Smoothies are very popular with people trying to eat well, but should we be drinking our calories? I decided to look into the scientific literature and learn more about how drinking vs. chewing our food affects our satiety signals and overall caloric intake.
For those of you looking for a quick answer, the literature seems to point to the fact that eating solid food makes you feel more full than drinking your meal. In most cases, it also leads to less caloric intake later in the day.
I’m sorry folks – I know how much you love your blenders. If it’s working for you, then that’s great! Don’t change a thing. However, if you are looking to lose weight, or if you are felling hungry between meals, your smoothie might be to blame. For people looking to gain weight, I suggest lots of smoothies between meals. Interestingly, in the case of soups, the research shows that soups are more filling than solid meals of the same components. There are many reasons for this, so I encourage you to read the rest of my research review for the full story:
Excessive caloric intake is one of the primary causes of obesity. There is a growing demand for convenience, as Americans spend more money on functional beverages. The Functional beverages category, consisting of energy drinks, sports drinks and nutraceutical drinks, grew by 7.4% in 2013 to reach a value of $27,049.6 million in the United States. 2 Energy-yielding beverage intake has been linked to positive energy balance. 3,4 Many of these beverages are marketed to overweight and obese adults as being helpful for weight loss.
How we feel full
Many factors influence our feelings of fullness and the amount of food we eat including our mental state, sleep patterns, physical activity, genetics, and hormone sensitivity. Food form (the physical state of the food: either in solid, semi-solid, or liquid) also impacts how much we eat. The effect of food form on satiety (the physical feeling of fullness), satiation (the end of desire to eat more after a meal, which can occur at any time after the onset of eating), and overall caloric intake is an important topic as nutritionists and food processors search for ways to help the public avoid over consuming calories.
Liquid vs. Solid Food
The role of food form in satiety and caloric intake has been studied in various ways. One systematic review on the role of liquid vs. solid carbohydrates found that liquid carbohydrates are less satiating than solids, and food intake is not adjusted when calories are consumed in sugar-sweetened beverage form. 5 Researchers hypothesize many different causes at play when it comes to satiety and food intake. Chewing whole food has been thought to increase physiological responses, which lead to satiety. Whole foods are more slowly digested than liquids, so gastric emptying has been studied in relation to satiety. Humans may have expectations that a food, which appears more solid, will be more satiating than a liquid. There could also be a psychological effect from subjects presuming solids are “food” where liquids represent “a drink”.
All 16 primary research papers in this review are randomized cross-over or partial cross-over studies. Two of the papers focus on older adults and one study was on adolescents. Solids were found to be more satiating than liquids in 11 of the studies. 6-16 Satiety scores were measured using subjective scores, comparing 3 or 4 ways in ANOVAs, so P values only have been reported for these values. Four of the studies found food form had no effect on satiety 17-20, and two studies found liquids were more satiating than solids. 16,21
Most of the papers also tested additional measures such as food intake, the difference between lean and obese participants, and appetitive serum markers. Test foods varied, and because of their application to real world settings, the eight studies that tested satiety using “real food” are discussed at the end of the paper.
Studies showing solid foods are more satiating than beverages
Of the 16 total papers reviewed, 11 studies found solid food to have a stronger effect on satiety than beverages. 6-16 Within these 11 studies, 9 also measured food intake. Of the nine that measured intake, five found that solid preloads led to less food intake at an ad libitum meal 7,8,11,13,14 and the four of the studies found that solid food form led to stronger feelings of satiety, but there was no effect on food intake. 6,9,15,16
Of the studies that found solids to be more satiating than liquids, measures other than food intake were investigated. Three looked at the difference in satiety response or caloric intake between lean and obese, 6,9,13 and four studies also looked at insulin, glucose and other serum markers. 7,9,10,12
Studies showing solids are more satiating that also looked at food intake
Nine studies that found solid foods are more satiating also measured food intake at an ad libitum meal following the test solid or liquid. Five studies found that a solid preload lead to less caloric intake 7,8,11,13,14, and four studies found that food form of preload had no effect on food intake of a test meal. 6,9,15,16
Studies showing solids are more satiating and also found solids decrease food intake.
