How the Ketogenic Diet Affects Hunger (Research Review)

Authors
Adam Tzur (Sci-Fit on FB),
Brandon Roberts (FBResearchgateThe Strength Guys)
Richard Nijholt (FB)

Reviewed by
Michael Hull (FB, Researchgate, Examine.com)
Brad Dieter (FBResearchgateScience Driven Nutrition)
Vegard Lysne (FBResearchgate)
Grant Tinsley (FBResearchgatewebsite)

Alan Aragon (FBResearchgateResearch Review)
Zad Chow (FBExamine.com)

Contributions by
David Nolan (FB, Synapse Performance)

Length: ~4000 words
Published: 14.03.2018 (updated: 20.03.2018)


Plain Language Summary


Disclaimer: the following information is not medical advice, please consult your doctor before making drastic lifestyle changes. This is very important if you have diseases that can be modified by ketosis, or if you’re on medication: “patients on diabetes medication who use [a ketogenic diet] should be under close medical supervision” - Yancy et al., 2005

By reading this article, you will:

  1. Learn how the ketogenic diet affects your hunger
  2. Find out whether keto is better than other diets for appetite suppression
  3. Get insight into how protein, ketones, and other factors affect hunger

What we did, and why

Hunger is very important because it affects how much you eat. People typically feel hungrier after going on a diet. This is because the body wants to regain lost weight. If you could control hunger, then maybe weight loss could be easier and you might prevent weight regain.

We have combed through the ketogenic literature. Our goal was to find studies with data on appetite, hunger, satiety, and cravings.

After analyzing the data, we answered the following questions: "Will I be hungry on keto?" and "Do I need to count calories?", and "Is the ketogenic diet superior compared to other diets for controlling hunger?"

People eat fewer calories on keto

When following a ketogenic diet, people spontaneously eat fewer calories.

“Spontaneous - happening or done in a natural, often sudden way, without any planning or without being forced”

- Cambridge Dictionary

In ketogenic studies, subjects only had to restrict carbohydrates, and did not need to count calories. On average, they ate about 600 kcal less than their regular diet:

Spontaneous calorie reduction on the ketogenic diet

Figure by Sci-Fit.

People feel less hungry on keto

When you go on a weight loss diet, you typically feel hungrier. Surprisingly, the ketogenic diet does not seem to follow this trend. In most studies, people felt less hungry on keto.

Is the ketogenic diet better than other diets for hunger control?

With the data currently available, the ketogenic diet seems superior for reducing hunger.

Less hungry on the ketogenic diet graph

Figure by Sci-Fit.

5 studies found that keto was superior to an alternative diet. 3 studies found no difference between groups. One study (Soenen 2012 in the graph) suggested that a low carb diet (80 g) might be superior to keto. We discuss it in greater detail here.

Why is keto effective at reducing hunger and calories?

We could not identify a single reason, so here is a list of several:

  1. Protein is the most satiating macronutrient. People on the ketogenic diet typically eat more protein than they normally do.
  2. Ketones could reduce hunger. However, the data on this is still in its infancy.
  3. Liquid calories are calorie dense and not very satiating. You can hardly drink sodas and juices on keto, so you get to avoid liquid calories. The food you eat will be more satiating and filling.
  4. The diet is simple. Several research teams speculate that the simplicity of the diet helps reduce hunger and caloric intake.
  5. Very low energy diets can reduce hunger and increase satiety. They are typically less than 800 kcal per day, and are almost always ketogenic.
  6. You do not need to count calories. It might be easier to eat until you’re full instead of having to think about calories. Most of the ketogenic studies were ad libitum, meaning that people ate as much as they wanted. Obviously, they had to restrict foods rich in carbs to be in ketosis.

“Although the hunger-reducing effect of [the ketogenic diet] is well-documented, its main mechanisms of action are still elusive”.

- Paoli et al., 2015

Note: fats and carbs also affect hunger, but we did not analyze this literature in-depth because it is too vast and warrants a separate article.

