Energy requirements

Appetite, Hunger, Satiation, and Satiety

Hunger and ghrelin

• Hunger is the desire to eat associated with an uncomfortable, empty or gnawing feeling in the stomach.

• Ghrelin, released by cells lining the stomach, rises as the stomach empties and its stretch decreases. Reduced protein in the stomach also contributes to ghrelin release.

• Eating stretches the stomach and helps lower ghrelin. The feeling of hunger may take time to subside because ghrelin must decrease and then be cleared from the body.

• Eating slowly may give satiety signals more time to develop, reducing the chance of eating past fullness before hormonal signals catch up.

Satiation and satiety

• Satiation is the feeling of fullness that ends a meal; it helps determine how much is eaten at that meal.

• Two signals contribute to satiation:

• Endocrine: Fat and protein entering the small intestine stimulate release of cholecystokinin (CCK), which promotes fullness.

• Mechanical/neural: Stomach stretch activates signals carried by the vagus nerve to the hypothalamus.

• Satiety refers to processes after satiation that inhibit eating and affect when someone eats again. It may occur before strong hunger returns.

• Appetite describes a desire or inclination to eat that is not necessarily driven by hunger.

• Leptin, released by adipocytes, reflects stored body fat and helps suppress appetite and increase satiety.

• GLP-1 (glucagon-like peptide-1) has effects at two sites:

• In the hypothalamus, it promotes satiety and suppresses appetite.

• From the ileum and colon, it delays gastric emptying, extending the time before hunger returns.

• GLP-1 agonist medications can reduce food intake through these appetite and satiety effects.

Energy Balance and Metabolism

Energy balance and excess macronutrients

• Energy balance compares energy intake with energy used. Energy consumed beyond expenditure is stored, primarily as fat.

• Excess protein: Amino acid oxidation changes relatively little. Some excess protein is excreted; some is deaminated and converted to glucose or ketones. Excess glucose derived from protein can ultimately be stored as fat.

• Excess carbohydrate: Storage as glycogen in liver and muscle is limited. Excess carbohydrate can be converted to fatty acids and stored as triglycerides. Glucose oxidation also increases, reducing fatty acid oxidation and favoring fat accumulation.

• Excess fat: Dietary fat is readily stored as triglyceride in adipose tissue.

• Sustained energy surplus increases fat mass, including visceral and subcutaneous fat.

Calories used by the body

• Exercise: Calorie use depends on duration, frequency, intensity, and body weight.

• Non-exercise activity thermogenesis (NEAT): Energy used for everyday activity that is not formal exercise, such as walking upstairs, physical work, and fidgeting. Some people increase NEAT unconsciously after eating more, which can limit weight gain.

• Adaptive thermogenesis: Changes in energy use in response to challenges such as injury, fever, or other stressors.

• Thermic effect of feeding: Energy used to digest and absorb food. A rough estimate is about 10% of calories consumed; it may be higher for large or protein-rich meals.

• Basal metabolic rate (BMR): Energy needed to maintain basic bodily functions. It is influenced by age, sex, hormones, growth, and body composition.

• Lean body mass, particularly muscle mass, is a major determinant of BMR.

• Children have higher energy needs associated with growth. BMR and muscle mass tend to decline with age, though activity and muscle maintenance affect the degree of decline.

• Lean body mass can be estimated using body-composition methods such as underwater weighing; changes in lean mass largely reflect changes in muscle when bone density is stable.

Weight Change, Dieting, and Fasting

Weight loss and weight cycling

• Weight loss occurs when energy expenditure exceeds intake. The lecture cautions against consuming fewer than 1,200 calories per day, because it can be difficult to obtain adequate micronutrients at that intake.

• Restrictive dieting can reduce muscle mass and BMR. Returning to the previous intake after dieting may then lead to rapid weight regain, sometimes beyond the starting weight; this is called weight cycling.

• A long-term lifestyle approach is presented as more sustainable than short-term restrictive dieting.

• Early weight loss can include loss of glycogen, water, and muscle—not just fat. Reduced food and fluid intake, as well as sweating without adequate hydration, can contribute to dehydration.

