First and foremost, I am grateful for Alan granting me the opportunity to write for his monthly Research Review. I also want to thank Menno Henselmans for introducing me to this topic.
Background story
Growing up with the flexible dieting movement back in 2010, I felt like being part of a group that valued science and had a no-bullshit approach to nutrition. At that same time, I was extremely confident in flexible dieting and saw little to no alternatives to how one could not use flexible dieting and still pursue weight-related goals. All diets were, in my eyes, doomed to fail, simply because everything except for flexible dieting appeared as a restricted way to go about food. How could the majority of diets simply ignore the Laws of Thermodynamics? Restricting foods, what kind of sorcery is that in our current environment? If adherence is key, why prohibit things? I was even going as far as saying flexible dieting wasn’t really dieting – it was a lifestyle.
Not long after practicing what I described as a number game, I developed this fascinating internal calculator in my brain that allowed me to master moderation. After all - if weight-related goals were all about getting the numbers right from a basic thermodynamic standpoint, what’s the point of dieting without knowing if the sum is either negative or positive? As much as I was making fun of people practicing clean eating or any diet that restricted food groups, looking back at it now I realized the truth lied somewhere in-between.
It wasn’t long ago that things started to change. I noticed a trend among the flexible dieting crowd, where both layman and experts were becoming more and more nuanced in their approach and view of flexible dieting. Where in the early days, flexible dieting was all about bragging how big of a pizza you could fit in your diet and still lose weight, more and more people started talking about the importance of plenty of fruits and vegetables, getting in sufficient fiber and micronutrients, as well as advocating for more than the recommended amounts of protein for other purposes than say muscle gain, such as for satiating purposes.
So where does this bring us now? Should we stick with the number games, eating more fruits and vegetables and start eating for other purposes than solely fitting your calories, or is there a possibility we could leave out the numbers and rely on more than just a few things outside of flexible dieting? Let’s name the approach the hunger games.
The hunger games
Have you ever woken up not feeling hungry at all, eating your meal whenever you felt like eating? Postponed a meal because you were still satiated from the previous one? What about food choices? Have you ever chosen one food over another for the sake of it being more satiating? Stopped eating for 5 days? Gained 20lbs after dropping My Fitness Pal? For the latter two statements, probably not. Congratulations, you’ve been ad libitum dieting all along.
Ad libitum (dieting) is defined as:
‘’(eating) at one’s own pleasure, or as much as one desires, or to the full extent of one's wishes’’.
In nutritional research in non-human animals, ad libitum refers to providing non-human animals with unlimited access to food (or feed) and water, allowing the organism to self-regulate their intake to meet their biological need[1]. A common example of ad libitum dieting in research is chow administered as feed to rodents. The advantage of using chow as feed for rodents as a control diet is that the ratio of nutrients have the tendency to induce neither weight gain nor weight loss[2],which is useful for minimizing variables and manipulating diet composition. This is a perfect example of an organism eating to meet their biological need.
In humans, the approach has relatively the same meaning in the context of dieting. Humans naturally eat in episodes (i.e. snacks and meals) until they’re comfortably full (satiation), after which they do not eat for an extended amount of time (satiety)[3]. After that, an ever-increasing intrinsic drive and motivation guides their eating behavior (hunger/appetite), which determines their next eating episode[]4.
Everything above closely resembles the same that takes place in non-human animals, consisting mostly predictable patterns. However, humans come with far more unpredictable patterns being more subject and responsive to internal and external factors. As one might point out correctly, the above scenario resembles textbook science and takes out the human part of the equation.
For example: humans don’t only eat to satisfy their appetite, but also for a variety of other factors, such as for sensory stimulation and hedonics[5-6] or as a coping strategy reducing tension or seeking relief from extreme boredom [7-8].
Another example of where humans differ significantly from non-human animals is the Hawthorne-effect 9. The effect describes how humans alter their behavior, sometimes because they’re subject to an intervention, other times because they’re prone to feeling the need to show desired (social) behavior. This boils down to humans not acting like they would normally do in their free-living environment, which makes ad libitum dieting in the context of observation harder to reality[10].