The five studies that found solids to be more satiating and found a solid preload to be more satiating than a beverage preload were Cassady et al, Yoemans et al, Flood-Obbagy et al, Houchins et al and Hogenkamp (a) et al. In all studies except Hogenkamp (a) et al, participants were given a test food of either a solid or liquid, asked about their feeling of satiety then, after a set period of time, given an ad libitum (as much as you can eat) meal. Participants were instructed to eat until “comfortably full” in all tests. The Hogenkamp (a) et al study tested intake on the actual test foods (no on a follow up meal after a preload). 14 Test foods (preloads) varied from study to study, however the ad libitum meal offered after the test was very similar in the four studies that offered a meal. Cassady et al and Houchins et al offered macaroni and cheese, while Yoemans et al offered pasta and ice cream and Flood-Obbagy et al served cheese tortellini with tomato sauce. Total intake of the meal was measured, and in these four tests, researchers found that those who consumed a solid test food consumed less of the ad libitum meal. Hogenkamp (a) et al also found that participants consumed more with the liquid test, even though the methods of their experiment differed. 14
The 52 healthy adults in Cassady et al, were exposed to four different preload foods. Participants were told that the liquids (water) or solids (gelatin) would either remain in it’s form (S-S) or (L-L), or turn into a liquid (S-L) or a solid once in the stomach (L-S). 7 This test was unlike most other research, in that it tested perceived food form. 7 The researchers found that when people consumed the liquid and were told it would remain a liquid in their stomach (L-L), they were significantly less satiated (greater hunger, desire to eat, and lower fullness) than in all other tests (P < 0.001). 7 The L-L test significantly led to the highest intake of the ad libitum challenge meal of mac and cheese (720 kcal +/- 40) compared to the L-S test food (583 kcal +/- 35 , P= 0.004) and the S-S (562 kcal +/- 38, P < 0.001). 7
Yoemans et al tested thin and thickened energy drinks on 48 healthy adults over multiple days. 8 Test lunch intake results were calculated by three-way ANOVA interactions between energy content of beverage, the sensory characteristics (thin or thickened) and test day, and no actual data was given on specific caloric intake. 8 In the first session, significantly less of the ad libitum pasta and ice cream lunch was consumed by participants that drank the thickened, high-energy drink than in either low-energy drink (P = 0.015). 8 In the final test, at the one-month follow up procedure, intake of lunch depended on which energy drink was consumed, regardless of the viscosity of the beverage. Those who drank the HE beverages consumed significantly less lunch intake compared to the LE beverages (P = 0.001). 8 The results suggest that learned satiety could overcome initial expectations of satiety in sensory enhanced (thickened) beverages. 8
Flood-Obbagy et al tested apples in the whole form, semi-solid (applesauce) and liquid (juice), and found apples to be significantly more satiating (hunger and fullness ratings) than juice (P < 0.05). 11 The study, with 58 healthy adult participants, also found that the solid apples reduced caloric intake of the ad libitum pasta meal by 15%. 11 Solid (562 kcal +/- 38) compared to liquid (720 kcal +/- 40; P < 0.001). 11
The Houchins et al test found that raw fruits and vegetables in solid form lead to significantly higher satiety (P = 0.030) and less intake (891 +/- 43 kcal) compared to the juice form (678 +/- 40 kcal, P < 0.0005). 13
In a study by Hogenkamp (a) et al, the differences in expected satiety and intake after repeated consumption of liquids and semi solid foods were tested on 53 healthy adults.14 The test foods were a very thin custard and a semi-solid pudding-like product. 14 Participants consumed these tests over several days. Expected satiety was higher for the semi-solid product on all test days (P < 0.001). 14 Instead of offering an ad libitum meal to measure intake, the researchers provided the test food (the custard and the pudding) ad libitum on days 1 and 5. 14 Intake of the thinner test food (custard) was higher on both of these days (P < 0.001 for both test days, though no actual figures were provided), compared to the semi-solid. 14
All five studies above, which showed that solids were significantly more satiating than liquids and led to less intake, were relatively small, ranging from the largest of 58 in Flood-Obbagy et al to 34 in the Houchins et al study and involved adults of normal BMI, except for the Houchins et al study, which compared the results from 15 lean and 19 obese adults. 11,13
Studies that showed solids are more satiating but food form of preload had no effect on intake.
