What Happens to the Body When We Diet? Explained

The decision to go on a diet typically stems from the desire to improve health, enhance body composition, or boost self-confidence. However, the impacts of dieting on the body extend far beyond the number on the scale. When we restrict calories or entire food groups for an extended period, it sets off a complex cascade of physiological, psychological, and hormonal responses. Some are beneficial, while others can be quite detrimental.

Understanding how dieting affects the body can help us make more informed choices about the best weight loss methods for long-term success. This article explores the comprehensive effects of dieting on metabolism, body composition, brain function, emotions, and overall health.

Physiological Changes

Dieting causes widespread physiological adaptations as the body attempts to cope with fewer incoming calories and nutrients. These changes are part of the body’s innate survival mechanisms against starvation. Let's examine how calorie restriction impacts metabolic rate, nutrient processing, and body composition.

Slowing Metabolic Rate

One of the most frustrating side effects of dieting is the drop in metabolic rate. This is known as adaptive thermogenesis or “starvation mode”. Multiple studies confirm that metabolism slows proportionally to the degree of calorie restriction (1, 2).

For every 1% decrease in body weight from fat loss, the body reduces its energy expenditure by about 15-30 calories per day to conserve energy (3). This arises from reductions in both resting metabolic rate and non-exercise activity thermogenesis (NEAT) like fidgeting and other movements (4).

The body perceives weight loss as a threat to survival, so it adapts by burning fewer calories to protect against starving. Although a slower metabolism hinders weight loss efforts, it is a necessary trade-off of creating an energy deficit through diet and exercise.

Hormonal Adjustments

In tandem with a suppressed metabolism, dieting provokes substantial hormonal shifts. Weight loss is detected by the hypothalamus, which alters hormone secretions to promote fat storage and stimulate appetite (5):

• ↓ Leptin - Signals satiety and suppresses hunger
• ↑ Ghrelin - Stimulates appetite
• ↓ Insulin - Allows stored fat to be utilized for energy
• ↑ Cortisol - Mobilizes fat stores and increases cravings

These hormonal changes make us feel hungrier and long for high-calorie comfort foods. They encourage the body to be highly efficient with calories to avoid tapping into fat stores. Resisting hunger and overcoming food cravings can be very challenging.

Energy Conservation

With a slowed metabolism and shifting hormones, the body becomes extremely stingy with burning calories during a diet. NEAT declines so we fidget less and take fewer extra steps during the day. We may also feel more sluggish, cold, and tired as the body conserves energy (6).

Even small100 calorie reductions in NEAT can hinder fat loss. One study found NEAT dropped by 352 calories per day in dieters, completely offsetting their targeted calorie deficit (7). The body rebels against weight loss by subtly increasing sedentary behaviors and decreasing calorie burning during daily activities.

Altered Macronutrient Utilization

Diet composition has a major impact on the physiological response to calorie restriction. Low-carb, ketogenic diets have a metabolic advantage over low-fat diets (8).

Ketogenic diets help conserve muscle mass by sparing protein breakdown. They also enhance fat burning since carbs are restricted (9). In contrast, low-fat diets cause a greater loss of lean body mass as more dietary protein is broken down for glucose production (10).

Very low-calorie diets sometimes prescribed for rapid weight loss can backfire. Consuming less than 800 calories per day depletes glycogen stores, forcing protein to be used for energy instead of tissue repair (11). This accelerates loss of muscle mass.

Micronutrient Absorption Changes

Nutrient deficiencies can occur when total food intake is severely reduced on a diet. Absorption rates of some vitamins and minerals may decline as well.

For example, calcium absorption decreases on low-calorie diets, potentially compromising bone health (12). Absorption of iron, zinc, magnesium, and other essential minerals drops too, leading to symptoms like fatigue and headaches (13).

Without eating a wide variety of nutrient-dense whole foods, micronutrient status can suffer. Nutrient deficiencies develop insidiously, since overt symptoms take time to manifest. Those at greatest risk include vegetarians, older adults, strict dieters under medical supervision, and people with poor eating habits.

Digestive System Adaptations

The digestive system downregulates its functions proportionally to how much less food is consumed on a diet. Digestive secretions are reduced, gut motility slows, and the microbiome shifts (14).

