GLP-1s for Weight Loss: How to Manage Side Effects, Nutrition, and Results

Weight loss is one of the most misunderstood areas in medicine. For decades, the standard advice was simple: eat less, move more. But for the millions of Canadians struggling with obesity, that advice has fallen short. As of 2022, roughly 30% of Canadians are living with obesity, and that number is projected to climb toward 50% within the next decade.

The emergence of GLP-1 medications has changed the conversation. These drugs target the biological root of obesity at the brain level. Understanding how they work, what to expect, and how to support your body throughout treatment can make a significant difference in your outcomes.

To view Harrison’s recent clinician-led webinar on this topic, click here.

Why Obesity Is a Medical Condition, Not a Lifestyle Choice

Obesity is a chronic, progressive, inflammatory condition; it is not a failure of willpower. It affects virtually every organ system in the body, from the cardiovascular system to the musculoskeletal system and even the reproductive system. People with obesity may face higher rates of cancer and often experience more challenging health outcomes.

The main symptom of obesity is heightened appetite, which begins in the brain. When the appetite system is dysregulated, it drives excess food intake, elevated BMI, chronic inflammation, and a cascade of complications.

Three key brain regions govern appetite:

• The hypothalamus: Controls basic hunger and fullness signals, regulated in part by GLP-1 hormones
• The mesolimbic system: The brain’s reward centre, driven by dopamine and opioid pathways (also the region implicated in addiction)
• The prefrontal cortex: Governs decision-making and executive function, which is why cognitive behavioural therapy can also support weight management

Why Many Weight Loss Attempts Fail

Your body is biologically designed to resist weight loss. For most of human history, food scarcity was a serious threat, and our bodies evolved powerful mechanisms to prevent starvation. Those mechanisms are still working, whether we need them to or not.

There are two main ways that this biological response prevents weight loss when a calorie deficit strategy is used:

1. First, as you lose weight, you lose both fat and muscle. Less muscle means your resting metabolism drops. The “calories out” side of the equation shrinks, and you can’t compensate indefinitely by eating less.
2. Second, the brain detects weight loss and responds by amplifying hunger signals and suppressing fullness hormones. At first, the signal is manageable. Then it becomes harder to ignore. Eventually, it becomes overwhelming and you are driven to eat. The weight returns and, because fat comes back faster than muscle, your body composition is actually worse than before.

Genetics also play a significant role. Research using functional MRI has shown that people with obesity have greater activation in the brain’s reward system when exposed to images of sweet, salty, and processed foods, suggesting this is a hardwired, biological trait. In short, “food noise” (the persistent, intrusive mental chatter about food) is measurably louder in those with obesity because of their neurobiology, not a lack of discipline.

The Three Evidence-Based Treatments for Obesity

There are only three approaches proven to produce meaningful, sustainable weight loss:

1. Psychotherapy: targets the prefrontal cortex to address the thoughts, emotions, and behaviours connected to eating.
2. Medications: target the hypothalamus to promote fullness, or the mesolimbic system, to reduce pleasure associated with food.
3. Bariatric surgery: changes the anatomy of the stomach and/or small intestine to reduce calorie absorption (appropriate for only a select group of individuals).

Proper nutrition and physical activity are essential complements to treatment, but evidence consistently shows they cannot produce significant, lasting weight loss on their own in most people with obesity.

How GLP-1 Medications Work

There are several medications currently used for weight management. Three deserve particular attention:

Bupropion-Naltrexone (Contrave)

This combination pill works primarily on the brain’s reward system. Bupropion, which is historically used for depression and smoking cessation, boosts dopamine activity to reduce hunger. Naltrexone, traditionally used to reduce alcohol cravings, blocks the opioid receptors that make eating feel rewarding. This medication is best suited for people who experience strong food cravings and emotional eating.

Semaglutide (Ozempic / Wegovy)

Semaglutide is a GLP-1 receptor agonist, which means it mimics the body’s naturally occurring GLP-1 fullness hormone. It works by increasing satiety, decreasing hunger, and slowing gastric emptying so food moves more slowly out of the stomach. It also appears to reduce food noise, suggesting some interaction with the reward system, though this mechanism isn’t fully understood yet.