Four studies found that, although solid test foods produced more satiety, intake of the ad libitum meal was not affected by food form of test food. 6,9,10,15 Mattes et al compared the results between 20 lean and 20 obese adults.6 Apolzan et al studied 18 sedentary and 16 weight trained older adults between the ages of 64 and 84. 9 The Hogenkamp (b) 15 et al study and Zhu (a) et al study were on adults of healthy BMI, with 27 and 15 participants, respectively .5, 10
In Mattes et al, the test food was apples in the whole, semi-solid (applesauce), and liquid (juice) form. 6 Although solid apples were found to be significantly more satiating than the juice (P = 0.026), no significant difference was found when participants reported their caloric intake with a 24-hour recall. 6
The Apolzan et al study tested the difference in solid meal replacement bars vs. liquid meal replacement beverages of the same nutritional content. 9 Although the bars produced a greater sense of satiety than the liquid (P < 0.01), there was no significant difference in consumption of test meal (a bowl of oatmeal) among the sedentary nor the weight trained older adults (data not given). 9
The Hogenkamp (b) et al study tested learned satiety over time with a high energy (HE) vs. low energy (LE) in beverage or semi-solid form over several consecutive days, followed by an ad libitum buffet lunch. 15 On the first day, participants had higher expected satiety thicker beverages (P < 0.0001) regardless of energy content, but satiety expectations did reduce by day 4, when expected satiety was still somewhat higher but not as high for the semi-solid compared to the liquid (P = 0.003) but the HE liquid beverage was expected to be more satiating than the LE foods (P = 0.01). Researchers found that participants consumed more calories at a buffet-style lunch on day 3 (1979 kcal +/- 567) compared to day one (1745 kcal +/- 577; P = 0.02) when participants consumed the low energy beverage, however no significant difference in food intake was found between the beverage or semi-solid (P = 0.56). 15 The researchers concluded that energy density and not food texture influenced ad libitum consumption after repeated consumption of the test food. 15 However, according to their data, expected satiety and fullness ratings were still higher after consuming thicker beverages compared to liquids throughout the experiment, even though actual intake changed. 15
Zhu (a) et al studied the difference a standard viscosity (custard) and high viscosity (pudding) meal. 10 Participants felt more satiated after the pudding (P < 0.001), however viscosity had no significant effect on intake of the ad libitum pasta meal, which was served three hours after the test food (pudding 493.5 g +/- 51.8 vs. custard 484.5 g +/- 50.0; P = 0.971). 10
Studies showing solids are more satiating than liquids, that compared responses between lean and obese
Of the studies that found solids to be more satiating than liquids, two studies compared lean and obese adults 6,13, and one other study compared responses between sedentary and weight trained older adults. 9 In both the Mattes et al study and the Apolzan et al study, no difference was found in caloric intake at the ad libitum meal between the two groups. 6,9
In the Houchins et al study, when the obese participants were given the fruit beverage preload they were significantly less satiated compared with the solid preload (P < 0.03). Obese consumed 547 kcal +/- 462 per day more with the beverage preload than with the solid (total caloric intake between leans and obese not provided; P = 0.03). 13 Lean participants who drank the beverage preload also ate more per day (218 kcal +/- 280 per day) than with the solid preload, but the difference between the lean and obese was not statistically significant (P=0.572). 13
Studies showing solids are more satiating than liquids, that also measured appetitive serum markers
Four of the studies that found solids more satiating than liquids also measured hormones and other serum markers. 7,9,10,12 Apolzan et al found lower blood glucose after the beverage (-11.0 mmol/L) vs. the solid (52.6 mmol/L, P<0.01). 9 Insulin was also lower after the beverage (63.04 ng/L) vs. solid meal replacement bar (10.65 ng/L, P<0.01). Leidy et al had similar findings. Glucose were both lower (1-h AUC, data not shown; P<0.05). 12
