Is Diet or Exercise More Important for Weight Loss?

Losing excess weight is crucial to decreasing your risk of heart disease and other chronic diseases. However, should you focus more on what you eat or how much you move? Recent research may have answered the debatable question: Is diet or exercise more important for weight loss?

Here, I’ll provide a registered dietitian’s perspective on a groundbreaking study published in Proceedings of the National Academy of Sciences of the United States of America (PNAS) that sheds new light on this topic.

Let’s Talk Weight Loss Basics

Weight loss isn’t always as simple as “calories in versus calories out”. There are other factors involved that make weight loss more complex, such as:

• Hormones- like insulin, cortisol, leptin, and ghrelin, which affect hunger, satiety, and fat storage
• Chronic stress and poor sleep, which can disrupt these hormones, increase cravings and make it harder to stick to healthy habits
• Genetics
• Underlying medical conditions
• Gut health

That said, weight loss still occurs when you consistently consume fewer calories than your body expends. This calorie deficit leads your body to tap into fat stores for fuel.

Research suggests a daily calorie deficit of 500 to 750 calories below your maintenance level or estimated daily energy expenditure (TDEE) is considered a safe and effective range for gradual, sustainable weight loss. This typically results in a weight loss of about 1 to 1.5 pounds per week, though individual results vary.

Understanding your body’s needs and how these factors impact metabolism can help create a more personalized and effective weight loss approach beyond simply counting calories.

Findings from the PNAS Study

A recent study published in the Proceedings of the National Academy of Sciences of the United States of America (PNAS) adds new perspective on whether diet or exercise is more important for weight loss.

After analyzing energy expenditure data from adults in 34 populations across six continents and accounting for differences in lifestyle and economic development, they found that while physical activity does increase energy expenditure in the short term, the body adapts over time by reducing energy spent on other biological processes behind the scenes—such as immune function, reproduction, and cellular repair.

In other words, our total daily energy burn doesn’t increase indefinitely with more exercise. Instead, the body compensates by conserving energy elsewhere, which helps explain why physical activity alone may not always lead to significant weight loss.

The study also pointed to a stronger connection between diet quality and obesity rates, especially in economically developed populations. These findings show how diets containing mostly ultra-processed foods can contribute to excess energy consumption and increased obesity risk. In addition, they support the idea that diet plays a more dominant role in driving weight gain and loss, while emphasizing the importance of food environment and accessibility.

Why Diet Plays a Bigger Role in Weight Loss

Diet plays a bigger role in weight loss for several key reasons.

1. It’s Easier to Control Calories Through Food Than Burn Them Off

Tracking and adjusting which foods and how much you are eating/drinking is far more manageable than trying to out-exercise a high-calorie diet. For example, consuming 300 calories may take just a few minutes, but burning them through exercise could require a 30-minute jog—under ideal conditions.

There are many meal tracking apps available that can help you track your intake. I personally recommend the MyNetDiary app, which is available in the App Store and on Google Play.

Disclosure: I work for MyNetDiary as a consulting dietitian, but for real, this app is easy to set-up and makes meal tracking a breeze. I’m happy to discuss more if you message me!

2. Relying on Exercise Alone Can Lead to Frustration

Exercise burns fewer calories than most people expect, and the body can adapt by reducing energy used for other internal processes. Without dietary changes, weight loss progress can stall, even with regular workouts.

Even if you wear activity trackers like a FitBit or Garmin, the accuracy of these devices at tracking energy expenditure is marginal at best. Research shows that wrist-wearable activity trackers have a 30% average margin of error!

3. Diet Quality Matters

Whole, minimally processed foods like fruits, vegetables, legumes, whole grains, and lean proteins are lower in calories and higher in fiber and nutrients. These foods support appetite regulation, blood sugar balance, and feelings of fullness.

If you are wondering which dietary patterns focus on these principles- there are many! The Mediterranean diet and the DASH diet are two of my top whole-foods-focused diet choices. You can lose weight by following either of these patterns, assuming a calorie-controlled plan is also applied.

4. Ultra-Processed Foods Promote Overeating

Highly processed foods are often high in added sugars, refined carbs, sodium, and unhealthy fats—and low in fiber and essential nutrients. Their hyper-palatable nature makes them easy to overconsume and less satisfying.

🔬 A small 2019 randomized controlled trial found that people eating ultra-processed foods consumed about 500 more calories daily and gained weight compared to those eating an unprocessed diet, even when nutrient content was matched.