Tirzepatide (Mounjaro / Zepbound)

Tirzepatide is the newest agent available and acts as a dual agonist, mimicking both GLP-1 and another hormone called GIP. On its own, GIP doesn’t strongly suppress appetite, but in combination with GLP-1, it significantly enhances weight loss and glucose control. GIP also appears to help mobilize stored fat directly and may reduce the nausea that some people experience on GLP-1 alone. Head-to-head clinical trials show tirzepatide outperforms semaglutide on weight loss outcomes.

Are You a Candidate for GLP-1 Therapy?

GLP-1 medications are generally indicated for individuals with:

• A BMI of 30 or higher, or a BMI of 27 or higher with a weight-related health condition such as high blood pressure, type 2 diabetes, or sleep apnea.

It’s important to remember that BMI is just one data point and has its limitations as a health metric. Your Harrison team will conduct a thorough assessment beyond just BMI. We look at your complete health picture, including your personal and family medical history, lifestyle factors, and other relevant tests.

Whether GLP-1 therapy is right for you is a deeply personal decision. Your team can walk you through what a realistic treatment commitment looks like.

What to Expect: Side Effects and How to Manage Them

The most common side effects of GLP-1 medications are:

• Diarrhea or constipation
• Abdominal discomfort and bloating
• Early satiety (feeling full quickly)
• Heartburn
• Nausea
• Vomiting (less common)
  
  

These effects are most pronounced when starting the medication or increasing the dose and typically ease over time. Here’s how to manage them:

Start low, go slow: Your physician will begin you on a low dose and increase it gradually; typically no sooner than every four weeks.

Stay hydrated: Aim for at least two litres of fluid daily. Sipping water consistently throughout the day helps with nausea, constipation, and diarrhea.

Stop eating when full: Eating past fullness significantly worsens symptoms. Try serving yourself smaller portions and checking in before going back for more. Using a smaller plate can also be helpful to prevent getting overly full.

Get enough fibre: Women should aim for 25 grams per day, men for 38 grams. Fibre helps regulate digestion in both directions. If you currently eat a diet low in fibre, speak with your dietitian to create a plan to slowly increase your fibre intake. Increasing your fibre intake too quickly can make your stomach upset.

Watch your trigger foods: Common culprits include high-fat or high-sugar foods, citrus, tomatoes, spicy food, garlic, onion, carbonated drinks, alcohol, and caffeine. You may not react to all of them, but it’s important to pay attention to patterns.

Separate food and fluids: If you are having symptoms, try leaving 30 to 60 minutes between eating and drinking.

Serious but rare complications include acute pancreatitis, gallbladder disease, and, in those with pre-existing severe GI motility disorders, potential bowel complications. There is also a theoretical risk of medullary thyroid carcinoma based on rodent studies, though no cases have been observed in humans to date.

Nutrition on GLP-1s: Eating Less, but Better

GLP-1 therapy typically reduces food intake by up to 40%. While that reduction is part of how the medication works, it also creates a real risk of nutritional deficiencies; particularly protein, fibre, iron, calcium, magnesium, and zinc. People with higher body weights already face elevated rates of nutritional gaps, and reduced intake can make this worse.

Your goals with nutrition during GLP-1 treatment are to:

1. Reduce fat mass and associated health risks
2. Preserve lean muscle mass
3. Ensure nutritional adequacy
4. Support overall wellbeing
5. Improve medication tolerance and adherence

Prioritize Protein

Protein is essential for satiety and maintaining muscle. For many people on GLP-1s, a target of 1 to 1.6 grams of protein per kilogram of body weight per day is appropriate, though the right amount varies based on age, size, and health conditions. High-quality sources include meat, eggs, tofu, beans, and lentils. Protein powder can be used as a supplement when appetite is low.

Choose Whole Foods

Ultra-processed foods including sugary drinks, packaged snacks, fast food, and processed meats are associated with higher body weights and poorer health outcomes. Reaching for fresh or frozen vegetables, lean proteins, and plant-based options supports both medication tolerance and optimal long-term health markers like blood sugar, cholesterol, and blood pressure.