The Cassady et al study, which measured perceived food form (participants were told certain foods would remain liquid or solid once in the stomach), found higher glucose after what participants thought would be a liquid in their stomach (3.02 +/- 0.14 pmol/L) compared to when the participants thought the food would remain a solid (gelatin) in their stomach (2.63 +/- 0.12 pmol/L, P = 0.012). 7 The perceived liquid resulted in lower insulin and GLP-1 AUC compared with perceived solid test (data not shown; P < 0.05). 7
Zhu (a) et al found that the high viscosity (HV) beverage produced significantly higher plasma glucose compared to the standard viscosity (SV) (P < 0.001). 10 Insulin was not significantly different, however GIP was higher after the SV (P < 0.001). 10 Gastric emptying rate was lower after the HV compared to the SV (P < 0.001). 10 No specific values were given in this study.
Studies showing food form has no effect on satiety
Four studies found that there was no difference in satiety between liquid or solid food forms. Three of these studies, Leidy (b) et al, Jones et al, and Akhaven et al involved a test food followed by an ad libitum meal in order to measure caloric intake. The 15 Leidy et al participants were age 14, and the test foods were a solid “pancakes and eggs” or a breakfast shake, followed four hours later with an ad libitum lunch buffet: crackers, string cheese, lunch-meats, fruit and vegetables. 17 The Jones et al study consisted of 104 lean or obese groups, and compared four different foods: low-energy beverage, high-energy beverage, low-energy solid, or a high-energy solid, followed one hour later by a challenge meal of macaroni and cheese. 18 The Akhaven et al study consisted of three experiments on healthy male subjects and included 300 kcal of the test food. 19 In experiment 1, there were 14 participants and the test foods were solid (gelatin) and liquid forms of sucrose. 19 Experiment 2 consisted of 15 participants and tested liquid and solid mixtures of 50% glucose and 50% fructose. 19 Experiment 3 tested acid and sweet whey protein in liquid and solid forms on 14 participants. After one hour, participants were served pizza ad libitum. 19
No significant difference was found in any of these studies with satiety ratings after consuming the liquid or solid preload. The Akhaven et al study found that there was also no significant difference in food intake after the liquid or solid preloads, and the researchers concluded, “macronutrient composition is more important than physical state of foods in determining subjective appetite and food intake.” 19 However, in both the Jones et al 18 and Leidy (b) et al 17 study, although solids were not found to have an affect on satiety, less of the ad libitum meal was consumed after the solid preload. The teens in the Leidy (b) at al et al study who consumed the solid breakfast ate approximately 480 kJ less compared to those that drank the shake (1418 kcal vs. 1900 kcal; P<0.05). 17 In the Jones et al study, both lean and obese participants consumed more of the challenge meal after consuming the beverage, compared to the solid preload (380 kcal +/- 171 vs. 337 kcal +/- 186; P < 0.001). 18
Ranawana et al did not find a difference in satiety with solid vs. liquid forms, but in contrast to the above three studies, the researchers did not measure food intake at an ad libitum meal following the test food. 20 This study compared the difference in satiety with 10 healthy adults, between two liquids (orange juice and a sugar-sweetened fruit drink) two solids (rice and spaghetti). 20 Water was added to each serving so that the volume of each test food was equal. 20 The only significant difference in feelings of satiety was found between the rice and the sugar-sweetened fruit drink (P = 0.05). 20 This study also looked at glycemic response, but only found a significant difference between the orange juice (45.0 mmol/L/min +/- 6.2) and the rice (96.0 mmol/L/min +/-15.9; P = 0.05). 20 Insulin response was also measured for all four test foods but no differences were found to be significant. 20
Studies showing liquids are more satiating than solids