Satiety hormones

We have briefly discussed satiety hormones. The hormone literature is much more complicated, and the results are not easy to interpret. No doubt, insulin, leptin, ghrelin, GLP-1, CCK, and PYY influence hunger. We might cover these in detail in a future article.

Does this imply that you should go keto?

We will answer this question in the final part of our series!

Next up in the keto series: how well people stick to the ketogenic diet over time (diet adherence).

Sign up for the newsletter and get notified when the article is out.

Do you want to learn more about how the ketogenic diet affects appetite and hunger?

Continue reading!

The Ketogenic Diet Series

Part 4

Is it Easy to Adhere to the Ketogenic Diet?

Part 5

Side Effects of the Ketogenic Diet

Part 6

Do Ketone Supplements Improve Athletic Performance?

Summary
Overview of Ketogenic Diet Science

Terminology

Here is a list of the definitions we use in the article:

  • Satiety: how satiated you feel after a meal
  • Appetite: your desire to eat
  • Hunger: your need to eat
  • Craving: an intense desire to consume a specific food
  • Fullness: the feeling of being full || having eaten enough
  • Ad libitum: eating as much as you want until you are full

Does the ketogenic diet suppress appetite?

People spontaneously eat fewer calories on keto

People tend to eat fewer calories while on the ketogenic diet (Gibson et al., 2015; Aragon et al., 2017). This happens spontaneously, because they aren’t asked to restrict calories. Hence, this diet could fit you well if you dislike calorie counting.

“The efficacy of [Ketogenic Diets] is (...) often attributed to a suppression of appetite resulting in spontaneous decrease in energy intake”.

- Gibson et al., 2015

In the graph below you can see how many calories subjects consumed. Across 11 studies, subjects ate, on average, 2,331 ± 473 kcal at baseline, and 1,694 ± 390 kcal on the ketogenic diet.

Remember, these are ad-libitum studies! The subjects were not asked to restrict calories (only carbs).

Ketogenic diet ad lib caloric intake 11 studies

Figure by Sci-Fit. We pooled the average caloric intake of the ketogenic studies in our analysis, and calculated the standard deviations. Data is extracted from 11 ad libitum ketogenic studies.

That is the good news.

The bad news is that people eat more calories and carbs after a while on the diet. In other words, adherence to the diet drops. This happens across all types of diets and isn’t unique to keto. We will cover adherence in the next part of our keto series!

Does keto make you eat fewer calories than other diets?

Sadly, most studies compare carb restricted keto groups to calorie restricted control groups. We can’t compare them directly.

Still, we have some relevant studies:

Study

Caloric intake

Time of measurement

Calorie restriction?

Johnstone et al., 2008

Crossover

Keto

~1732 kcal

Moderate carb diet

~1899 kcal

Week 4

No

McAuley et al., 2005

RCT

Atkins (keto)

−~500 kcal

Zone

−~500 kcal

Week 8

No

Dansinger et al., 2005

RCT

Atkins

−193 kcal

Ornish diet (low fat vegetarian)

−554 kcal

Week 4

No

Gardner et al., 2007

RCT

Atkins

−507 kcal

Ornish diet

−442 kcal

Week 8

No

 

Non-ketogenic studies:

Study

Caloric intake

Time of measurement

Calorie restriction?

Gardner et al., 2018

Low fat

−633 kcal

Low carb

−642 kcal

Week 12

No

Weigle et al., 2005

High protein

−441 kcal

Week 12

No

Overall, the data on this specific question isn’t settled yet. We might do a full analysis in the future.