The effect extends as far as participants eating less or more food if they’re aware of being observed in a lab, knowing that they’re portions are measured[11]. Another example is participants that are being asked to fill in a questionnaire, trying to gratify the researchers by giving socially desirable answers[12-13[.
An example that is known to perhaps some readers are the effects of participants reducing their energy intake if they engage in solely reporting their energy intake [14-15]. Without setting up a dietary protocol for weight loss, participants engaged in completing food diaries have heightened awareness of their own intake, as for having to justify their food intake. This leads to participants eating less, resulting in weight-loss, even without a protocol focused on weight loss[16-17].
Taken together, it appears that the vast differences between humans and non-human animals are problematic in the context of ad libitum dieting. Is there even such a thing as ad libitum dieting in the context of humans, since humans tend not to act ‘naturally’ to begin with? How would this type of dieting look in our current modern environment? Could a human in our current environment, despite all differences, still be able to meet their biological need, despite the rise of obesity?
Now that we’ve got a theoretical basis that describes the fundamentals of ad libitum dieting and how it differs between humans and non-human animals, it’s time to explore how we should view this approach to dieting in the 21st century.
Ad libitum dieting in the 21st century
Substantial effort has gone into research investigating the effects of diets varying in macronutrient composition, and the accompanied effects that those macronutrients exert on the control of appetite and energy intake[18]. As a result, body weight can be gained, lost or maintained in diets varying in macronutrient composition[19]. But if we strip a diet from all its common components, what are the sole concepts we’re left with?
Looking at dieting stripped from its sole core, there are two major approaches to setting up a diet. Simply put, one is counting calories, while the other’s not. One can theoretically eat anything on an energy-restricted diet, including going low-carb or low-fat, but one cannot eat more than the select number of calories given. This is solely what defines the caloric approach to dieting.
The other approach is ad libitum dieting, restricting either carbohydrate or fat intake and/or eliminating certain food groups, foods or nutrients, allowing ad libitum intake of what’s available after restriction. This excludes any Calorie counting. Popular examples of diets using this approach are Atkins (LC: low carb ad libitum), South-Beach (LC/HP: high-protein ad libitum) and Zone (LC ad libitum), which all have shown long-term weight loss success, as for having varying differences in overall diet quality[20-21].
In terms of weight goals and adherence, most people would view the manipulation of calories to be the best approach. After all, the theoretical basis for this argument is very straight forward since body weight is a function of energy intake. How could going low carb or low fat and/or restricting food groups, foods or nutrients be a feasonable option in the long run if you would ignore numbers? Allowing ad libitum intake of the remainder would surely make people overeat by the thousands, gulping down olive oil and eating down pounds of steak if they’re allowed to eat those without restrictions, right? But if one were to ask what fits best, one would mostly give the answer that dieting is highly individual. This is in accordance with recent research showing larger inter-individual differences in total weight lost in ad libitum vs caloric restriction trials[22]. This simply means that there is a large portion of people that is either slightly more or less receptive to ad libitum dieting.
A recent study by Otten et al (2016)[23] made participants adhere to a Paleo diet for 12 weeks in ad libitum fashion (e.g. unrestricted consumption of lean meats, fish, fruits and vegetables, nuts) excluding product groups such as cereals, legumes, dairy and refined fats and sugars. Even the group that completed the study without an exercise regimen only lost 15% less fat mass compared to the combined intervention (5.7kg vs 6.7kg). Another example is another 12-week ad libitum diet, this time performed by Jönsson et al (2010)[24] comparing an ad libitum Paleolithic vs an ad libitum Mediterranean-like diet and found that despite the larger energy deficit in the Paleolithic-group (1385kcal vs 1815kcal in the Mediterranean-like diet), the Paleolithic diet was more satiating per Calorie than a Mediterranean-like diet[25].
