The Science of Hunger
By Aimee Gallo
Many factors — including our hormones, food choices, habits and more — influence our bodies’ feelings of hunger and satiety. Learn more about what motivates hunger and improve your ability to fully nourish yourself and maintain a healthy weight, without second-guessing every bite. As a nutrition coach, I frequently see women looking to manage their weight. Inevitably, the term “portion control” comes up in our initial conversations. Everyone seems to believe smaller meal sizes must be the key to effective weight management. I’m happy to tell them that, by changing what they eat, most women can eat plates full of food, feel reduced hunger, and finally see the results they’ve been working toward in the gym come to light.
Chronic deprivation rarely leads to lasting results, so one of the focuses in my practice is to educate people on what foods are most satisfying so they needn’t worry about “managing portion control,” but rather focus on enjoying their meals. Most of us have enough on our plates to manage without having to manage what’s on our plates!
Yet, while the tools to managing hunger can be simple for many people, the triggers of hunger are complex and multifaceted. It all begins in the head. Read on to learn about the many cues and triggers that influence our levels of hunger and satiety, and be better equipped to properly feed and care for yourself.
The Hunger Sensation
Known as the cephalic phase of hunger, simply thinking about or seeing food begins the hormonal cascade that prepares our bodies for nourishment. Hunger is triggered by sensory, cognitive, digestive and hormonal stimuli.
You’ve probably experienced the sensory part walking by a bakery: The scents and sights of the tasty baked goods elicit salivation and may trigger a craving for a food you weren’t even thinking about mere seconds before. As the senses trigger a craving, the digestive process begins to prepare for the anticipated treat by releasing salivary amylase, an enzyme that helps break down carbohydrates as we chew. The senses also send signals to the stomach to release gastric juices. Your stomach may begin growling. Your body is now expecting you to step into that bakery! This response is conditioned, and we feel it most strongly when food we enjoy is brought to our awareness.
Our physical senses are key to our sense of satiety. How we see our food, visually and perceptively, makes a difference in how satisfying it is. Brian Wansink has done numerous studies on appetite and shares some of his work in his book, Mindless Eating. In one study, eating in complete darkness led participants to overeat (consuming a “super-sized” meal), underestimate how much they’d consumed and, despite consuming more food, to not report feeling fuller than a control group. In another study, incorporating air into a milkshake so it appeared twice the size but contained the same amount of calories resulted in a 12-percent reduction in food intake at the next meal, as well as lower reports of hunger.
So, there is something to those visual tricks of eating on smaller plates to increase satisfaction. We can also take advantage of the link between our senses of sight and smell and our sense of satiety by slowing down before and while we eat to fully appreciate the way our food looks and smells, allowing our bodies to absorb the meal with additional senses alongside taste.
Meet the Hunger Hormones
Although our physical sensations involve many bodily systems and external factors, in the end most of them depend upon hormones. “Sleep, hunger, mood, energy and cravings are hormonal sensations,” says Jade Teta, a naturopathic doctor and fat-loss expert. Hunger hormones impact appetite from numerous internal and external cues. They act as messengers to deliver information to the cells based upon information received in the surrounding environment — in this case, information pertaining to the drive for food procurement.
The main hormones that influence hunger are insulin, leptin and ghrelin. Insulin and ghrelin impact hunger levels hour to hour. A storage hormone, insulin is released in the presence of glucose or amino acids (carbohydrate or protein), delivering these nutrients to the cells to be used for fuel or stored. When insulin levels are high, hunger is suppressed in healthy individuals. When they’re low, hunger and cravings ensue. In individuals with type 2 diabetes, metabolic syndrome or insulin resistance, the cells don’t receive the message that insulin is present, and so hunger may continue. Insulin will typically rise to a lesser degree with protein than carbohydrates, and will rise in a more gentle curve with whole foods than processed foods, which are more quickly absorbed into the bloodstream.
Ghrelin is released in the stomach and signals the body’s need for fuel, rising between meals to stimulate appetite. Eating stretches the stomach, in turn reducing ghrelin levels. Various types of foods affect ghrelin release differently. Carbohydrates and protein contain more volume than fat, thus both lower ghrelin more effectively. While carbohydrates shut down ghrelin more rapidly than proteins, proteins take longer to digest than carbohydrates and may ultimately be more satisfying. Fibrous carbohydrate sources such as fruits, vegetables, beans and unprocessed whole grains provide the most volume and nutrient density of all carbohydrate sources.