Interestingly, the two studies that found liquids to be more satiating than solids compared solid meals to soups. 10,21 Both Clegg et al (a study on 12 healthy adults) and Zhu (b) et al (a study on 19 healthy adults) investigated the difference in satiety and appetitive hormones after a test meal of a solid food or the same nutrient components pureed as a soup. Clegg et al also tested a chunky version of the soup. 21 The Zhu (b) et al study’s test soups consisted of: chicken broth, carrots, broccoli, cauliflower, potatoes, chick peas, and butter. 16 The solid test was these ingredients presented as cooked whole pieces placed in the chicken broth, and the liquid test was a pureed version of the same ingredients. 16 The Clegg et al study tested similar items in the soup: rice, chicken, carrot, peas, onion, mushrooms, and celery. 21 The first test meal was presented as a solid (not in soup, but 400ml water was provided). 21 The second test was a chunky soup, where the contents were pureed except the rice, which was added in whole cooked form at the end to make a chunky texture, along with 250 ml of water. 21 The liquid soup was the third test, where all of the ingredients were blended together, with the addition to 50ml of water, to form a smooth soup. 21
In the Zhu (b) et al study, three hours after the soup test (which was given at breakfast), participants were given an ad libitum pasta meal. 16 Subjective satiety ratings such as: hunger, fullness, preoccupation with food, desire to eat were measured in addition to appetitive serum markers. 16 No ad libitum meal was provided in the Clegg et al study, but the researchers did test glycemic response, gastric emptying and satiety ratings similar to the Zhu (b) et al study.
In the Zhu (b) et al study, although there was no effect on actual hunger ratings, fullness ratings were higher following the liquid soup (P = 0.001). The study also showed the liquid soup significantly increasing insulin (6.55 +/- 1.15 nU/mL) vs solid soup (5.05 +/- 0.89; P > 0.05), but no significant difference on ghrelin with either test food. 16 No effect on food intake was found in the Zhu (b) et al study at the ad libitum pasta meal following the test foods (P = 0.859). 16
The Clegg et al study also showed a significant difference in fullness ratings between the smooth soup (939 +/- 297) and the solid meal (774 +/- 314; P = 0.033), with the larger values indicating greater satiety. 21 Gastric emptying half time (the time it takes for half of the meal to leave the stomach) was significantly greater for the smooth soup (96.9 min +/- 10.3) compared to the solid meal (86.3 min +/- 12.6; P = 0.022). 21 Glycemic response (GR IAUC) was significantly higher after participants consumed the smooth soup (87.0 +/- 49.5 mmol/L/min), compared to the solid meal (61.6 +/- 36.8 mmol/L/min; P = 0.040). 21 The researchers were surprised by this finding, but concluded that the smooth soup was digested faster because the particle sizes were smaller, allowing carbohydrates to be absorbed more quickly. 21 Previous research had shown that food producing a lower GR lead to increased satiety, however the researchers found that because of increased availability of nutrients in the duodenum, soups trigger a faster release of satiety hormones. 21
Studies that tested “real life” foods
Of particular interest were the studies that tested foods that would be typically eaten as a snack or meal because of their impact in “real life” settings. Most of these studies found that the whole form was more satiating and led to less caloric intake than the beverage test. The two studies that tested meal replacement bars to beverages, found bars to be more satiating. 9,12 One study found that solid raw vegetables and fruits were more satiating than the juice form. 13 Pancakes and eggs were not more satiating than a breakfast shake, however the solid breakfast did lead to less intake at the test meal. 17 Both studies on apples found that the whole form is more satiating, and led to less intake than the juice form. 6,11 In contrast to the above studies on real foods, the two studies on soups found that liquids are more satiating than the same calories consumed in solid form. 16,21 However, in this case, the soups were presented as a warm “meal”, so perhaps expected satiety was a factor in these studies.