Although the examples only sufficed to show that ad libitum diets are able to reduce energy intake by picking a random ‘fad’ diet, and can be more satiating on a Calorie by Calorie basis, there is actually a lot of logic to why ad libitum dieting works. This isn’t to say that all ‘fad’ diets were right after all, but that they’ve often failed to correctly address the underlying physiological reasons why forcing the elimination of a particular food group(s), food(s) or nutrient(s) is so successful.
A portion of the readers is probably aware that protein is a satiating nutrient, and thus limiting either carbohydrate or fat inevitably leads to increased protein intake[26]. This is a clear example of a particular nutrient exerting a change in overall satiety, which indirectly lowers energy intake[26].
What defines the ad libitum approach to dieting to be so successful, ‘is’ actually the underlying physiological mechanism by which dietary variables are manipulated in such a way that satiation and satiety are maximized, and feelings of appetite minimized. What is mistakenly seen as a ‘fad’ diet or approach is actually a reasonable approach to dieting for some people, despite the fact not being focused on calories per se.
Having said all this, is there a possibility that we could shift our attention from a calorie-counting dominant approach to a combined approach, focusing on the underlying physiological processes responsible for satiation and satiety with a little help from our internal calculator in emergency cases? Should we or people that we help with dieting focus on mastering Calorie counting first, after which ad libitum dieting might become a more optional approach in a later stage? To answer these questions we first need a theoretical background to work from.
The psychoneurobiology of satiety
I bet you’d spit out your coffee after reading that term. In reality, it only accounts for the big players that come into play when talking appetite. I doubt using socialpsychoenvironmentaleconeuroendocrinobiology to describe any field intertwined with appetite is going to prove useful. To get a grasp of understanding the science of appetite and how it relates to ad libitum dieting, we first need to understand how appetite is measured.
Objective and subjective appetite
Appetite can be measured directly in two ways:
Via subjective (appetite) ratings
Via actual food (caloric) intake
Humans have the unique capacity to rate their appetite via introspection, and serves as a function of subjective appetite. Subjective appetite is often determined by questioning the participants by rating the food or beverage prior, immediately after and in intervals of 15 minutes after the consumption for up to 240 minutes on a scale from 1-100mm. This scale is called the Visual Analogue Scale (VAS). It allows us to ask how participants feel and view food based on their hunger, fullness and their ability to review and then anticipate their timing and/or size of their next meal based on the meal they ate previously, during and/or after making the rating. These methods have shown to be reproducible and can act as a predictor of energy intake[27-28].
After the initial rating period varying from 15 up to 240 minutes, another meal is often consumed to assess objective food intake. This is done by providing the participants with a range of foods or meals that they’re allowed to consume ad libitum. This serves as a proxy for possible compensation for energy intake later that day, and is often continued with more interval ratings via the VAS-scale.
Combined, both subjective and actual food intake serve as a proxy for the overall satiating capabilities of meals, foods or nutrients, as well the effects on energy intake. Because participants eat for a variety of other reasons, these scales and intake measurements are preferably used in within-subject design, which means participants often participate in more than a single experimental condition to make sure participant 1 rates both A, B and C, so that inter-individual differences can also be taken into account.
There are however, multiple things to note with assessing both subjective and objective satiety, especially if we go by the premise that appetite is only slightly coupled with metabolic regulation[29]. Besides being influenced by the earlier mentioned differences which make people stand out from other non-human animals, there are more problems with introspection (e.g. the use of words to describe appetite).
For example: when we feel hungry, what are we actually telling ourselves or others? The Dutch tend to say ‘Ik heb honger’ (I’m hungry), but they’re told as a counter argument by older Dutch people ‘No you’re not, that’s what people in a famine feel’. The English in this context would say ‘I’m hungry’ or ‘I’m starving’. Although the latter is an extreme example, they all have something in common. The older Dutch people were right after all, where ‘hunger’ and ‘starvation’ refer to compensation that arises from homeostatic mechanisms responsible for energy regulation. But what everyone on a day to day tends to experience is not the underlying homeostatic force (we’ll get to that in the next part), but actually the absence of fullness in the gut and the lack of reward in the brain from acute food events[30].