The hunger hormone leptin helps regulate weight and appetite day by day, rather than hour by hour. It is secreted from fat cells when energy intake becomes too high, suppressing hunger in an effort to maintain weight. Leptin received widespread attention in the ’90s when Jeffrey Friedman discovered the hormone was behind the massively obese “ob” lab mice — three times the weight of normal mice with an insatiable appetite. Friedman discovered these mice were genetically leptin-deficient. Leptin injections dramatically lowered their appetite and normalized body weight.
Hopes that supplemental leptin could solve the rising obesity epidemic were dashed when studies failed to produce the same results in humans. In humans, obesity may be due to leptin resistance rather than deficiency; studies reveal most obese individuals produce enough leptin, but their bodies do not respond to the hormone. Those with leptin resistance have very high circulating leptin yet remain hungry.
Patients I’ve seen with suspected leptin resistance have difficulty feeling satisfied; are overweight or obese; struggle with weight loss; and may have polycystic ovarian syndrome (PCOS) and/or infertility, as leptin also impacts ovarian and testicular function. For these individuals, eliminating high fructose corn syrup is crucial and minimizing fruit may also be needed to achieve results, as fructose (sugar from fruit), unlike glucose, lowers leptin levels in some studies and also appears to increase ghrelin release. Individuals who think they may be leptin-sensitive often do better on a lower-carbohydrate, higher-protein and -fat diet. It’s best to discuss dietary changes with a health professional and check in with your own body’s response to various meal types before trying such a program.
Other hormones are released during meals: When we eat fat and protein, we release cholecystokinin (CCK) to signal the release of digestive pancreatic enzymes and suppress the appetite. “The more CCK you have floating around, the less hungry you are, and the less you’re likely to eat,” writes Helen Kollias, exercise physiologist and writer for Precision Nutrition. “This is why a lower-carb, higher-protein, higher-fat diet tends to make people feel fuller longer.” Because CCK can shift significantly meal to meal, manipulation of carbohydrates, fats and proteins can impact one’s hunger and cravings within a 24-hour period. Finally, glucagon-like peptide-1 (GLP-1) and peptide YY (PYY) are released in the gut and signal satiation. They are released 10 to 15 minutes after a meal, and GLP-1 is released again after 30 to 60 minutes. Both hormones are influenced by meal size and composition.
Eating for Satiety
“Satiation and satiety have taken a back seat in the nutrition world to things like glycemic index, insulin load and antioxidant concentrations in foods,” Teta writes. “This is a shame, because your ability to get and stay full is directly related to your ability to make the right choices at your next meal, and the rest of the day.”
Many women I work with are shocked at how much they are able to eat and still lose weight. Those with a history of dieting often struggle with fear of feeling deprived or believe that they’ll need to be extra vigilant in counting carbohydrate grams or controlling their portions. By emphasizing fibrous vegetables, protein and unprocessed starchy carbohydrates such as beans and whole grains, they enjoy full plates of food and do not need to worry about portion control. Unprocessed foods and emphasis on nutrient-dense produce easily allows them to find stable energy levels and experience no challenges with hunger.
Upon revisiting how the hormones work, it is easy to see that increasing protein and fiber is most likely to increase satisfaction. Whole grains, beans, fruit and especially vegetables have high volume, triggering ghrelin in the stomach and GLP-1 in the intestines. These fibrous foods also contain carbohydrates, releasing insulin into the bloodstream. Protein adds volume as well and has a more potent impact on hunger hormones, signaling GLP-1, CCK and insulin.
A review of the research also supports protein and fiber foods as a satisfying component of diet over other nutrients. In studies where protein in the diet is increased but calories remain the same, participants experience greater satiety. A diet high in unprocessed, fiber-rich foods is believed to be best for overall weight management because, ounce for ounce, they are more satiating for fewer calories than processed foods. Fruits and vegetables also contain an appreciable amount of water, whereas grains have more starch and less water. The key to satisfaction is finding the appropriate balance of fiber, starch and protein to stave off hunger and cravings while maintaining a healthy body weight.
