary benefits (i.e. lower glycemic index, lower salt intake, and excellent micronutrient nutrition) due to the exclusion of many processed foods.


That being said, I do think it is important to be familiar with the limitations of calorie density and to realize that it does not work perfectly for every food.  It is probably significantly inaccurate for specific foods like nuts, popcorn, and potatoes as discussed above.


Home







"Low-fat dairy products, eggs, and legumes enhance satiety. Although energy dense, nuts have some satiety-inducing effect, when included in the diet by isocaloric replacement of usual foods." (58)