There aren’t massive differences between hunger and starvation, but the way in which people differ between the two meanings implies there is. We should abandon those terms altogether and describe our tendency to eat on a day-to-day basis as ‘appetite’, rather than ‘hunger’. It helps us understand that what we feel is not starvation, but the absence of fullness and reward – which is very relevant to understand if you’re anticipating future food events and have to cope with increased appetite, perhaps in the case of an unforeseen fast, or due to a lack of satiating foods that you’ve eaten.
Furthermore, extremes in the VAS-ratings are often avoided by participants, as due to the inconsistencies between categories[31]. This would mean that if I were to ask you on a 1-10cm scale how much hungry you were, you would evade both 1 and 10 – where 1 would stand for total starvation and 10 for a total buffet massacre.
Now that we’ve got an idea how subjective satiety looks in the context of rating scales, and objective food as a function of energy intake, we’re applying this knowledge to a practical model of short- and long term energy homeostasis.
Short- and long term energy homeostasis
The observation that bodyweight is a function of energy intake is a generally accepted idea supported by the scientific literature, but the assumption that bodyweight can remain very similar from one year to the next is not evidence that bodyweight is actively regulated, because this consistency is to be expected from a person’s lifestyle, environment and physiology[32-33]. Introduce a variable such as going to school[34], migrating to another location[35] or something like getting married[36], and you’re more prone to weight change.
This suggests that appetite is, after most things being equal, governed by short-term signals. In other words: the major contributor to appetite is not the overall energy balance, but rather determined by past food events. Although a major portion of obesity research focuses on eating behavior, obesity is rather the result of defective regulations in the energy homeostasis and bodyweight, instead of primarily weak controls in eating behavior.
To put this topic in perspective, we’re going over an analogy postulated by Rogers and Brunstrom (2016)[37] consisting of a bathtub and a saucepan both filled with water, where the bathtub represents the body’s energy stores and the saucepan represents the gut.
The thick arrow reflects the acute suppressing effect of the saucepan (gut) that provides instant short-term fullness and dietary induced satisfaction (short-term appetite), which can be viewed as a single lunch consumed, digested and absorbed - where the thinner arrow reflects the long-term inhibition of appetite based on the body’s energy stores (bathtub).
Both the saucepan and the bathtub resist significant overfilling as for drainage, representing the natural positive and negative feedback that occurs when for example under- or overeating.
To further build on the analogy: the average 65kg lean male has about a 55 day’s supply worth of fat in his system, with a corresponding energy expenditure of around 2400kcal[38]. The bathtub in the same context is 180 times the size of the saucepan, where the saucepan would represent a single medium-sized lunch (800kcals).
Since a whole lunch would only contribute to 1:180th of the bathtub, excluding the daily energy requirements of a lean adult male which can offset the ability of the saucepan to add anything at all to the bathtub, one can image that this adds little to nothing to the bathtub itself.
This analogy highlights why appetite is primarily governed by recent eating events, and why ad libitum dieting is especially useful since it pre-dominantly focuses on short-term appetite. It also explains why diets very low in calories can still be more satiating on a Calorie by Calorie basis.
This generally tends to explain why when people who suffer from impaired signals (e.g. low insulin sensitivity and/or high insulin resistance), tend to respond very good to diets high in volume and high in protein[39]. You’ve just created counter-signals to the ones that are impaired, that were responsible for the whole offset of homeostasis in the first place.
Tons of examples of long-term weight loss trials tend to show that ultimately, there is a point of plateaued weight-loss, often occurring at the 6-8 month time mark. A depressing example of this low-acting force has recently been demonstrated by Polidon et al., (2016), where participants were treated with canagliflozin for 52 weeks[40]. Canagliflozin has the ability to inhibit the sodium glucose co-transporter, which leads to urinary energy excretion without the awareness of the participants. Despite the unawareness of the participants losing weight, for every kilogram of fat mass they lost, compensated an average of ~100kcal a day in energy, 3-fold higher than the adaptations that normally occur with weight-loss.