Chewing is an often overlooked but potentially important factor in triggering satiety. As hunger begins in the head, we can also use the head to begin the signal to satisfaction. Chewing thoroughly slows rates of consumption and increases satiety by stimulating GLP-1 and PYY. Given these hormones are released 10 to 15 minutes after one begins eating, chewing has the potential to induce satiety before the plate is empty. Macrobiotic dietary theory has been strongly supportive of chewing, recommending each bite be chewed a minimum of 50 times. In a clinical setting, I have seen the required volume of food to achieve satisfaction go down significantly when chewing becomes a focus of the meal, although I have yet to see anyone reach more than 30 chews per bite. Yet, while chewing has been associated with satiety-related responses throughout the body, the jury is still out on whether or not it makes a significant impact.
Sleep and Hunger
With all this talk about food, it can be easy to overlook other factors that drive or suppress appetite. Yet calorie-free experiences such as stress, sleep and exercise greatly impact how much, when and what we desire for our next meal.
“Get at least eight hours of sleep a night,” Teta says. “Sleep is a hormonal reset button for your body. It lowers cortisol and balances ghrelin and leptin, which means less hunger, balanced energy and decreased cravings the next day.”
Research confirms that sleep deprivation has a definite impact on hunger hormones. Sleep deprivation inhibits the suppression of ghrelin after meals and keeps ghrelin levels elevated between meals, leading to less satisfaction from meals and more hunger throughout the day. Widespread research also affirms that sleep deprivation has negative impacts on insulin and glucose balance. This can drive cravings, especially for sugars and other foods with a high “reward value.” In the long term, low-quality sleep or outright sleep deficits can also increase risk of insulin resistance and type 2 diabetes. Other researchers have found additional appetite hormones altered by chronic sleep deprivation. The work of Hibi, Kubota, Mizuno and colleagues showed with just three days of limited sleep, participants experienced increased hunger, decreased satiety and had lower fasting levels of PYY and GLP-1.
Set aside time for sleeping by setting a specific bedtime each night and following a bedtime ritual. Reducing evening screen time and managing stress can positively impact sleep. I often encourage clients who have trouble sleeping to end the day by drinking herbal tea such as chamomile or a sleep blend, journaling and taking a magnesium glycinate supplement.
Stress and Hunger
Cortisol is a stress hormone, and stress has a powerful impact on hunger levels. Yet some people under stress completely lose their appetite, while others become ravenous. What gives? Immediate, acute stress shuts down appetite, but chronic stress can lead to increased hunger for some but not others. It seems that the key differentiation driving stress eating may be the amount of insulin present in the individual.
Cortisol raises blood sugar in response to stressful stimulation (this can be positive stimulation, like exercise, or negative stimulation, like losing a job). If circulating insulin levels are elevated, the insulin quickly pulls the glucose out of the bloodstream for storage, stimulating the appetite to demand more glucose intake. If insulin levels are lower, the blood sugar is not pulled out of the bloodstream as quickly. Overweight individuals, especially those with the “apple-shaped” body type (a wider torso, broad shoulders and full bust, with thinner arms, legs and hips), are shown in some studies to have a higher response to cortisol-driven hunger than others. This demographic is also at higher risk for diabetes, demonstrating that insulin resistance may be at play here. If you get hungry when stressed, focus on eating protein and healthy fats. One study observed a decrease in cortisol after protein and fat intake but not carbohydrate intake.
Exercise and Hunger
Exercise is a frequently recommended solution to achieve and maintain healthy weight, yet there is controversy about its effect upon weight due to the influence of exercise upon appetite. In 2009, Time magazine published the article “Why Exercise Won’t Make You Thin,” which threw the national health and fitness industries into a heated debate for quite some time.
The article claimed that exercise can make us hungrier and lead to increased calorie intake, and also indicted exercise as being responsible for loss of self-control and a sense of entitlement to eat fattening foods later in the day.
The reality is, like weight loss, the relationship between exercise and weight is more complicated than calories in, calories out. The Time article was pretty skewed in the research selected and focused exclusively on the caloric aspect of food and exercise while neglecting the hormonal aspect. So let’s take it a little deeper: “Exercise in itself can affect hunger — either make you hungrier or make you not have an appetite,” says Stacy Sims, Ph.D., an exercise physiologist, researcher, and author of ROAR, a sports nutrition book for women.