“ketogenic diets have shown appetite-suppressing potential exemplified by spontaneous caloric intake reductions in subjects on ketogenic diets without purposeful caloric restriction” - Aragon et al., 2017

“The efficacy of [Ketogenic Diets] is (...) often attributed to a suppression of appetite resulting in spontaneous decrease in energy intake”. - Gibson et al., 2015

“ad libitum intake of low-carbohydrate food that is high in fat and protein produces a weight loss that is more rapid and pronounced than with a conventional low-fat diet. This weight loss is most dramatic after 3 and 6 months of low-carbohydrate diet, there being no difference after 12 months between a conventional low-fat diet and the low-carbohydrate diet. The main reason for weight loss is a suppressed appetite, both through the high-protein content of the diet and the ketogenic nature of the diet.” - Erlanson-Albertsson and Mei, 2005

“Volunteers consumed significantly (P = 0.020) more energy (0.7 MJ/d) when following the MC nonketogenic diet than when following the LC ketogenic diet (Table 2).” - Johnstone et al., 2008

People feel less hungry on keto

Hunger is often increased when dieting (MacLean et al., 2011; Gibson et al., 2015). Yet, that may not be the case for the ketogenic diet (Gibson et al., 2015). In almost all of the studies we analyzed, the ketogenic diet lead to a decrease in hunger.

This is quite unexpected, and very promising.

In a few studies (seen below), the scientists reported how hunger changed over time. For example, in Nymo et al, hunger increased at day 3 but slowly dropped thereafter. Yet, subjects in the other three studies dropped down and stayed at low hunger levels.

Hunger over time ketogenic diet, tinified

Figure by Sci-Fit. Each bar represents a point at which hunger was measured. All four studies used the Visual Analogue Scale for rating hunger. Read more about how scientists measure hunger (internal link).

A high protein diet makes you satiated

A large body of evidence demonstrates that protein is the most satiating macronutrient.

Why does this matter? Because the ketogenic diet is typically moderate or high in protein. This could be one explanation for why people eat fewer calories and feel less hungry on keto.

The average american diet is typically low to moderate in protein. It is difficult to quantify exactly what “low protein” means, because it is somewhat arbitrary. With that said, anything below 1 g protein per kg bodyweight can be considered low. Especially if your goal is to maintain or increase muscle mass.

Yet, it’s hard to say whether it’s ketosis or the high protein that causes people to eat fewer calories. For example, in Weigle et al., 2005 people ate −441 kcal per day when put on a high protein, normal carb diet. In McAuley et al., 2005, both the Zone group and the Atkins (keto) group ate ~−500 kcal after 8 weeks. This might be because both groups were high in protein. So in studies where the keto group ate more protein than the control diet, we would expect greater satiety for keto.

Exceptions to the rule:

“[the ketogenic diet] has demonstrated hunger-suppressive effects independent of protein content.”

- Aragon et al., 2017

  • Protein intake wasn’t “high” in Nickols-Richardson et al., 2005 (94 g or 1.2 g/kg for keto), or in Foster et al., 1992 (70–90 g or ~0.7–0.9 g/kg). They still had a large decrease in hunger.
  • In Saslow et al., 2014, the control group ate ~68 g or 0.7 g/kg protein and had the same reduction in hunger as the keto group (~106 g or 1.12 g/kg protein).
  • Protein was moderate in Johnstone et al., 2008 (123g or 1.2 g/kg for keto vs 135 g or 1.3 g/kg for control). If protein is the main reason for appetite suppression, why didn’t the control group reduce their hunger to the same extent as keto?
  • In studies where subjects eat Very Low Energy Diets (VLEDs), keto groups increase satiety and decrease hunger on 500-800 kcal and 50-60 g protein per day (i.e. Kovacs et al., 2004). If protein is so important, how come people feel so satiated on these types of diets (Gibson et al., 2015)?

These examples suggest that many factors affect satiety, beyond protein. We’ve covered some of the factors further down. [link to section].

Read more: a high protein diet does not prevent ketosis.