Depressing as the truth is, keeping the weight off is another way of dieting on itself. Weight gain is the rule rather than the exception. In this context, tracking your calories is worth the bang for your buck – but ad libitum dieting does a great job at minimizing the chances of both under- and overeating.
The technique isn’t meant to reach low competition levels of bodyfat, nor is it appropriate to gain a lot of weight since the dietary discomfort would be staggering. Ad libitum dieting is the mastery of appetite, learning from your own satiety signals and knowing how to influence them.
Now that we understand the relevance of short- and long term homeostatic mechanisms, we can start how to ad libitum dieting, or more specially – how to manipulate dietary variables in such a way that satiation is maximized and feelings of appetite are minimized – into action.
Ad libitum strategies: trusting our signals
Everyone can benefit from ad libitum dieting strategies, even those practicing solely Calorie counting. In this chapter, we’ll go over strategies that have shown promise in maximizing satiation and satiety, while reducing feelings of appetite.
Please note that not a lot of these techniques have been investigated in the long run. Ultimately, we’re left with a primitive brain, and the chances that the low-driving forces of our energy homeostasis will get the overhand, or the ability of the environment to affect us to a greater degree, is present. Furthermore, both overweight and obese people would respond far more to these techniques than solely lean people, because they suffer from impaired homeostatic signals to begin with. However, this doesn’t imply that these strategies can’t provide meaning to your existing approaches to dieting or those of your clients or patients.
We’re going to work our way down the digestive system and focus mostly on the early pre-absorptive phase[41]. There appears to be accumulating evidence to suggest that small nudges (i.e. targeted interventions or cues) can have a modest but cumulative effects on food intake and body weight[42]. The following strategies will stem from the same principle, where focusing on as much of these strategies can function as a foundation for weight-loss and maintenance.
Eating rate, bite-size and chewing speed
Both eating rate, bite size and the amount of chews consuming a food or meal play a role in the control of energy intake due to increased oreo sensory time[43].
There are multiple studies assessing eating rate, but few take into consideration the earlier points we discussed how humans have the tendency to behave differently because awareness. The study by Petty and colleagues (2007)44 is one of the few authors that attempted to both look and investigate laboratory and free-living conditions in self-reported eating rate as either slow, medium or fast eaters in 60 male and female students. The major finding was that there were no large discrepancies between self-reported eating rate and what was observed in the lab, which is surprising - reflecting on other findings how participants normally under-report data. Another interesting finding from this study was that men tended to eat significantly faster than women (80.6±30.7kcals/min in men vs 52.0±21.6kcals/min in women).
A study by Andrade et al (2008)[45] altered bite size, pauses between chewing and eating rate in 30 healthy young women who all described themselves to be either a slow, medium or fast eaters. All participants ate a test meal containing 870 kcal of ditalini pasta with diced tomatoes with Italian seasoning, celery, and minced garlic sauteed in olive oil, and Parmesan and Romano cheeses.
Although neither groups ultimately fully ate the test meal, the QUICK-group ate significantly more calories (645.7±155.9 vs 579±154.7), while being less satiated than the SLOW-group, which ate approximately 21 minutes. Although the SLOW-group drank more water and thus consumed an overall bigger volume, I hope it now comes to you as no surprise that per Calorie of weight the QUICK-group showed lower satiety ratings.
Another study from the same lab by Andrade et al (2012)[46] assessed the self-reported eating rate as for altering both eating rate and chewing speed, controlling for various factors that are known to influence appetite, satiation and satiety such as social eating in group setting, the menstrual cycle and the ad libitum water consumption during the experiment. This study only investigated 1 participant at a time for a total of 30 healthy women. They found that although having smaller bites, pausing between them and chewing thoroughly was associated with higher satiety ratings, total caloric intake wasn’t affected. The study suggested that inter-meal consumption might be more important than we think, besides eating rate, bite size and chewing speed.