When people experience excess hunger after exercise, it’s often due to a combination of three factors: low blood sugar; high stress (i.e., cortisol); and low-intensity workouts. Together, various combinations of these factors can trigger the brain to desire more food. The remedy? The opposite form of exercise. “If you use high-intensity, hot exercise, you will delay hunger for two to three hours, and the return to baseline will still result in a muted appetite,” Sims says. “If someone is trying to lose weight, it is much more effective to eat a whole diet (for example, eating low on the food chain, no processed food, low sugar intake, etc.) and use meals around exercise for fueling. This way overcompensation of calorie burn doesn’t happen, and appetite hormones stay in check.”
Higher-intensity, shorter-duration exercise may blunt appetite, while low- to moderate-intensity, longer-duration exercise can increase appetite. For those who tend to have higher levels of circulating cortisol, long-duration exercise sessions can drive cortisol levels even higher and impact hunger and weight. Working in the sports nutrition sector, I’ve met many women who have trained for marathons and triathlons yet have difficulty maintaining or losing weight. When they switch to higher-intensity, lower-duration workouts and bring in rest as a constructive, intentional part of their program, they often discover their appetites calm down and their body visually appears more athletic with far fewer hours per week exercising.
When beginning an exercise regimen, some women find it difficult to achieve the higher intensities in training that can lead to appetite suppression. This is because physically untrained women tend to rely more on fat as a fuel source, rather than the carbohydrates men and more fit women use. Fats are digested more slowly by the body when consumed; when being used for energy, they take longer to be broken down into a useable fuel source. Carbohydrates are much easier to access as energy. It’s like the difference between using kindling versus logs in a fire. As women continue to train, their bodies adapt to utilize more carbohydrates during activity. Then intensity can increase and the appetite-dulling effect can take hold. If appetite suppression is desired, the key is to be consistent and, regardless of fitness level, utilize higher-intensity training such as track sprints, Tabata intervals or Crossfit-style workouts. Although they may provide other benefits, long yoga sessions, distance running or low-impact aerobic-style exercise classes won’t achieve the same appetite-suppressing results.
“Blood sugar regulation is key in making the brain feel satisfied,” says Jade Teta of Metabolic Effect. He recommends the following tips to keep satisfaction high:
“Chew your food for longer, and choose solid calories over liquid ones whenever possible.” Take advantage of the effect chewing has on nutrient absorption and satiety.
“Protein is the king of reducing hunger. Carbs next. Fat last.” If you want to reduce hunger, increase protein intake above all else. If you have PCOS or suspect leptin sensitivity may be a problem, you may do better to emphasize fat over carbohydrate. Tinker with your plate and do what works for you.
“The combination of fat and starch, or sugar, may actually trigger cravings, especially for more highly palatable food, according to some studies.” Choose one or the other at a meal, but not both, if you have trouble with cravings.
“Fiber is a great hunger fighter, but only the highly viscous or sticky fibers seem to do the trick.” Oat bran, apples, and other sources of soluble fiber such as chia seeds, beans, lentils, okra and acorn squash are great choices.
“Adding fiber and protein together would seem wise, as doing so provides great hunger suppression with a low calorie load. ” The highest fiber sources per volume are found in the produce aisle. Load up on greens and beans.
“Push until you can’t, rest until you can,” suggests Jade Teta when it comes to exercise. Short, intense workouts less than 45 minutes long can be effective in dulling the appetite.
Mind Over Matter
Our perceptions about our food can influence our satiety levels.
What you believe about the food you are eating can affect how satisfying it is. Research reveals meals or snacks we perceive as indulgent and rich are far more satisfying than those marketed as “low-calorie” or “diet.” In one study, the same milkshake was given on two separate occasions. One time it was labeled as “indulgent” and 600 calories, the second time as “sensible” and 140 calories. Ghrelin, a hunger hormone, was tested after consumption. Those who believed they were having the indulgent shake had a sharp decline in ghrelin. When they consumed the “sensible” shake these same individuals had very little change in ghrelin. Their beliefs about what they were eating had physiological and hormonal impacts on their appetite.
Aimee Gallo is a functional nutrition and fat-loss coach whose nationwide practice, Vibrance Nutrition, focuses on sustainable, tailored nutrition for long-term results. Her passion for food as medicine began as a child, and she still delves into current research regularly. When she is not with clients or studying functional medicine and nutrition, she enjoys cooking with her toddler, frolicking in the Pacific Northwest forests and fermenting various vegetables. Find her at Vibrance Nutrition and Fitness.
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