The main ingredients
in a smoothie intended for weight control are fruits and vegetables.
Protein powders often figure in recipes but I want to consider them separately
because in most cases they are not used for weight control specifically. There
are exceptions to that such as protein-sparing fasts, but it is the fruits and
vegetables that most advocates of blender foods are talking about for the most
part.
In general we think
of fruits and vegetables as healthy. Not much to argue with there, but I
want to first examine the basis of this idea so we have a clear understanding
of what we are talking about because in this case the details matter and not just
the general principle that fruits and vegetables are mostly very healthy
food.
In general we tend
to accept the assumption that fruits and vegetables are healthy compared to
most of what most people eat. The most convenient and least expensive
foods and ones we value most tend to be the least healthy in general.
Compared to those, fruits and vegetables are clearly more conducive to good
health.
There are two
important points of contention though:
- "Smoothies" often contain ingredients for flavor that are not just blended fruits and vegetables and maybe some protein powder. They can contain a lot of added sugar, and significant added fats because those often make the drink a lot more appealing. However that is a concern mainly if you buy commercial drinks or if you are careless about the recipes you use.
- There is also evidence that drinking food can sometimes have completely different effects on satiety than eating it. This is a much more important concern to me and one that I want to examine in more detail. The tricky part is that the effects of drinking calories can be good or bad depending on other things.
For the moment let's
assume we are intelligently blending fruits and vegetables, and that the
resulting drink is healthier in most ways than commercial soft
drinks. That assumption is probably not too much of a
stretch. The question I'd like to address is specifically whether we can
generalize that adding daily smoothies to our diet can improve our ability to
regulate our weight, whether it can be a detriment, or whether it tends to be a
neutral or indeterminate factor.
First, let's get a
scientific handle on fatness. The problem of obesity at a population
level has a lot of complicated aspects and the biology of metabolism is also
very complex, but advocates of various products and services have often used
that complexity to distract us from what are fairly simple overall facts of the
matter. So let's start by taking a very simple high level cut at the
problem of human fatness from a biological perspective.
Kelly Brownell, an
expert in nutrition and weight disorders, describes the overall situation as
clearly as any problem description can be stated:
"The
simple story is that biology seeks out an energy-dense diet, the environment
provides it, and we have runaway obesity." [1] (p. 35)
This is sometimes
known as the "mismatch" framework because it reflects our observation
that our environment has become less well matched over time with our biology in
some ways. We have made great strides in exploiting the widespread animal
evolutionary selection for efficiency in the form of preferring an energy dense
diet when it is available. We seek out sugar, fat, variety, and the
flavors we associate with fats and carbohydrates especially. Our cultural
environment exploits this in selling food. That's the simple
truth of why so many of us are fat. Our stable biology preferring energy
dense foods provides a vulnerability that our environment has come to
exploit.
Obesity, or runaway
excess fat tissue, is relatively uncommon outside of humans, the companion
animals of humans, and the animals domesticated by humans. In general
when animals have equally palatable or equally unpalatable choices of
different macronutrients, they tend to balance them pretty well rather than
becoming malnourished or overnourished. Under conditions of
their natural environment, animal preferences in food tend to serve them well
most of the time, just as we would expect. Weight is regulated in a very
stable way under those conditions. Animals evolved to
eat in a way that helps them survive in their natural habitat, by taking
advantage of the food sources available to them.
The bad news is that
something relatively uncommon in nature is now very common in human
environments: abundance in the form of an amount and variety of energy
dense foods that was very rare in earlier times. The seemingly good
intake regulation animals do in natural environments, where scarcity is the
rule, is easily overridden by simply making greater amounts and greater variety
of foods available of the sort we tend to prefer.
We are not wired to
regulate our weight, we are wired to thrive in natural environments by strongly
preferring energy dense foods in order to take advantage of them when we find
them, and there is apparently no natural mechanism that effectively compensates
for that preference by eating less of them under those conditions.