“[the ketogenic diet] has demonstrated hunger-suppressive effects independent of protein content.” - Aragon et al., 2017

“An increase in dietary protein from 15% to 30% of energy at a constant carbohydrate intake produces a sustained decrease in ad libitum caloric intake that may be mediated by increased central nervous system leptin sensitivity and results in significant weight loss. This anorexic effect of protein may contribute to the weight loss produced by low-carbohydrate diets.” - Weigle et al., 2005

“We speculate that it is the protein, and not carbohydrate, content that is important in promoting short-term weight loss and that this effect is likely due to increased satiety caused by increased dietary protein. It has been suggested that the increased satiety might help persons to be more compliant with a hypocaloric diet and achieve greater weight loss.“ - Schoeller and Buchholz, 2005

“Weekly ratings of perceived hunger did not differ by diet group during the trial, which suggests, as discussed by others (31, 32), that it is the protein content of the diet and not the severity of dietary carbohydrate restriction that affects perceived hunger.” - Johnston et al., 2006

“Subjects in this group (i.e. keto) may have experienced greater satiety on a diet with liberal proportions of protein and fat.“ - Samaha et al., 2003

“This anorexic effect of protein may contribute to the weight loss produced by low-carbohydrate diets.” - Weigle et al., 2005

“...when reducing carbohydrate, a diet that reflects both a higher protein and a higher fat intake may be more effective in increasing satiety and resulting in a spontaneous calorie reduction than simply a high protein diet.” - Chatterton, 2015

“This study showed that individuals are significantly less hungry and exhibit significantly greater fullness/satiety while adhering to VLED compared with when they are in energy balance at baseline” - Gibson et al., 2015

“increased protein intake cannot explain why appetite is suppressed during VLEDs, which, while relatively high in protein, provide absolute protein intakes of only ∼ 50–60 g day –1 (which for obese persons is lower than the suggested normal intake of 0.8–1.2 g kg − 1 body weight per day [39]).

Additionally, increased protein intake cannot explain the observation of an ‘absence of hunger’ during starvation or fasting regimes (35,61) – an observation that historically led to the initial development VLEDs to mimic this benefit (15). Further, well-controlled studies have shown that when protein intake is matched, a ketogenic high-protein diet suppresses appetite more so than a non- ketogenic high-protein diet in obese (20) and in lean subjects (62), highlighting ketosis as a plausible factor common to both VLEDs and KLCDs that could suppress appetite.

Besides protein, another factor that could conceivably explain the effect of ketogenic diets on appetite is dietary fat (63). However, the contrasting low levels of dietary fat intake during VLEDs with the relatively higher levels consumed during KLCDs suggest that dietary fat intake is not a common factor explaining the appetite-suppressive effect of both diets.” - Gibson et al., 2015

Ketone bodies might suppress your appetite

Several studies mention that ketone bodies could suppress appetite (Jabbek et al., 2010; Veldhorst et al., 2012; Pesta and Samuel, 2014; Gibson et al., 2015; Karl et al., 2016). It is possible that ketone bodies accomplish this by altering hunger hormones (Sumithran et al., 2013; Stubbs et al, 2017). For example, Sumithran et al., 2013 reported correlations between hunger hormones and ketones.

Although, this doesn’t explain why subjects on moderate and high carb diets feel less hungry. This suggests that ketones might play a role in appetite suppression, but they’re not the sole reason. Researchers generally agree that this is a hypothesis (Brehm et al. 2005; Gibson et al., 2015).

There are some interesting studies out there, such as Stubbs et al., 2017. The subjects were given a ketone drink which reduced their hunger and desire to eat over 4 hours. Yet, we need to see more long-term data on this topic.

“The increased concentration of beta-hydroxybutyrate may act as an appetite suppressing substrate [62].” - Pesta and Samuel, 2014

Correlating carbs and hunger

We graphed hunger and carb intake to see if they correlate. In our analysis we excluded studies with more than 80 g of carbohydrates. We found a moderate linear correlation (Pearson’s R = 0.54) between hunger change and carb intake. If we assume that there is a connection between ketosis and hunger, then carbs might influence hunger on a ketogenic diet.

Ketogenic diet carbs hunger correlation

Figure by Sci-Fit. Note that we should interpret the correlation carefully. By itself, it does not suggest that fewer carbs = less hunger.