• Calorie restriction can reduce thyroid hormone release and conversion of T4 to the more active T3. Decreased muscle mass and thyroid activity can lower energy expenditure and make continued weight loss harder.

Fasting and low-carbohydrate or ketogenic diets

• Liver glycogen may be depleted within about 12 hours of fasting. The body then increases gluconeogenesis to maintain blood glucose.

• During the first one to three days of fasting, amino acid release from muscle can be substantial, contributing to muscle loss. With longer fasting, amino acid use decreases as ketone production and use increase.

• Fat breakdown releases fatty acids and glycerol. Fatty acids can be converted to ketones, which the brain can use during prolonged fasting.

• Low-carbohydrate diets can produce some similar metabolic changes to fasting, generally to a lesser extent. Lower carbohydrate intake reduces insulin and increases glucagon, promoting glycogen breakdown, gluconeogenesis, and lipolysis.

• Ketogenic diets increase ketone production by reducing carbohydrate intake. The notes describe a target blood ketone concentration of 3–8 mM and warn that higher levels may be associated with ketoacidosis.

• The lecture’s central point is that ketosis itself has not been shown to produce greater fat loss than other diets when calorie intake is matched. Reduced appetite and changes in fat use are attributed largely to reduced carbohydrate intake, not to ketones themselves.

• The energy cost of gluconeogenesis is small relative to the glucose produced.

• Early weight loss on a ketogenic or low-carbohydrate diet can reflect depleted glycogen, associated water loss, and muscle loss. Weight may return as glycogen, muscle, and hydration are restored.

• Reported side effects include diarrhea, constipation, vomiting, nausea, fatigue, low blood pressure, elevated uric acid, and “keto flu” symptoms. Prolonged fasting can increase ketones and make the blood more acidic.

Healthy weight-loss benchmarks

• A commonly cited healthy rate is 0.5–2 pounds per week.

• A 5–10% body-weight reduction over six months is presented as a longer-term benchmark.

• A daily deficit of 500–750 calories below expenditure is described as a reasonable weight-loss target. The comparison is with calories burned, not simply BMR.

• Restricting intake too aggressively can cause fatigue, impair exercise, and increase the risk of nutrient deficiencies.

• The notes report that about 17% maintain a 10% body-weight loss after one year.

Adipose Tissue and Fat Distribution

Adipocyte size and number

• White adipocytes store triglycerides and have a limited capacity to expand. When enlarged, they can release growth factors that stimulate formation of additional adipocytes.

• Weight loss generally reduces adipocyte size, but the number of fat cells tends not to decrease; adipocytes are relatively resistant to apoptosis.

• Childhood obesity is a concern in part because adipocyte number increases especially rapidly during late childhood and early puberty. A greater number of fat cells may make long-term weight maintenance more difficult.

• The notes report that adults with fewer fat cells are more successful at maintaining weight.

Lipid storage and release

• Endothelial lipoprotein lipase (LPL) helps make fatty acids from circulating triglycerides available for uptake and storage by tissues.

• LPL activity is reported to be higher in adipose tissue of people with obesity. The notes describe sex-related patterns in fat storage: greater abdominal activity in males and greater activity in the breasts, hips, and thighs in females, potentially influenced by testosterone and estrogen.

• Hormone-sensitive lipase (HSL) helps release stored lipids from adipocytes. The notes report similar release rates across upper and lower body regions in males, but lower release from women’s lower-body fat than from their upper-body fat.

• Lower-body subcutaneous fat, more typical of a “pear” distribution, is associated in the notes with lower heart-disease risk than central abdominal fat.

Visceral and ectopic fat

• Visceral fat is fat stored around abdominal organs. It is a significant risk factor independent of BMI and is associated with cardiovascular disease, type 2 diabetes, and cancer.

• Central obesity is linked to elevated LDL and hypertension. Weight gain can raise cardiovascular risk, while weight loss can reduce it.

• Ectopic fat is fat stored in tissues not normally intended for substantial fat storage. It can disrupt those tissues’ metabolism and contribute to inflammation and metabolic problems.

• Waist circumference is listed as a risk indicator above 35 inches for women and 40 inches for men.

BMI, Chronic Disease, and Health Risk

Body mass index

• BMI is weight in kilograms divided by height in meters squared (kg/m²).