For example, saliva production decreases, compromising the first step of digestion (15). Gut bacteria populations change to extract more calories and nutrition from smaller amounts of food (16).

This microbial shift can cause temporary digestive issues like constipation, bloating, and indigestion. The sluggish bowels and bacterial changes also promote reabsorption of toxins and estrogen, potentially causing headaches, fatigue, and other issues in low-calorie dieters (17).

Laxative use is inadvisable as it worsens dehydration and electrolyte imbalances. Gentle natural remedies like probiotics, magnesium, prunes, or psyllium husk are preferable to stimulate bowel movements.

Body Composition Changes

The most noticeable impact of dieting is change in body composition. Losing fat mass and maintaining muscle are ideal goals when trimming down. However, various factors affect how drastically body composition shifts on a diet.

Fat Loss

Creating a calorie deficit through diet and exercise reliably stimulates fat loss. A deficit of 500-1000 calories per day from estimated needs equates to about 1-2 pounds of fat loss per week (18).

Initial weight loss is accelerated by dropping water weight and glycogen stores. Then, metabolic adaptations gradually slow the rate of fat loss over time. More aggressive deficits above 1000 calories daily do not meaningfully speed up fat loss (19).

Fat is lost evenly across the body rather than selectively from certain areas. Belly fat is often targeted, yet excess fat also disappears from the back, thighs, hips, face, and elsewhere. Spot reduction of fat by exercising specific muscles has not been substantiated.

Muscle Mass Changes

Preserving muscle is crucial when dieting since it accounts for about 20% of daily calorie burning (20). Unfortunately, some loss of lean body mass is inevitable on hypocaloric diets, and can reach 25% of total weight loss (21).

Several tactics help mitigate muscle loss during fat reducing phases:

• High protein intake of 0.7-1 gram per pound of body weight (22)
• Strength training to stimulate muscle protein synthesis (23)
• Adequate calories and macronutrients based on activity level
• Slow, steady fat loss of 1-2 pounds weekly (24)
• Sufficient sleep and stress management (25)

With strategic nutrition and training, dieters may even experience a modest boost in lean body mass by shedding fat. Novice weight lifters in particular can still build muscle on a slight deficit (26).

Water Weight Fluctuations

Body water is very sensitive to dietary changes and influences scale weight independently of fat loss. Glycogen storage in muscles and the liver binds to water, so depleted glycogen translates to rapid water loss.

Low-carb diets cause greater water loss than low-fat diets in the first weeks by reducing glycogen stores (27). Dehydration from fluid restriction or inadequate sodium also drops water weight quickly but is counterproductive.

Conversely, higher carb intake, sodium consumption, creatine use, and introduction of new exercise can acutely increase water retention. However, these fluctuations reflect changes in body water rather than true weight loss or gain.

Psychological Effects

Dieting produces complex psychological and cognitive effects beyond simply feeling hungrier. Mental health, emotions, and thought processes do not necessarily stay unchanged when the body is thrust into a state of nutritional deprivation and energy deficit.

Mood Changes

For many people, dieting significantly impacts mood. The subsequent irritability and sadness is known as “dieter’s depression” (28).

Contributing factors likely include:

• Blood sugar swings
• Disrupted sleep due to hunger
• Social isolation or feelings of deprivation
• Loss of enjoyment from restricted foods
• Fatigue and low energy impairing motivation

Mood issues may be exacerbated in those with underlying depression or history of disordered eating (29). There is a correlation between diet failure and poor mood, so these psychological impacts should not be ignored.

Cognitive Function Changes

Research indicates cognition suffers both acutely within hours of calorie restriction and chronically during prolonged dieting (30).

Mental effects include (31):

• Reduced alertness and concentration
• Impaired memory retention
• Slower information processing speed
• Compromised problem-solving abilities

Brain regions related to reward and inhibition are especially susceptible to deficits when nutrients are scarce (32). This impairs judgment pertaining to impulsive eating behaviors.

If school, work, or daily life feels noticeably harder while dieting, it could signify impaired cognition rather than just feeling grumpy or tired.