In experiments with
rats, the preference is so strong that they eat themselves into protein
deprivation when either more fats or more carbohydrates or both are provided
than proteins. [2] Rats may seem pretty far removed from us in some
ways, but the pattern is suspiciously familiar in human environments as
well. Greater availability of higher energy density foods leads to eating
more of those and neglecting other sources of nutrition, to the detriment of
our health.
So we don't need to
look at a lot of complicated issues in nutrition to see why fruits and
vegetables are offered as
favored foods for weight control. However I think we do need to be
suspicious of whether simply adding more of those to our diet will have
the desired effect of competing successfully with the higher energy density
foods that make us fat. Will drinking more green smoothies lead to eating
less loaded fries and mega burgers and drinking less gargantuan soft
drinks? That's the promise of the blender as a weight control tool, at
least in the ads for smoothies and blenders and smoothie recipes.
We have clear-cut
evidence that fruits and vegetables are in general less energy dense and yet
are still satisfying sources of nutrition compared to most of the food that
comprises the average American diet. The strategy of reducing energy
density in general has been supported by research and argued by leading experts
in weight regulation such as Barbara Rolls :
"A
growing body of laboratory-based, clinical, and epidemiological data suggests
that low-energy-dense diets are associated with better diet quality, lower
energy intakes, and body weight. Dietary energy density can be lowered by
adding water-rich fruits, vegetables, cooked grains, and soups to the diet, and
by reducing the diet’s fat content." [3] p. S98
So the argument for replacing at least some portion of our
energy dense foods filled with added sugars and fats with satisfying but far
less energy dense foods like fruits and vegetables appears to be very
defensible, so long as we are also somehow still getting the nutrients we need
that might not be easily found in fruits and vegetables. The advocates of
smoothies rarely suggest that they should entirely
replace other foods with smoothies, so that doesn't seem like a big concern to
me so long as people are not relying entirely on smoothies for their nutrition.
Replacing some of
our energy dense convenience foods with less energy dense fruits and vegetables
certainly seems reasonable. So we might well agree in principle that
adding fruits and vegetables to our diet can help regulate our weight.
But does it actually work that way if we simply add fruits and vegetables to
diet otherwise filled with convenience foods? Do we actually start
eating less of other things if we somehow get ourselves to eat more fruits and
vegetables? Or do we end up just adding more "healthy" calories
on top of what we already eat?
The question is not
just the trivial one of whether forcing ourselves to eat tons of veggies
temporarily prevents us from eating a cheeseburger, or whether that would be a
good strategy. The question is whether adding fruits and vegetables in
some enjoyable way actually helps us eat less of other things in the long
run in a way that causes us to take in less total energy. That
would be a legitimate aid to weight control.
Or are green
smoothies a minor convenience that still relies on brute force self-control to
replace the more attractive foods we crave?
And if eating more
fruits and veggies does help, does it still help if the fruits and vegetables
are eaten as a liquid?
There is a real possibility that it might make a difference.
These are the real questions I want to explore.
Phrased this way,
many of the vast complexities of nutrition and metabolism are mostly
irrelevant. What I want to know is whether smoothies can actually help
with weight control, which means the ultimate deciding factor is whether adding them to our diet causes us to
take in less energy overall,
without taking other measures. That's a strongly stated but relatively
common version of the claim made for why smoothies are supposed to be useful
for weight control.
Fruits and
vegetables are low in energy density mostly because of their high water content
and their low fat content. Their fiber content contributes as well, but
to a lesser and more variable extent. The low energy density due to high
water content and low fat content also seems to be the primary reason why
fruits and vegetables are relatively high in satiety
(we tend to compensate for eating them by eating less later) as well as satiation (we tend to find smaller amounts
satisfying and stop eating sooner). [4] p. 6
If fruits and
vegetables are generally useful in regulating our weight, the best argument I
can find is that they replace higher density nutrition with lower energy
density nutrition without motivating us
to eat more to compensate. If we ended up more hungry a few hours
later as a result of taking in less energy dense foods now, we would still be
relying on our willpower to lose weight and the fruits and vegetables would not
actually be helping us lose weight in the strongly stated sense.
