Lyme disease weight gain is one of the least talked about—and most misunderstood—symptoms of Lyme disease.

For many patients, weight gain and weight loss become confusing and distressing parts of the illness. Some people lose weight quickly without trying. Others gain weight despite eating the same—or even less. These changes reflect how Lyme disease affects inflammation, metabolism, hormone signaling, gut function, sleep, and the autonomic nervous system.

Patients often describe similar patterns in different ways. Some say, “I keep losing weight and I’m not dieting.” Others report, “I’ve gained so much weight since getting Lyme, and nothing else changed.” Many notice appetite swings that range from nonexistent to unusually strong. These shifts are common in Lyme disease and arise from real physiologic mechanisms—not lifestyle choices or lack of willpower.

If you want to understand how Lyme affects the nervous system more broadly, see Signs and Symptoms of Untreated Lyme Disease.

Why Lyme Disease Causes Weight Changes

Lyme disease can drive metabolic changes in both directions. Early in the illness, inflammation and infection-related stress may push weight downward. As the disease evolves, autonomic instability, hormonal disruption, gut changes, sleep disturbance, and reduced activity can shift the body toward weight gain.

These patterns reflect how the body adapts to ongoing physiologic stress rather than a single, fixed metabolic state.

Why Lyme Causes Weight Loss

Some patients experience unintentional weight loss early in infection or during flares because several mechanisms often converge at once.

Inflammatory Metabolism

Active infection increases metabolic demand. Inflammation raises resting energy expenditure and alters how the body uses fuel, making it harder to maintain weight even with normal intake.

Loss of Appetite

Inflammation disrupts appetite-regulating hormones and affects gut function. Many patients describe nausea, early fullness, altered taste, or simply feeling too unwell to eat.

Increased Energy Expenditure

Immune activation, fever, and sympathetic overdrive burn more calories even without increased physical activity. Fighting infection requires substantial energy.

Gut Dysfunction

Lyme and associated infections can slow motility, disrupt digestion, and impair nutrient absorption. Bloating, diarrhea, constipation, and gastroparesis-like symptoms can reduce intake and absorption at the same time.

Lyme Disease Weight Gain: Why It Happens

Just as commonly, patients develop gradual—or sometimes sudden—weight gain as the illness progresses.

Autonomic Nervous System (ANS) Dysfunction

ANS disruption can slow metabolic rate and alter how the body uses and stores energy. Even without dietary changes, the body may shift toward conserving calories and storing fat.

Inflammation and Hormonal Effects

Inflammation affects insulin sensitivity, cortisol rhythms, and fat distribution. This can promote central weight gain and fluid retention.

Thyroid and Hormonal Disruption

Even mild thyroid or cortisol irregularities can slow metabolism. Many patients experience hypothyroid-like symptoms despite laboratory values that fall within reference ranges.

Reduced Physical Activity

Pain, fatigue, dizziness, and post-exertional worsening reduce daily movement. As activity decreases, total energy expenditure falls, increasing the likelihood of weight gain.

Sleep Disturbance

Poor sleep alters appetite and metabolic hormones. Lyme-related insomnia or circadian disruption can increase hunger, reduce satiety, and impair glucose regulation—conditions that favor weight gain.

Why Lyme Disease Weight Gain Gets Missed

Weight changes may be overlooked or dismissed because they develop gradually or appear nonspecific. Standard blood tests—including thyroid panels, glucose levels, and cortisol—may look normal even when the body is under significant metabolic strain. Clinicians unfamiliar with infection-related metabolic changes may not immediately connect weight instability to Lyme disease.

Normal laboratory results do not rule out infection-related metabolic or autonomic dysfunction.

For general medical reference, see the Mayo Clinic overview on Lyme disease.

Why Weight Can Swing in Both Directions

Many patients experience early weight loss followed by later weight gain, or the reverse. Early infection may accelerate metabolism and suppress appetite. Later, autonomic dysfunction slows metabolic rate, hormones fluctuate, activity levels drop, sleep becomes irregular, and gut function may change.

Weight often mirrors the body’s attempt to adapt to these shifting physiologic pressures.

What We Still Don’t Know

Although Lyme disease weight gain and weight loss are frequently reported, formal research specifically examining weight changes in Lyme disease remains limited. Much of what we understand comes from established effects of inflammation, autonomic dysfunction, hormonal disruption, sleep disturbance, and consistent clinical patterns.

More research is needed to clarify why these metabolic shifts occur, who is most affected, and how best to support patients experiencing them.

What Can Help

For patients navigating these symptoms, understanding the full range of Lyme disease treatment options can help guide conversations with your care team. As underlying infection, inflammation, and autonomic dysfunction are addressed, many patients find that weight begins to stabilize. Recovery is possible, though it often requires patience and individualized care.

Your Experience Matters

Unexpected weight loss or weight gain can feel unsettling and discouraging. These changes reflect the body’s response to infection, inflammation, autonomic instability, and disrupted metabolism. Understanding the physiologic reasons behind them can reduce self-blame and provide clarity.

Has Lyme disease changed your weight over time? Share your experience in the comments—your story may help someone else feel understood and less alone.

Frequently Asked Questions

Is weight loss or weight gain common in Lyme disease? Yes. Both unintentional weight loss and weight gain are commonly reported by patients with Lyme disease and may occur at different stages of illness.

Can Lyme cause weight changes even if my diet hasn’t changed? Yes. Lyme disease can affect inflammation, metabolism, hormone signaling, gut function, sleep, and autonomic regulation—all of which influence weight independent of diet.

Why do my lab tests look normal if my weight is changing? Standard blood tests may not capture infection-related metabolic strain or autonomic dysfunction. Normal results do not exclude physiologic causes of weight instability.

Can weight changes reverse over time? For many patients, weight stabilizes or shifts again as inflammation improves, sleep normalizes, autonomic balance returns, and activity levels gradually increase.

Should weight changes in Lyme disease be taken seriously? Yes. While common, unexpected weight changes can be distressing and should be evaluated in clinical context, especially if they are rapid or persistent.

References

1. Cell Microbiol (2017): Zlotnikov N, et al. Infection with the Lyme disease pathogen suppresses innate immunity in mice with diet-induced obesity.
2. Med Sci Sports Exerc (2015): D’Adamo CR, et al. Supervised resistance exercise for patients with persistent symptoms of Lyme disease.