Taken together, men tend to eat faster than women. Men also tend to be more affected by eating slowly than women, suggesting that sex-specific advise should be considered as well as a strategy[47]. Women tend to eat slower, but effects on energy intake remain to be discussed. Furthermore. inter-meal water consumption should be viewed as a possible strategy to increase inter-meal and post-meal feelings of fullness. More research further supports that slower eating only reduces energy intake in normal-weight, but not overweight/obese participants, which supports the earlier concept mentioned that short-term appetite is impaired in overweight/obese participants, while normal healthy (lean) participants appear to be more reactive to short-term signals[48].
Now that we’ve finished discussing the first step in the pre-absorptive phase, we’re ready to move down the digestive system.
A volume-dose response of food
Just like hypertrophy (hey there Brad Schoenfeld), volume is one of the most important foundational aspects to consider as a strategy to impact short-term satiation and satiety, and is one of the sole concept that defines ad libitum dieting to be successful (oh the puns).
To start off with a simple argument: if something takes up space, there is less room for other things.
Second, a mechanistical explanation, gastric stretch receptors respond to the volume of food/beverages rather than its energy content in the initial period after consumption, communicating with the vagal nerve[49]. Simply blowing up a synthetic balloon into someone’s stomach decreases hunger and increases satiety, even in a totally fasted state[50]. This effect even extends as remarkable to show no differences between lean and obese participants, where infusing different volumes independent of energy content[51] or blowing up a synthetic balloon, filling it with anywhere from 400-800ml of water has the ability to significantly reduce food intake[52].
Another feature of the stomach has been highlighted by a recent discovery that showed ‘taste’ not only to occur in the mouth, but that our gut has the capacity (ha) of tasting our food via mechanosensors and chemoreceptors capable of sensing the volume and nutrient content of consumed food[x], which communicates via the same vagal sensory nerve the gastric stretch receptors communicate with.
Recent research by Camps et al (2016)[53] altered both the viscosity (thin and thick) and caloric content (100 and 500) of shakes and simultaneously assessed the sensory characteristics.
They found that increasing viscosity is less effective than increasing the energy density in slowing gastric emptying, but that altering viscosity was able to significantly influence satiation and satiety via mouth feel and oral exposure, independent of caloric content. This meant that a 100-kcal thick shake was better at promoting satiation and satiety than a 500-kcal thin shake, termed as the phantom fullness effect. A limitation from this study was that the participants lied in a suspense position for the MRI. We know from earlier research that sitting upright also promotes satiety[54], where the position might have complimented the ability of the volumes. Furthermore, other research contrasted the findings where soups were able to induce more satiety compared to the same food products taken separately in either chunky or solid form, which was attributed to the delayed gastric emptying[55].
Summarized, volume and viscosity are able to significantly impact short-term satiation and satiety. Combined with oral stimulation, both strategies are key foundations for setting up a successful (ad libitum) diet.
Conclusion
At any given moment in non-dieting conditions, appetite is primary governed by past meal events, while the urge to compensate driven by the low-acting forces of energy homeostasis that make us overeat when losing weight and overeat when gaining weight will slightly gain the overhand, although the latter is less prohibited because our natural tendency to carry around more bodyfat due to our primitive brain.
Despite this depressing facts, this means that in any given dieting context, there should be a large focus on dieting strategies (nudges) to have a cumulative effect on energy status and weight control. Since appetite resolves around countless pathways, indirectly lowering caloric intake by practicing an ad libitum approach should be considered an option. Especially in overweight and obese, the ability to offset imbalances by maximizing satiation and satiety while minimizing feelings of appetite provides countless opportunities in weight management and energy maintenance, especially in those that have a tough time tracking their calories.
Since not all people are able to (accurately) track their calories, macronutriënts, and are far better able to adhere to an ad libitum approach to dieting, ad libitum dieting is a scientifically plausible approach to a successful diet.
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