5. Portion Sizes Are Larger Than Ever

Over the past few decades, portion sizes (especially in restaurants and packaged foods) have increased dramatically. Larger portions skew perceptions of normal eating and contribute to excess calorie intake.

You see, portion size differs from serving size. A serving size is a standardized amount of food, basically the amount we SHOULD be consuming. Portion size is the amount of food you actually eat, which many times is larger than the serving size.

6. Food Additives May Influence Weight Regulation

Ingredients like high-fructose corn syrup, emulsifiers, and artificial sweeteners may increase energy density, alter gut microbiota, and impact hunger signals, which makes weight management more difficult.

Efforts continue around the globe to eliminate artificial ingredients for health reasons. Earlier this year, we celebrated a win here in the U.S. when the FDA banned red dye number 3 from foods and supplements. YAY! 🎉

Still, we need to read food labels carefully, including the ingredient list, to be sure food items do not contain any questionable additives.

Don’t Discount Exercise — It Still Matters

While diet plays a leading role in weight loss, exercise is still an essential part of the equation, especially when it comes to long-term success and overall health. Here’s why staying active matters:

• Improves overall health: Regular physical activity supports cardiovascular fitness, helps regulate blood pressure, improves insulin sensitivity, and boosts mental well-being.
• Preserves lean muscle mass during weight loss: Without resistance training or movement, some of the weight lost through dieting may come from muscle. Exercise helps maintain muscle mass, which supports metabolism and functional strength.
• Boosts mood and reduces stress: Physical activity stimulates the release of endorphins, which can elevate mood, reduce anxiety, and help you better manage stress—making it easier to stick to healthy eating habits.
• Helps prevent weight regain: Studies show that people who maintain long-term weight loss often engage in regular physical activity, which is arguably more important than diet in preventing weight regain.
• Encourages a healthier lifestyle mindset: Incorporating movement into your daily routine can reinforce psychological resilience, mental health, and health-related behavior.

In short, exercise may not be the primary driver of weight loss, but it plays a powerful supporting role in helping you feel better, move better, and maintain your progress.

Best Approach for Combining Diet and Exercise

For sustainable, long-term weight loss and overall health, the most effective strategy is combining a healthy diet with regular physical activity. Each plays a unique role in the following ways:

1. Diet creates the caloric deficit needed for weight loss.
2. Exercise supports:
   • Metabolism (by preserving muscle mass)
   • Body composition (more lean tissue, less fat)
   • Mental well-being (reducing stress and improving mood)

Together, they create a powerful synergy that boosts results and increases the likelihood of losing weight.

Tips for Successfully Combining Both:

• 🥗 Focus on whole foods – Build meals around lean proteins, fiber-rich vegetables, whole grains, and healthy fats.
• 🏃 Move consistently – Aim for at least 150 minutes of moderate-intensity exercise per week (e.g., brisk walking, biking, or swimming).
• 💪 Incorporate strength training – Aim for 2–3 times per week to preserve or build muscle.
• 😌 Stay flexible and realistic – Choose foods and activities you enjoy to build habits that last.
• 📅 Plan ahead – Prep meals and schedule workouts into your calendar like appointments.
• 🧠 Listen to your body – Prioritize rest, hydration, and sleep for recovery and appetite regulation.

Real-Life Takeaways from a Dietitian

My professional recommendation is to focus first on cleaning up your diet. This could mean many different things- reducing ultra-processed foods, being mindful of portions. Literally wherever you’re willing to start.

Next, add consistent, enjoyable physical activity. Diet and exercise go hand-in-hand, and you’ll soon find that these healthy habits will lead to others! 🤩

Tips for beginners: 1) Don’t rely on exercise to “outwork” a poor diet. 2) Don’t ignore movement—find everyday ways to stay active for overall health.

The Bottom Line on Diet or Exercise

So, is diet or exercise more important for weight loss? Ultimately, diet has a bigger impact on weight loss, but exercise still plays a critical supporting role. BOTH are recommended as part of any weight loss regimen. And no matter which route you take to lose weight, be sure to do it in a sustainable way, focusing on long-term behavior change, not extremes!

What healthy habits are you working on? Want to learn more about which areas of health to focus on? Check out 5 Things You Can Do to Support Heart Health NOW or seek personalized guidance from a registered dietitian near you!