The effect of food form on satiety is a complex topic to research. Factors such as food aversions and preferences, cognitive attitudes towards food, time of day, intake of alcohol the previous day, insulin sensitivity, level of sleep the previous night, mental attitude on test day, level of exercise, genetic predisposition, and the palatability of the test food all effect food intake and satiety ratings. The timing and amount of preload, and time of ad libitum test meal also affect satiety and food intake. Liquid foods can be more acutely satiating (<1h) when provided in high volume (>600ml). 5 Additionally, real life foods as opposed to isolated nutrients such as sugar solutions, tested outside of laboratories, may have a different satiety and ad libitum intake effect.
Although at first glance, the literature seems to point in different directions, due to limitations of the different study designs, it appears that in general, solids foods do have a stronger affect on satiety than liquids, except in the case of soup. In contrast to many of the liquid test foods, which were presented in cups as a beverage, the soups were served warm, in bowls with spoons, which may have altered the participant’s attitude towards the liquid test food, considering it not a “drink” but as a “meal”. Certainly perception is a strong factor effecting not only satiety ratings, but also appetitive serum markers, as illustrated by the Cassidey et al study. 7
Overall caloric intake however, is not as clearly associated with food form as satiety. Of the studies showing solids significantly increased satiety, four studies showed solid preloads were responsible for lower ad libitum intake, and four studies showed solids had no effect on intake at the test meal. Timing of ad libitum meal, and type of food offered varied greatly between the studies, with some studies offering an ad libitum meal after one hour, and others offering the meal after four hours. Furthermore, it’s commonly known that people will eat more if there is a wider variety of food offered, so the studies that offered an ad libitum lunch buffet may not be as accurate as the studies that offered one or two options for ad libitum intake. Some of the test meals were highly processed (puddings, sugar solutions) and could be considered hyper-palatable, which could increase food desire, and subsequently increase consumption at the test meal.
Small study size was a limitation in all of these studies. The smallest being 15 participants in the Leidy 2011 study and the largest was 104 in the Jones 2014 study, with a mean of 35. There also can be differences in satiety and food intake between participants based on sex, race, and baseline body weight 5, and not all studies provided even distributions in these areas. Another limitation was that most studies did not ask participants to refrain from drinking alcohol the night before. Alcohol consumption decreases insulin sensitivity and can increase food intake, and lower satiety the following day. Additionally, most studies did not ask participants about their activity level, which could confound the results.
Of the studies that compared the results between lean and obese participants, no significant differences were found in satiety or food intake. This conflicts with established theories that obese people are generally less satiated than lean people, and tend to consume more calories. However, some of the researchers added that their obese participants were “weight stable” and free of disease, which is not representative of the greater obese population. 6 Also, overweight and obese weight stable individuals could potentially have the same caloric intake regulation patterns as lean adults. 6
With studies this small, and so many confounding factors not being checked, it’s difficult to draw solid conclusions from the literature, however it makes sense that in general, whole foods are more satiating than beverages, and could potentially lead to less intake at the next meal. Further research should be conducted with larger pools of study participants, at least four hours between test food and an ad libitum meal focusing on food that is not hyper-palatable nor widely varied. There also needs to be stricter controls on confounding factors.
Based on the literature reviewed, solid foods have stronger effect on satiety beverages. Soups seem to be more satisfying than solid meals, although there were only two studies, so more literature and larger studies are required to make a clear conclusion. Food intake is not necessarily directly tied to food form in the short term, however, beverages lead to excess overall caloric intake. Many companies are marketing sugar-sweetened beverages and highly caloric meal replacement beverages as health foods, targeting those who are overweight. With many so Americans struggling to maintain a healthy weight, caloric beverage consumption should be replaced with water, and meal replacement beverages should not be considered good solution for those looking to lose weight.
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