So do fruits and
vegetables have this effect of helping us take in less energy while not
compensating later?
And do they still
have this effect if prepared in a blender first?
In the latter 20th
century, the rate of obesity rose dramatically and unexpectedly along with the
amount of money we spent marketing and buying convenient foods that are very
high in added sugars, added fats, enhanced flavors, portion sizes, and energy.
It is very unlikely that this was a coincidence. We started eating more
because foods that exploited our preferences became more readily
available and appealed particularly to our decision making by appealing to our
taste preferences and our preference for economic value. Nor did we
compensate for the increased intake by moving more. If anything
technology changes have led us to move less and exert less physical effort in
our daily lives. As a result the environment came to overwhelm our
ability to regulate our own weight.
Popular theories
that preferentially blame fats or carbohydrates for obesity are mostly missing
the point. We became fat when we started eating more of everything, and
we did that because of increased availability of high energy foods that suit
our natural preferences, not simply because fats or carbohydrates are
fattening.
The crux of the
problem of fatness is increased intake, not whether we eat "healthy"
foods. That's why the question of whether fruits and vegetables help us
eat less is crucially important. We eat more now across all of the major
food groups, not just the "unhealthy" foods, so we can't lay the
blame for obesity entirely on those. The increased intake that led to
increased obesity included fruits and vegetables, not just French Fries
and soft drinks. Adding fruits and vegetables to our diet in
general as a population has not by itself magically pushed out junk food or
reduced our overall calorie intake, and there is little population evidence to
suggest that it should. The evidence that simply adding fruits and
vegetables to our diet would compensate for overeating is not terribly
compelling at a population level. But what about experimental
evidence?
Considering how
commonly it is recommended, there is surprisingly little direct evidence
regarding the effect on weight control of adding fruits and vegetables to our
diet. Most research where higher energy density foods were replaced with
fruits and vegetables was relatively short term and also included explicit
instructions and assistance in avoiding compensating for the added
calories. So we don't really know whether (or how much) adding fruits and
vegetables really helps us eat less or helps us eat less in a later meal.
We strongly suspect it is at least a factor though because low energy density
foods do tend to result in both higher satiety and higher
satiation. The fiber content of those foods may also play a secondary
role.
For the most part
when we are not relying on external cues for how much to eat and we make use of
internal sensation, it is the weight
and volume of what we eat that makes the
difference in how much we eat rather than the amount of energy it contains or
the glycemic loading, so long as it has the right sensory properties that we
experience it as substantial food. [5] Surprisingly, since so much
diet advice mentions glycemic index, it does not appear that carbohydrate
content or glycemic index are reliable predictors of satiation or satiety
compared to energy density and fiber content. For example, boiled
potatoes, which are relatively high in glycemic index, are also particularly
satiating.
The available
evidence from intervention studies seems to support the idea that adding fruits
and vegetables to meals can assist in weight control by adding water and fiber
and reducing energy density, increasing satiation and satiety, and helping us
to eat less overall while still getting good nutrition. Supporting this
idea, restricting high energy density foods while allowing unlimited amounts of
fruits and vegetables has sometimes been a successful weight control
strategy. [3]
This doesn't
necessarily tell us that that adding fruits and vegetables to our diet causes
us to eat less of other things, but it does tell us that we tend not to overeat
fruits and vegetables, which suggests that many people find them either
relatively satiating or relatively unpalatable. So it leaves the
door open to the possibility that they can be useful for weight control for
those who do find them palatable as well as satiating.
So let's assume for
now that fruits and vegetables do help us with weight control by helping us eat
less of other things. That being the case does this still apply when the
fruits and vegetables are prepared in a blender?
The answer to this
might seem obvious depending on how you think about satiation and
satiety. The counter-intuitive reality though is that some foods increase
in satiety when in liquid form and some foods decrease in satiety in liquid
form. The case is most clearly established for high sugar drinks, which
have been unambiguously established to have very low satiation and satiety and
are believed by obesity researchers to be an important contributor to
obesity. The case is more equivocal for liquid meals that also
contain more satiating ingredients such as fiber and protein. In
those cases, the variation in outcomes may be because the behavioral context
plays a crucial role in their effect on intake.