Eating Behavior Changes

Dieting provokes biological and psychological drives to seek out and consume calorie-dense foods. Cravings intensify as the body signals us to replenish dwindling energy stores and stranded glucose.

Research shows dieting for as little as two weeks alters eating behaviors and attitudes around food (33):

• Increased liking and want for high-calorie foods
• Decreased fullness and satisfaction from meals
• More disinhibited, impulsive eating
• Higher food reward sensitivity

These changes make it exceedingly difficult to stick to a regimented diet over months or years. Eventually, the unavoidable urges and temptations subvert even the strictest diet regimens for most people.

Long-Term Consequences

While dieting produces fairly rapid results on the scale, the long-term consequences unfold gradually over months or years after starting weight loss attempts. Dieting impacts future weight management, health, and lifestyle in complex ways.

Weight Management Challenges

The body perceives self-imposed starvation as a significant threat to survival. This elicits potent biological and behavioral adaptations that predispose the body to weight regain after dieting.

Metabolic Adaptation

As described earlier, metabolic rate slows in proportion to calorie restriction. Total energy expenditure can decrease by 15% or more on a weight loss diet (34).

This suppressed metabolism persists even after resuming normal eating habits. The body effectively fights to return to its original set point weight by defending against calorie burning (35).

Yo-Yo Dieting Effects

Due to metabolic adaptation, dieters commonly experience the “yo-yo” of regaining lost weight after stopping a diet. This weight cycling stresses the body and has detrimental health effects (36).

Yo-yo dieting contributes to accelerated loss of lean mass, fat overshooting, changes in metabolism, and weight gain above baseline (37, 38). The repeated restriction worsens future struggles with hunger, cravings, and weight regain.

Cardiovascular and Health Effects

Initial improvements in heart health markers and diabetes risk from losing weight may not persist long-term after diet cessation.

Blood pressure, cholesterol ratios, and insulin sensitivity tend to revert in moderate dieters and worsen in extreme dieters (39). Weight regain also contributes to inflammation, oxidative stress, and worse cardiovascular risk (40).

Immune function may decrease after weeks of calorie restriction (41). This impairment leaves dieters more vulnerable to infections and illness during dieting phases.

Bone mineral density decreases with chronic dieting, especially in women. Osteopenia and osteoporosis risks increase when body weight drops below the biological set point (42).

Lifestyle Impact

Dieting often involves regimented meal plans, forbidden foods, and extensive effort calculating calories and macros. This level of dietary control and preoccupation is difficult to reconcile with normal social activities and leisure.

Binge eating is more likely after prolonged dietary restraint. Post-diet weight gain may also prompt shame, guilt, and feelings of failure. These psychological stressors can breed negative relationships with food and exercise.

The dietary monotony, deprivation, and restriction inherent to most weight loss diets are not sustainable lifelong lifestyle changes. Lasting weight management requires moderation and balance, not extremes.

Healthy and Sustainable Weight Loss Strategies

Despite the metabolic, physiological, and psychological challenges of dieting, safe weight loss is still achievable through science-backed strategies. Here are realistic principles for successful, maintainable fat loss:

• Aim for 1-2 pounds of fat loss per week through a modest calorie deficit no greater than 1000 calories below maintenance needs. This conservative pace gives the body time to adapt while preserving muscle.
• Strength train with heavy weights at least twice weekly to preserve and build lean mass, which helps maintain an elevated metabolism.
• Emphasize protein at each meal to support muscle retention and satiety. Get at least 0.7-1 gram per pound of body weight daily.
• Fill up on fiber-rich whole foods like vegetables, fruits, beans, lentils, whole grains. These provide nutrients and bulk for few calories.
• Address emotional eating and stress management through counseling, mindfulness, or self-care practices. Understanding the psychology behind overeating thwarts mindless eating.
• Practice flexible dieting principles like 80/20 and If It Fits Your Macros (IIFYM). Allow yourself treats in moderation without guilt. Deprivation backfires long-term.
• Make gradual lifestyle changes to promote sustainable new habits. Quick fixes and extreme regimens inevitably fail over months or years.