• The lecture gives these categories:

• Underweight: below 18.5

• Ideal range: 18.5–24.9

• Overweight: 25–29.9

• Obesity: 30 or higher

• BMI is moderately correlated with body fat and more strongly correlated with chronic-disease risk at a population level. It does not directly measure body fat or body composition.

• Muscular athletes or people with heavier bones may have an elevated BMI without excess body fat. Conversely, someone with a normal BMI may still have metabolic risk.

• Physical activity can reduce risk at a given BMI; cigarette smoking can increase it. BMI alone should not determine a person’s health status.

Obesity-related health risks

• Metabolic syndrome involves a combination of insulin resistance, abnormal blood lipids, and hypertension.

• Visceral fat is associated with reduced glucose tolerance/insulin resistance and increased risk of type 2 diabetes. Weight loss can reduce diabetes risk and may help reverse type 2 diabetes in some people.

• Obesity is linked to chronic inflammation and increased cancer risk; the notes emphasize that risk is related to both duration and severity of obesity.

• Adipocytes release signaling molecules, including adipokines, that can promote inflammation or reduce anti-inflammatory signaling. Adipocytes also release estrogen; increased estrogen may contribute to estrogen-sensitive reproductive cancers.

• The notes distinguish people with normal BMI but metabolic syndrome from people with elevated BMI who have relatively better metabolic health and more adiponectin. These are examples of why BMI does not capture every individual’s risk.

Weight-Loss Products and Treatments

Fad diets and over-the-counter products

• Fad diets may rely on exaggerated or incorrect theories and may not provide adequate nutrients. Weight-loss claims for products that do not require a prescription may not be evaluated by the FDA in the same way as pharmaceutical claims.

• Some products do not clearly disclose active ingredients. Stimulants are common and can cause serious effects; the notes report thousands of emergency-room visits, including for chest pain or seizures.

• The lecture argues that consumers would benefit from reliable, well-controlled evidence about average outcomes and long-term chances of success.

Prescription medications

• The notes describe weight-loss medications as generally modestly effective and intended for people with significant health risks, alongside dietary intervention—not simply for people who want to lose weight.

• Listed medications and mechanisms:

• Contrave: Combination of an antidepressant and an opioid antagonist; described as suppressing appetite and increasing metabolism.

• Ozempic/Wegovy: GLP-1 agonists that reduce appetite, increase satiety, and may alter food preferences. The notes cite 12–15% weight loss after 1.25 years and weekly injections; the stated cost is approximately $1,000 per injection.

• Belviq: Serotonin-receptor agonist described as increasing satiety and reducing food intake.

• Alli: Reduces pancreatic lipase activity, limiting fat digestion and absorption.

• Qsymia: Described as increasing norepinephrine to suppress appetite and increase fullness.

Bariatric surgery

• Options listed include gastric bypass, gastric banding, and gastrectomy. These limit food intake by reducing stomach capacity and can also change hormone release.

• The notes cite a 20–30% weight reduction after 10 years.

• Surgery is described as an option for people with severe BMI elevation (over 40) who have repeatedly been unable to lose weight and understand the risks and required postoperative plan.

• Diet changes substantially after surgery; returning to previous eating patterns may cause problems and undermine the treatment.

Causes and Environmental Influences

Genetic and biological factors

• Obesity susceptibility reflects interactions among genes and environment. Single-gene causes are rare; multiple genes can influence susceptibility and response to excess calories.

• Prader-Willi syndrome, involving multiple genes on chromosome 15, is associated with excessive appetite and obesity.

• Leptin or leptin-receptor defects can contribute to severe obesity.

• Adoption and twin observations are cited as evidence of genetic influence: adopted children’s weight may resemble their biological parents’ more than their adoptive parents’, and identical twins may respond similarly to excess calorie intake.

• Body weight may diverge with age as people accumulate different environmental and epigenetic influences.

• Ghrelin tends to be higher during negative energy balance. Lack of sleep may increase ghrelin and decrease leptin, increasing hunger.

• Brown adipose tissue (BAT) is described as lower in people with obesity and in older adults. Exercise-related myokines and a more diverse gut microbiome are noted as factors that may promote “beiging” of white adipose tissue.