“Some have speculated that [dietary] self-restriction is a result of the effect of circulating ketones on appetite or other satiating effects of low-carbohydrate diets, but this remains unproven.” - Brehm et al. 2005

“A decreased feeling of hunger seems to be an advantage with the low carbohydrate diet [49-51], an effect attributed to increased levels of ketone bodies [50], reduced levels of neuropeptide Y and leptin levels and decreased insulin levels [1].” - Jabbek et al., 2010

“An increased concentration of β-hydroxybutyrate may have contributed to appetite suppression on the high-protein diet.” - Veldhorst et al., 2012

“The increased concentration of beta-hydroxybutyrate may act as an appetite suppressing substrate [62].” - Pesta and Samuel, 2014

“The supposed appetite suppression seen with both VLEDs and KLCDs is often hypothesized to be due to ketosis (15,34–39). Ketosis is a condition where the production of ketone bodies or ‘ketones’ (β -hydroxybutyrate, acetoacetate and acetone that are produced in the liver by ß-oxidation of free fatty acids) is increased, leading to elevations in their circulating concentrations” - Gibson et al., 2015

“From the current studies, it was not possible to determine whether the appetite suppression seen with ketogenic diets is indeed due to ketosis, or due to other factors such as an increased or decreased content of protein or fat in the diet or the restriction of carbohydrate.” - Gibson et al., 2015

“No significant correlations were found between energy, protein, fat or carbohydrate intakes and either the hunger or fullness scores.” - Krebs et al., 2010

“Ketone bodies have also been suggested to act as satiety signals, the elevation of ketone bodies in the low-carbohydrate group hence contributing to satiety” - Erlanson-Albertsson and Mei, 2005

“Ketone bodies including beta-hydroxybutyrate (BHB) circulate at very low concentrations during energy balance, but rise during energy and carbohydrate restriction and are thought to suppress appetite [15,16]” - Karl et al., 2016

“individuals with the lowest glucose concentrations during ED also had the highest BCAA and BHB concentrations, and that FFA concentrations demonstrated a trend to be positively associated with satiety. Therefore, our findings are consistent with animal models [34,35] and clinical trials [36,37] which reported that increased FFA and BHB concentrations are associated with reduced appetite and food intake. Further, these findings suggest that appetite suppressing effects of ketogenesis and gluconeogenesis may override any glucostatic regulation of appetite during severe energy deficit.” - Karl et al., 2016

“One line of evidence that ketosis plays a role in appetite suppression during both VLEDs and KLCDs is the observation that changes in perceived appetite and circulating concentrations of hormonal regulators of appetite coincide with increased or decreased concentrations of circulating ketones. For instance, appetite is increased during the first couple of days on VLED (or on a fasting regime), before marked increases in circulating ketone concentrations would be expected to occur (13,15,25,58). However, after the time when elevated circulating ketone levels are observed, perceived appetite (10,29,55) and circulating concentrations of hormonal regulators of appetite, namely, ghrelin (29,55) and CCK (10,14,29), have been shown to be no different from pre-VLED and/or pre-KLCD levels” - Gibson et al., 2015

“AUC ghrelin increased significantly between weeks 0 and 8 in participants who did not achieve ketosis (BHB >0.3 mmol/l) at week 8, but the weight loss induced increase in ghrelin was completely suppressed in subjects who were ketotic. There were significant inverse correlations between BHB and AUC ghrelin at week 8 (P=−0.34; 95% CI (−0.62, −0.04)), and between changes in BHB and AUCs for ghrelin (P=−0.48, 95% CI (−0.70, −0.22))” - Sumithran et al., 2013

“The mechanism whereby ketones could decrease appetite may be via central actions in the brain or by changes to peripheral hormone secretion [5].” - Stubbs et al., 2017

Keto might affect cravings

Below, you see a graph of cravings. Every sphere represents a ketogenic diet group. The size of the sphere reflects how many subjects were in the study. For instance, the Foster et al., 1992 study had over 200 participants, so the sphere is much larger than the Rosen et al., 1982 study which had 8 participants.

