Why It Can Be So Hard to Lose Weight, According to Science

Reviewed by Dietitian Jessica Ball, M.S., RD

Key Points

• Weight loss can be challenging because the body resists it by increasing hunger and slowing metabolism.

• Sustainable habits like eating protein, managing stress and sleeping well can support weight loss.

• Minimally-processed foods help regulate hunger and energy, making weight loss easier to maintain.

Weight loss can sound straightforward in theory, but sometimes it’s much harder in practice. Even with consistent changes to diet and exercise, progress may slow or stall, leaving many people wondering what they’re doing wrong. While lifestyle habits do play a role, research shows that weight loss is influenced by complex biological systems that regulate hunger, metabolism and energy balance.

As you lose weight, your body responds with changes that can make continued weight loss more difficult. Hormones shift, metabolism slows and hunger often increases—all of which can work against your efforts. Here’s what the science says about why losing weight can be challenging.

Your Body Actively Defends Against Weight Loss

One of the most important—and often overlooked—reasons weight loss can be difficult is that the body is biologically wired to resist it. From an evolutionary perspective, protecting against weight loss helped humans survive periods of food scarcity. But today, those same survival mechanisms can make it harder to lose weight and keep it off.

“As [people] lose weight, hunger hormones like ghrelin increase, while satiety signals such as leptin decrease, creating a powerful biological drive to regain weight,” says Flavio Mitidieri Ramos, M.D., M.Sc., FASGE. “This is not a failure of motivation, but a normal physiological response rooted in survival mechanisms designed to prevent starvation.”

Because these responses are part of the body’s built-in survival system, they can’t be eliminated—but how you approach weight loss can influence how strongly they occur. Significant calorie restriction and rapid weight loss may amplify hunger, which can make progress harder to maintain. Instead, eat regular, balanced meals that include protein, fiber and healthy fats to help you stay satisfied, maintain consistent energy levels and support changes that are easier to sustain over time.

Metabolism Slows During Weight Loss

In addition to increasing hunger, the body also adapts to weight loss by reducing how many calories it burns. This process, known as metabolic adaptation or adaptive thermogenesis, occurs when the body conserves energy in response to reduced calorie intake and weight loss.

“What people need to understand is that weight loss is more than just calories in versus calories out, because your body actually fights to hold on to energy when you reduce your calorie intake,” says Hector Perez, M.D., a physician board-certified in obesity and metabolic diseases.

Research shows that resting metabolic rate (RMR)—the calories your body burns at rest—often decreases with weight loss, meaning your body requires fewer calories to perform basic functions. This reduction in energy expenditure can slow progress over time, even when eating and exercise habits remain consistent.

Lean muscle plays a major role in metabolic rate, so preserving muscle mass during weight loss may help limit the extent of metabolic slowdown. Strength training, eating enough protein and avoiding prolonged periods of undereating can help support muscle maintenance and overall metabolic health. While these strategies won’t prevent metabolic adaptation entirely, they may help support a higher metabolic rate and make weight loss easier.

Sleep and Stress Directly Affect Appetite and Insulin Sensitivity

Sleep and stress play an important role in regulating hormones that control hunger, metabolism and fat storage. “Poor sleep directly alters leptin, ghrelin and insulin pathways, leading to increased cravings and impaired glucose control,” says Perez. Research also shows that sleep deprivation is associated with increased calorie intake, particularly from foods high in fat and carbohydrates.

Inadequate sleep has also been linked to reduced insulin sensitivity, which affects how efficiently the body uses and stores glucose. When insulin function is disrupted, the body may be more likely to store excess energy as fat rather than use it for fuel. Chronic stress can further compound these effects by keeping cortisol levels elevated, reinforcing signals that increase hunger and energy storage.

To make weight loss more achievable, make getting enough quality sleep non-negotiable, and find ways to manage daily stress. Even small changes—like sticking to a consistent bedtime or building time into your day to unwind—can help support appetite regulation, metabolism and weight loss.

Ultra-Processed Foods May Disrupt Appetite Regulation

Biological changes aren’t the only factors that make weight loss difficult. “Constant exposure to ultra-processed foods can further disrupt appetite regulation,” says Nicole Avena, Ph.D..

Ultra-processed foods—such as chips, soda, fast food and packaged snacks—are designed to be highly palatable and easy to eat quickly. Compared to minimally processed foods, they are often digested more rapidly and lower in nutrients that promote fullness, like fiber and protein. This combination can make hunger return sooner and increase the likelihood of eating more throughout the day.

Research also suggests that ultra-processed foods can alter signaling in brain regions involved in appetite and reward, which may increase cravings and make it harder to regulate food intake, explains Avena.

To help support appetite regulation, aim to build most meals around minimally processed foods such as vegetables, fruits, whole grains, legumes and lean proteins. These foods digest more slowly, support steadier energy levels and help promote more consistent hunger and fullness signals.

Tips for Healthy, Sustainable Weight Loss

While weight loss can be challenging, certain habits can help. Focus on these consistent, sustainable changes:

• Prioritize protein and fiber at meals. Protein and fiber increase satiety and promote stable blood sugar, which can help reduce cravings. Protein also supports lean muscle mass, which is associated with a higher metabolic rate.

• Incorporate resistance training. Aim for 2 to 3 sessions per week, focusing on major muscle groups using bodyweight exercises, resistance bands or weights.

• Get enough quality sleep. Aim for 7 to 9 hours of sleep each night and try to go to bed and wake up at consistent times, even on weekends. Creating a wind-down routine—such as dimming the lights, limiting screen time before bed or reading—can help improve sleep quality.

• Manage stress in small, consistent ways. Take a walk, spend time outdoors, practice deep breathing or set aside time to relax. Even just a few minutes per day can help support healthier routines and make weight loss easier to maintain.

• Choose mostly minimally processed foods. Aim to build most meals around whole or minimally-processed options. Keep convenient staples on hand—such as frozen vegetables, canned beans or pre-cooked grains—to make it easier to prepare balanced meals at home.

• Avoid restrictive dieting. Instead of drastically cutting calories, focus on building balanced meals and maintaining a consistent eating schedule. Eat regular meals and snacks throughout the day to stay energized and limit cravings.

Our Expert Take

Weight loss isn’t about willpower—it’s influenced by hormones, metabolism and other biological systems that help regulate hunger and energy balance. As you lose weight, your body naturally adapts in ways that can make continued weight loss more challenging.

The most effective approach is to focus on sustainable habits, such as eating balanced meals, preserving muscle through resistance training, getting enough sleep and managing stress. These strategies can help support your body’s natural systems and make weight loss easier to achieve and maintain over time.

Read the original article on EatingWell