Eat Smaller, More Frequent Meals

Rather than two or three large meals, aim to distribute your intake across the day. This approach helps manage side effects, supports adequate protein intake, and reduces the risk of nutrient gaps.

Your Harrison Registered Dietitian can assess your current intake, identify deficiencies, and build a plan that fits your preferences, schedule, and health goals. Research consistently shows that greater dietitian involvement during GLP-1 treatment leads to better outcomes across the board.

Exercise: Protecting Muscle and Building Longevity

Across GLP-1 clinical trials, roughly 25 to 30% of total weight loss can come from lean mass, not just fat. This matters for more than aesthetics. Low muscle mass is associated with increased risk of diabetes, falls, and significantly higher all-cause mortality. If you’re on a GLP-1, protecting your muscles has to be part of the plan.

Strength Training

Aim for at least two structured strength training sessions per week, targeting all major muscle groups: shoulders, arms, back, chest, core, and legs. The key to avoiding a plateau is progressive overload: gradually increasing the resistance over time. This means that if a weight no longer feels challenging by the last couple of repetitions in a set, it’s time to go slightly heavier.

While generic plans can be a starting point, a program designed specifically for you by a qualified professional will always be more effective. Your Harrison Exercise Physiologist can create a plan tailored to your needs and lifestyle.

Cardiovascular Exercise

Some patients reason that since GLP-1s are already helping with fat loss, cardio isn’t necessary. The evidence says otherwise. A 2024 study published in The Lancet found that patients who combined GLP-1 therapy with regular cardio maintained at least 10% of their total weight loss after treatment ended, compared to those who didn’t exercise.

Beyond weight maintenance, cardiorespiratory fitness (which can be measured as VO₂) is one of the strongest predictors of long-term health and longevity. This does not mean one must train to become an elite athlete. Moving from low to above-average fitness is associated with a 10 to 15% reduction in mortality risk.

Frequently Asked Questions

Can I stop my GLP-1 medication once I’ve reached my goal weight?

This is the most common question, and the honest answer is: probably not. GLP-1 medications work by regulating the brain’s appetite system. Stop the medication, and appetite typically returns, along with most of the weight lost. There is a small exception for patients who maintain a high volume of cardio exercise, but regaining weight is the norm. Your physician will help you understand what a long-term commitment looks like before starting treatment.

How much weight should I expect to lose?

Rather than focusing on an absolute number, consider what percentage of body weight loss is needed to improve your specific health conditions. For context:

• 5% weight loss can lower blood pressure and improve blood sugar
• 5–10% can prevent diabetes, improve fertility, reduce cholesterol, and address fatty liver
• 10–15% begins to improve cardiovascular disease, sleep apnea, and acid reflux
• Over 15% can produce complete remission of type 2 diabetes and reduce cardiovascular mortality

My appetite has dropped significantly since starting my medication. Should I be concerned?

Not necessarily. The medication’s appetite-suppressing effect is strongest in the early weeks. This is expected. Work closely with your dietitian to ensure you’re still meeting your nutritional needs, but don’t panic if hunger feels very low at first. It tends to level off as your body adjusts.

How often should I check my body composition?

This varies by individual and treatment plan. Home bioimpedance scales can provide useful trend data over weeks. Don’t rely on day-to-day fluctuations. For a more precise picture, a DEXA scan is the gold standard, though it isn’t necessary for everyone. At Harrison, we also offer a SECA scans at all locations. Your exercise physiologist can help determine the right monitoring schedule for your situation.

Making the Most of GLP-1 Therapy

GLP-1 medications represent a genuine shift in how we treat obesity, grounded in biology, not blame. They work because they address what actually drives weight gain: an overactive appetite system that, for many people, is genetically wired to resist change.

If you’re considering GLP-1 therapy or want to optimize your current treatment, our integrated team of physicians, dietitians, and exercise physiologists at Harrison is here to support you with a personalized, evidence-informed approach.

Click here to watch our clinician-led webinar for a deeper look at GLP-1 therapy.