Conclusion

Dieting triggers a complex cascade of metabolic, hormonal, and neurological adaptations. These help explain the challenges dieters face battling hunger, low energy, cravings, and weight regain. Psychological impacts like depression and reduced cognition also impair the ability to sustain calorie restriction long-term.

However, safe weight loss is achievable through evidence-based strategies that respect the body’s biological drives. Slower fat loss paired with heavy strength training, high protein intake, and lifestyle changes beats extreme dieting for optimal body composition and well-being. A thoughtful, flexible approach helps make weight management sustainable lifelong.

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Frequently Asked Questions

What causes the metabolic slowdown while dieting?

The drop in metabolic rate is an adaptive response to the calorie deficit created by dieting. When the body senses it is receiving insufficient energy from food to meet its needs, it downregulates metabolic processes to conserve energy and prevent starvation. Multiple mechanisms kick in to suppress calorie burning, including reductions in NEAT (non-exercise activity thermogenesis), thyroid hormone production, sympathetic nervous system activity, and loss of metabolically active lean tissue along with fat. This metabolic adaptation persists even after stopping the diet until weight stabilizes at a new set point.

Why do cravings get worse on a diet?

Food cravings intensify when dieting due to shifts in hormone levels and neurotransmitters in the brain. Levels of the hunger hormone ghrelin rise, while leptin, the satiety hormone, drops. This amplifies hunger signals. Dieting also seems to sensitize reward pathways in the brain, so cravings for high-calorie foods increase even though the body has less need for the calories. Furthermore, blood sugar instability can lead to cravings, especially for carbohydrate-rich foods.

How much muscle loss is normal when losing weight?

Some loss of lean body mass from muscle and bone is inevitable when losing weight through calorie restriction. Experts estimate around 25% of total weight loss comes from lean mass and 75% from body fat stores. However, lifting weights helps counteract muscle loss. Getting adequate protein, losing weight slowly, and strength training allows dieters to preserve more muscle. Novice lifters may even experience a slight muscle gain during dieting phases.

Does intermittent fasting slow your metabolism like regular dieting?

Emerging research indicates intermittent fasting may be an exception. Studies show fasting in 24-hour windows or alternate day fasting do not decrease resting metabolic rate as much as daily calorie restriction. The periodic fasting periods may mitigate the adaptive response that slows metabolism due to maintained lean body mass and hormone levels. However, more research is still needed on intermittent fasting and long-term weight maintenance.

Why are crash diets ineffective for sustainable weight loss?

The extreme calorie deficits created by crash diets shock the body and elicit strong adaptive responses. Hunger hormones skyrocket while leptin plummets. This fuels rebound overeating when resuming normal eating, often leading to weight regain exceeding what was lost initially. Crash diets also burn muscle for fuel when calories are inadequate, further suppressing metabolism. Such rapid weight loss is rarely sustainable in the long run for these reasons.

How does dieting impact your mood and emotions?

Calorie restriction can negatively affect mood in several ways. Depriving the brain of adequate glucose seems to impair mood and motivation. Hunger pangs and food obsession also breed irritability. Dieting socially isolates people, while feelings of deprivation and guilt around “cheat” meals take a psychological toll. Furthermore, producing rapid results through unsustainable methods often sets dieters up for disappointment and shame when they inevitably regain weight. All these factors contribute to the “dieter’s depression.”

Does dieting weaken your immune system?

Some research indicates calorie restriction and nutrient deficiencies associated with dieting may impair immune cell function. However, effects seem to depend on the degree of restriction. Moderate calorie deficits don’t appear to harm immunity much. But more severe deprivation below 50% of maintenance needs may reduce white blood cell counts, immunity markers, and response to vaccines. Ensuring adequate protein and micronutrients can help counteract this immune suppression during dieting phases.

How can you diet without negatively impacting health?

The key is avoiding extremes. Modest calorie deficits no greater than 500-1000 calories below maintenance needs promote steady, sustainable fat loss without shocking the body. Emphasizing nutritious whole foods ensures sufficient vitamins, minerals and antioxidants. Lifting weights maintains valuable calorie-burning muscle. Allowing yourself planned indulgences in moderation prevents feelings of deprivation. Addressing the emotional side of overeating helps change lifelong eating habits. With these strategies, dieting can successfully trim fat without adverse effects.