First, on the plus
side, the water content of foods is one of the main things that increases how
well they satisfy our appetite. This happens by increasing their volume
and their weight. When you make a soup out of ingredients, you are getting
both a greater weight and greater volume of food than when you eat the
ingredients without the liquid, and in general that tends to increase the satiation of the same food without increasing
the energy intake. More interestingly, and more surprisingly, it can also
increase the satiety of the same
energy-equivalent of food, causing us eat less later. [6] For foods
that are already satiating, adding water while still making them palatable and
perceived as food, tends to increase satiation and satiety.
The same effect is not seen simply by drinking water with
a meal or before a meal as when the water is part of the food. Hunger and
thirst are regulated separately in the body, the satiating effect of fluids are
because we experience the food as heavier and higher volume (and as
food!), not simply because there is more water in our stomach.
Blending fruits and
vegetables into a drink obviously increases the water content, and they are
already satiating, so we have reason to suspect it might increase the satiety
and satiation. Assuming we experience it more as food rather than more as
water. So the case for losing weight with green smoothies seems plausible
scientifically.
On the minus side,
we don't seem to regulate our own intake as well with a liquid diet as we do
with a solid food diet.
Under controlled
conditions, where we are not inundated with abundance, variety, and other cues
that tell us eat more, we tend to regulate our intake from one meal to the next
during the day to eat relatively the same amount from day to day, and we
also seem to regulate out intake to some extent from day to
day. This is especially true of the volume of food we eat, but under some conditions it is also
true of calories.
Given the same
weight and volume of solid food, we also tend to eat more or less from meal to
meal to take in about the same amount of energy every day. In
experiments, secretly adding more calories to the same amount of food each day
results in people eating less in subsequent meals. This phenomenon of
energy-specific satiety is sometimes known as dietary
compensation. [7],[8] The argument against
liquid diets is based on the finding that dietary compensation seems to be much
weaker with liquid meals than with solid meals. [9] However this is
mostly based on findings regarding fruit juices vs. fruit and sugary drinks vs.
sugary solid foods, and almost entirely based on liquid vs. solid carbohydrate
intake. Liquid diets have also been used successfully for weight control
under some conditions. [10]
This means that
different forms of a food (at least a carbohydrate) can alter its satiety and
satiation, and the liquid form of carbohydrates in general seem to bypass our
tendency to compensate by eating less. With fruit for example, the case
is quite clear, the liquid form is considerably less satisfying to our hunger
when used as a "preload" just before eating.[11] As
usual, the energy density plays a big role, and fiber plays a smaller role, but
simply drinking calories rather than eating them seems to have an independent
effect on satiation as well. This may be due to structural factors
involved in eating and digestion or it may be due to expectations we have regarding how satisfying the food will be
and the context in which we are eating.
We probably don't
expect fruit juice to satisfy our hunger as well as fruit, and that may in part
be why it doesn't. Do we expect
smoothies to satisfy our hunger? That might tell us whether they can
serve us in weight control by helping us eat less in total.
One strategy for
eating less is sequencing. Starting
a meal with a low energy density food (as an appetizer or "pre-load")
seems to reliably help us reach satiation with less total energy intake, but
starting with solid low energy
density food seems significantly more effective than starting with liquid low
energy density food, regardless of fiber content. This is in direct
contrast to the popular advice to drink water prior to eating in order to fill
up. That seems relatively ineffective even if we replace the water a high
fiber carbohydrate drink.
Using a blender to
conveniently add fruits and vegetables to our diet seems a reasonable strategy
for weight control, by providing satisfying nutrition at lower energy intake,
but the way we use it probably matters a lot. It appears that blender meals
are best used as weight control aids when:
- We enjoy them and find them palatable and satisfying and expect them to be satisfying while still keeping them at low energy density.
- We do not make them energy-dense with sugars and fats, even "healthy" ones.