Obesogenic environment and activity

• An obesogenic environment includes circumstances that encourage excess energy intake or reduce energy expenditure.

• High-calorie foods that are inexpensive and readily available can increase how often and how much people eat. Modern technology and work patterns can reduce physical exertion and therefore reduce energy needs.

• Past and present eating patterns both contribute to body weight; a person with obesity may not be in a positive energy balance at the moment their diet is assessed.

• Watching television can influence food choices, including increased intake of high-calorie, fatty, or processed foods.

• The notes state that maintaining weight may require about 60 minutes of moderate-intensity activity daily and that general daily activity can account for about 350 calories per day.

Eating and Behavior Strategies

Food choices and eating patterns

• Nutrient-dense foods help provide micronutrients within an appropriate calorie intake. “Empty calories” provide fewer nutrients.

• Eating slowly may allow time for ghrelin to decrease and CCK-related fullness signals to rise.

• Water can help with fullness; replacing sugary drinks and choosing plant-based foods are listed as strategies.

• Protein-rich breakfasts are noted as being associated with lower BMI, blood pressure, and blood lipids.

• The notes recommend choosing fats sensibly and selecting carbohydrates carefully; low-fat diets are described as not necessarily superior to other diets for weight loss.

• Eating patterns and food accessibility matter: keep healthy foods readily available, remove food from the table after a meal, choose smaller portions, and consider smaller plates or packages.

• Distractions and socializing can reduce attention to food intake and fullness. Having many food choices can encourage eating more, as can large portions and package sizes.

Behavior modification and self-monitoring

• Identify behaviors that contribute to overeating and change them using specific, measurable, attainable, relevant, and time-bound (SMART) goals.

• Psychotherapy may help people understand their relationship with food. Cognitive strategies include problem-solving and replacing negative thoughts with positive ones.

• Frequent self-monitoring, such as weekly monitoring, is listed as a weight-management strategy.

Physical Activity and Maintaining Weight

Activity and body composition

• Physical activity can help reduce fat while maintaining lean body mass and BMR. Maintaining muscle is particularly important because muscle mass is a major determinant of BMR.

• The notes give a current recommendation of 200–300 minutes per week of moderately intense activity.

• Energy expenditure depends on activity duration, frequency, intensity, and body weight. Calorie expenditure for walking or running is described as depending largely on distance and body weight; traversing the same distance at lower intensity may use a greater proportion of fat as fuel.

• Starting activity may reduce appetite and energy intake among previously sedentary people.

• Aerobic exercise is noted as helping release abdominal triglycerides; lower-body fat may be harder to mobilize in women.

• Enjoyable activities and increased movement throughout the day may help sustain activity.

• Weight maintenance may become harder after weight loss because appetite can rise and BMR can decline. The notes cite 2,500 calories of exercise per week as a maintenance requirement.

Underweight, Relative Energy Deficiency, and the Female Athlete Triad

Underweight and malnutrition

• Low body weight may reflect high activity and energy expenditure, malnutrition, smoking, substance use, or illness.

• Malnutrition can impair the body’s response to stress. Older adults may be especially vulnerable to losing muscle during illness.

Relative energy deficiency in sport

• Relative energy deficiency occurs when calorie intake is inadequate relative to exercise expenditure; it affects both men and women.

• Attempts to make weight by restricting food or sweating can be counterproductive, reducing strength, anaerobic power, and aerobic capacity.

• Inadequate carbohydrate intake can reduce muscle glycogen and contribute to exercise-related fatigue. Inadequate iron can contribute to anemia and fatigue.

• Low intake of milk or other nutrient sources can reduce calcium and vitamin D intake. Inadequate protein can reduce protein synthesis, bone growth, and strength gains.

Female athlete triad

• Inadequate energy or carbohydrate intake can reduce gonadotropin-releasing hormone (GnRH), lowering follicle-stimulating hormone (FSH) release from the anterior pituitary.

• Reduced follicle development and estrogen production can disrupt ovulation and menstruation, including amenorrhea.

• Reduced estrogen can impair peak bone-mass development and contribute to osteoporosis risk.