There is a lot of variability in how keto affects cravings. There is data to suggest (not pictured) that those on a keto diet (Martin et al., 2011) have lower cravings for carbs or sweets compared to people on a low-fat diet.

Note that a craving is different from hunger. You can crave a specific food while feeling satiated, in general.

Ketogenic diet cravings graph

Figure by Sci-Fit. The cravings were measured using a 5-point scale. A drop in cravings by ~33% (as seen in Saslow et al., 2014) is notable, because they went from rarely feeling carb cravings to almost never feeling a craving. It’s hard to say what the practical implications of this is in the long-term

Is keto better than other diets for hunger control?

People on the ketogenic diet reduced their hunger to a greater extent than people on control diets, in several studies:

Study

Subjects

Hunger

Nickols-Richardson et al., 2005

RCT, 6 weeks

28 women, mean age = ~39–40

⬇ ~50% for keto, ⬇ 17% for control

McClernon et al., 2007
Retrospective Analysis of 2 studies, 24 weeks

170 men and women, age range = 18–65

Estimated ⬇ ~65% hunger for keto, ⬇ ~38% for Low-Fat Diet

Johnstone et al., 2008

Randomized Crossover, 4+4 weeks

17 obese men, mean age = 38

⬇ ~56% for keto, ⬇~24% for medium-carbohydrate diet. Averaged across 4 weeks

Martin et al., 2011

RCT, 104 weeks

270 men and women, mean age = ~45

-6 decrease in hunger (VAS) for keto vs ~-1.5 for control

Lobley et al., 2014

Crossover

12 obese men, age range = 50–74

14% lower for keto vs. control

 

There are also studies with no or minimal difference in hunger between diets:

Study

Subjects

Hunger

Johnston et al., 2006

RCT, 6+4 weeks

19 overweight and sedentary men and women. Mean age = ~38

“hunger ratings did not differ significantly between diet groups”

Soenen et al., 2012

VLED, 52 weeks

132 overweight and obese men and women. Mean age = 50

~35% hunger ⬇ for keto

~49% hunger ⬇ for low carb

Data from week 12

Saslow et al., 2014

Randomized Pilot Trial, 12 weeks

32 obese men and women with T2D

Hunger ⬇ ~17% for keto and ⬇ 11% for control

Krebs et al., 2010

RCT, 12+24 weeks

33 obese boys and girls. Mean age = ~14

No between-group difference for hunger. Keto vs low fat.

Notably, no studies found that moderate or high carb diets were superior to keto.

The only study that supports non-keto is Soenen et al., 2012. They showed that the control group had a greater non-significant hunger reduction than keto. Though, the control diet ate ~80 grams of carbs, which is borderline keto. Also, there was a 10+ year age difference between the groups. This is not good, since we want to compare people of the same age. Both diets were very low energy, which reduces hunger regardless of macronutrient composition.

Overall, the data show that the ketogenic diet can lead to impressive reductions in hunger. Compared to other diets, keto seems to be superior for curbing hunger.

How scientists measure appetite

Appetite is hard to measure objectively. After all, how hungry or full you feel is subjective. However, scientists have developed several tools to measure satiety, hunger, appetite, and cravings.

Visual Analogue Scale (subjective)

The Visual Analogue Scale uses a line that is 100 mm in length. Zero on the line represents “not hungry” and 100 represents “extremely hungry”. This tool can be used for hunger, appetite, satiety, cravings, etc. Study participants are asked to make a mark on the line corresponding to how hungry they feel. You can see an example of the scale below:

Visual analogue scale, hunger

Visual analogue scale ratings for hunger, desire to eat, and appetite can be related to subsequent food intake (Flint et al., 1998).

Appetite hormones (objective)

We can measure appetite objectively by looking at hormones. Hormones are measured in blood and can help explain why our appetite changes during or after a diet.