- We use them to replace rather than just add more intake to higher energy density sources
- They contain satiating ingredients such as high fiber carbohydrates and lean protein
- We don't rely on them as our only strategy for getting good nutrition while taking in less energy
References
[1](2004) Brownell,
Kelly and Katherine Battle Horgen, "Food Fight: The Inside Story of the
Food Industry, America's Obesity Crisis, & What We Can Do About
It." McGraw-Hill
[2]
Michael G. Tordoff (2002) "Obesity
by choice: the powerful influence of nutrient availability on nutrient
intake"
American
Journal of Physiology - Regulatory, Integrative and Comparative Physiology
Published 1 May 2002 Vol. 282 no. 5, R1536-R1539 DOI:
10.1152/ajpregu.00739.2001 URL: From http://ajpregu.physiology.org/content/282/5/R1536
[3] (2005) BARBARA
J. ROLLS, PhD; ADAM DREWNOWSKI, PhD; JENNY H. LEDIKWE, PhD "Changing the
Energy Density of the Diet as a Strategy for Weight Management"
Supplement to the Journal of the AMERICAN DIETETIC ASSOCIATION, May 2005
S98-S103
[4]
(2004) Barbara J. Rolls, Ph.D., Julia A. Ello-Martin, M.S., and Beth Carlton
Tohill, Ph.D., M.S.P.H. "What Can Intervention Studies Tell Us about
the Relationship between Fruit and Vegetable Consumption and
Weight
Management?" Nutrition Reviews , Vol. 62, No. 1 January 2004: 1–17
URL: http://www.researchgate.net/profile/Barbara_Rolls/publication/8674390_What_can_intervention_studies_tell_us_about_the_relationship_between_fruit_and_vegetable_consumption_and_weight_management/links/5405d2cb0cf2c48563b1ba87.pdf
[5] (2005)
Tohill, Beth Carlton, "Dietary intake of fruit and vegetables and
management of body weight," World Health Organization , ISBN 92 4 159284 2
URL: http://www.who.int/dietphysicalactivity/publications/en/f&v_weight_management.pdf
[6]
(1998) Barbara J Rolls, Victoria H Castellanos, Jason C Halford, Arun Kilara,
Dinakar Panyam, Christine L Pelkman, Gerard P Smith, and
Michelle L Thorwart Am J Clin Nutr 1998;67:1170–77.
"Volume of food
consumed affects satiety in men" URL: http://ajcn.nutrition.org/content/67/6/1170.full.pdf
[7]"Short Term
Dietary Compensation in Free-Living Adults"
F. McKiernan, J.H.
Hollis, and R.D. Mattes
Physiol Behav. 2008
March 18; 93(4-5): 975–983.
[8] "Dietary
compensation in response to covert imposition of negative energy
balance by removal
of fat or carbohydrate"Gail R. Goldberg*, Peter R. Murgatroyd, Aideen P. M. McKenna, Patricia M. Heavey
and Andrew M. Prentice
British Journal of Nutrition (1998), 80, 141–147
http://www.researchgate.net/profile/Peter_Murgatroyd/publication/13457513_Dietary_compensation_in_response_to_covert_imposition_of_negative_energy_balance_by_removal_of_fat_or_carbohydrate/links/0deec52cbd1ac48ea3000000.pdf
[9] (2000)
"Liquid versus solid carbohydrate: effects on food intake and body
weight"
International
Journal of Obesity (2000) 24, 794±800DP DiMeglio and RD Mattes
http://cibr.refrescantes.es/ka/apps/cibr/docs/06_2000_Solido_y_liquido_efecto_peso.pdf
[10]
(2007) "Liquid calories, sugar, and body weight"
Adam Drewnowski and France BellisleAm J Clin Nutr March 2007 vol. 85 no. 3 651-661
URL: http://ajcn.nutrition.org/content/85/3/651.long
[11] (2009)
Julie E. Flood-Obbagy and Barbara J. Rolls "The effect of fruit in
different forms on energy intake and satiety at a
meal" Appetite. 2009 April ; 52(2): 416–422.
doi:10.1016/j.appet.2008.12.001.
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