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."  (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 unpalatable 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.  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."  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).  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.  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. 
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.  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. , 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.  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. 
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. 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
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