Below is a table of hormones and their effects on appetite:

Satiety hormones overview  table

Table by Sci-Fit, adapted from Pliquett et al., 2006

 

For more information on what these hormones do, 

  • Leptin is a feedback mechanism to inhibit food intake and regulate body weight (Klok et al., 2007).
  • Ghrelin increases before a meal and decreases after eating (Cummings et al., 2009).
  • Cholecystokinin (CKK) is released in response to food intake and decreases appetite (Kaviani et al., 2017).
  • PYY 3-36 is low in a fasted state and rises rapidly following a meal. PYY is elevated for several hours after a meal (Adrian et al., 1985).
  • Glucagon-Like Peptide 1 (GLP-1) rises after a meal and returns to baseline in a fasted state. It may also rise in anticipation of a meal (Williams 2009).

Three Factor Eating Questionnaire (subjective)

Aside from the visual analog scale, the nutrition field has other methods to quantify food cravings and hunger. One of those is the Three Factor Eating Questionnaire. It measures three dimensions of eating: disinhibition, hunger, and cognitive restraint (Stunkard and Messick, 1985).

Food Craving Inventory (subjective)

The Food Craving Inventory is a self-reported measure of specific food cravings (White et al., 2002).

Subjects rate how frequently they feel cravings to eat certain types of foods:

  • Carbohydrate-cravings
  • Fast-food cravings
  • Sweet cravings
  • Fat cravings

They use a 5-point scale:

  1. Never feel a craving
  2. Rarely feel a craving
  3. Sometimes feel a craving
  4. Often feel a craving
  5. Daily/continual cravings

Things to keep in mind

Many factors affect appetite

Appetite is complicated (as reviewed by Bellisle, 2008; Pesta and Samuel, 2014). We have previously mentioned some things that could affect hunger, such as calories, protein, and ketosis. Yet, there are many other factors.

“Satiety is multifactorial and influenced by many components including but not limited to the endocrine system, the cognitive and neural system as well as the gastrointestinal system.” - Pesta and Samuel, 2014

“The reason for decreased energy intake in the low-carbohydrate group, even in the face of no restrictions on energy, remains to be explained.” - Brehm et al. 2005

“Although the hunger-reducing effect of KD is well-documented, its main mechanisms of action are still elusive. The global picture is complicated by the contradictory role of ketosis on anorexigenic and orexigenic signals (summarized in Figure ​4). Ketones (mainly BHB) can act both orexigenically or anorexigenically.” - Paoli et al., 2015

“Brehm et al. (6) and Sondike et al. (29) stated that a reduction in caloric intake during a CRD may be because of a reduction in food choices while attempting to limit carbohydrate intake.” - Sawyer et al., 2013

“The mechanism responsible for the decreased energy intake induced by a low-carbohydrate diet with unrestricted protein and fat intake is not known but may be related to the monotony or simplicity of the diet, alterations in plasma or central satiety factors, or other factors that affect appetite and dietary adherence” (Foster et al., 2003)

“... other potential explanations include the simplicity of the diet and improved compliance related to the novelty of the diet.” - Samaha et al., 2003

“These results underscore the lack of the satiating efficiency of empty calories in quickly ingested drinks such as sodas. The increase in perceived fullness that is due solely to the increased viscosity, which is a phenomenon that we refer to as phantom fullness, may be useful in lowering energy intake.” - Camps et al., 2016

Hormonal changes on a ketogenic diet

Your body controls your appetite and hunger via hormones such as leptin, insulin, ghrelin, GLP-1, cholecystokinin, and peptide YY (Austin and Marks, 2009, Marić et al., 2014; Gibson and Sainsbury, 2017)

In some keto studies, leptin decreased while hunger decreased (Chearskul et al., 2008) or stayed the same (Kovacs et al., 2004; Sumithran et al., 2013). These signals are contradictory, because the decreased leptin favors weight gain (Chearskul et al., 2008), while the drop in hunger favors weight loss.

This suggests that leptin is a long-term signal that slowly makes you eat more calories to regain weight. We see that diet adherence drops over time, and people regain weight.

Overall, the hormone literature is complicated, and we will not examine it in detail for this article.

“One possible reason for weight regain is that leptin concentrations profoundly decrease after weight loss (...) After this drop, the dieting person then experiences the effects of leptin deficiency, namely hunger and lethargy (16)”. - Chearskul et al., 2008

“Throughout the study period, a mismatch was apparent between subjective appetite feelings and anticipated relevant appetite-related hormones. Among others, there was an increase in hunger on day 3 (2±1% WL), despite no changes in appetite hormones except for insulin, and 5% WL, despite an increase in GLP-1 AUC and a reduction in CCK. Even though this mismatch can derive from lack of power, it is well known that the appetite control system is extremely complex and subjective feelings of appetite are not always correlated with the concentration of appetite-related hormones.42 ” - Nymo et al., 2017

Do you only eat when hungry?

While hunger is important, it’s not the only thing affecting how much you eat. Think about what you eat or drink at parties, birthdays, holidays, and social occasions. In these circumstances, do you only eat out of hunger?

Furthermore, you probably also have food habits. You might enjoy a beer while watching your favorite TV show, or you celebrate an achievement by rewarding yourself with something to eat or drink. The point is that hunger is important, but other factors also affect how many calories you consume per day.

Study limitations

  • Most studies are short-term. What matters most is long-term diet adherence, not short-term hunger/calorie reductions.
  • Protein is not matched in many studies. Yet, this isn’t necessarily a limitation, because you can’t match protein intakes if people are given an ad libitum diet. By matching protein, you increase internal validity, but decrease ecological validity.
  • Many studies compare calorie restricted control diets to ad-libitum ketogenic diets
  • The ketogenic diets are sometimes given better and more extensive training than the control diets
  • Underreporting is typical. Researchers ask subjects to recall or report how many calories they ate during a day. The subjects then tend to report that they ate fewer calories than what they really did. This put some of the caloric data into question, but we still see that ketone bodies increase with lower levels of reported carb intakes. Hence, the self-reported calorie or macronutrient intakes do have validity, but they are not perfectly accurate.

Excluded studies

Study

Reason(s)

Willi et al., 1998

No relevant data

Westman et al., 2002

No relevant data

Volek et al., 2002

No relevant data

Sondike et al., 2003

Not enough data

Meckling et al., 2004

No relevant data

Volek et al., 2004

No relevant data

Stern et al., 2004

Not keto

Yancy et al., 2004

No data on main outcomes

Dansinger et al., 2005

Only keto for a short period, no data on satiety

Noakes et al., 2006

No relevant data

Stimson et al., 2007

No relevant data

Shai et al., 2008

No data on main outcomes

Foster et al., 2010

Not relevant

Jabbek et al., 2010

No relevant data

Veldhorst et al., 2010

Too many methodological and reporting issues

Paoli et al., 2011

No relevant data

Paoli et al., 2012

No relevant data

Wood et al., 2012

No relevant data

Ruth et al., 2013

No relevant data

Rhyu and Cho, 2014

No relevant data

Hu et al., 2016

Not keto

Meirelles and Gomes, 2016

No relevant data

Hall et al., 2016

No relevant data

Veum et al., 2017

No relevant satiety data + calorie restriction

McSwiney et al., 2017

Issues with dietary reporting, no data on satiety/hormones

Wilson et al., 2017

No relevant data

Tsaban et al., 2017

No relevant data

Kephart et al., 2018

Insufficient dietary reporting (4 participants) and no hunger/satiety data

The Novartis study cluster (VLED studies) has been excluded because the data are too similar across different studies using different subjects: Lejeune et al., 2003, Lejeune et al., 2005, Kovacs et al., 2004, Westerp-Plantenga, Lejeune, Kovacs, 2005, Diepvens, Soenen, Westerp et al., 2007, Hursel et al., 2009