Researchers followed over 7,000 adults for about 7 years to see how changes in eating habits and exercise affected their body weight and fat. They found that people who improved their diet quality and increased their physical activity both lost more weight and had healthier body composition than those who only did one or the other. The study suggests that combining better eating with more movement creates the strongest results, especially for people who were overweight or inactive at the start.

The Quick Take

  • What they studied: Whether improving diet quality and increasing physical activity together help people lose weight and reduce body fat more effectively than doing just one or the other
  • Who participated: 7,256 adults from the UK, averaging 49 years old, about half men and half women, tracked over 7 years with measurements taken at the beginning and end
  • Key finding: People who improved both their eating habits and exercise lost significantly more body fat and dangerous belly fat than those who only changed one behavior. The combination was more powerful than either change alone.
  • What it means for you: If you want to lose weight or improve your health, focusing on both better nutrition and regular exercise together appears more effective than choosing just one. This is especially true if you’re currently overweight or inactive, though results take time and require sustained effort.

The Research Details

This was a long-term follow-up study of real people living their daily lives, not an experiment where researchers controlled everything. Researchers measured participants twice—once between 2005-2015 and again between 2014-2020—giving them about 7 years between measurements. They tracked what people ate using food questionnaires, measured their physical activity using special devices that monitored movement and heart rate, and measured their body composition using imaging technology and body measurements. The researchers then looked at how changes in diet and exercise connected to changes in weight, body fat, and belly fat.

This approach is valuable because it shows what happens in real life over many years, rather than in a controlled lab setting. By measuring the same people twice, researchers could see how individual changes in behavior connected to individual changes in body composition. The long time period (7 years) is important because it shows lasting effects, not just short-term results.

This study is relatively strong because it included a large number of people (7,256), had detailed measurements of both diet and activity, used objective body composition measurements rather than just weight, and adjusted for many other factors that could affect results. However, people who stayed in the study may have been more health-conscious than those who dropped out, which could affect how the results apply to everyone.

What the Results Show

The study found clear connections between behavior changes and body fat changes. When people improved their diet quality by one standard amount (about 1.3 points on a 15-point Mediterranean diet scale), they lost about 0.5 kilograms of body fat on average. When people increased their physical activity by one standard amount (about 19 kilojoules per kilogram per day), they lost about 1.4 kilograms of body fat. Importantly, people who improved both diet and exercise at the same time saw even bigger reductions in body fat—about 149 grams more reduction in dangerous belly fat compared to those who only improved one behavior.

The benefits were especially strong for people who started out overweight or obese. For example, people with a BMI of 25 or higher who increased their physical activity lost about 1.74 kilograms of body fat for each standard increase in activity, compared to only 0.96 kilograms for people who started at a healthy weight. This suggests that if you’re currently overweight, increasing exercise may help you lose weight faster than if you’re already at a healthy weight.

The study also looked at different types of body fat. People who improved their behaviors reduced not just total body fat, but also dangerous belly fat (visceral adipose tissue) and fat under the skin (subcutaneous adipose tissue). This matters because belly fat is linked to more health problems than fat in other areas.

The research also examined whether diet and exercise worked together in a special way (called an ‘interaction’), but they didn’t find strong evidence for this. Instead, both behaviors worked independently to reduce body fat, and the benefits simply added together when people improved both. The study also measured liver fat in some participants and found similar patterns—better diet and more activity were associated with less fat in the liver.

Previous research has shown that both diet and exercise help with weight loss, but this study adds important information by showing what happens when people change both behaviors over many years in real life. It confirms that the combination is more powerful than either alone, which aligns with what health organizations recommend. The finding that overweight people see bigger benefits from increasing activity is consistent with other research showing that people further from a healthy weight often see faster initial improvements.

The study has several important limitations. First, people who stayed in the study for 7 years may have been more motivated about health than those who dropped out, so results might not apply to everyone. Second, the study only shows associations (connections) between behavior changes and body fat changes—it doesn’t prove that changing diet and exercise caused the fat loss, though this is very likely. Third, the study was conducted in the UK, so results may differ in other countries with different food cultures and activity patterns. Finally, the study didn’t track whether people maintained their behavior changes long-term, so we don’t know if the benefits lasted beyond the 7-year period.

The Bottom Line

Based on this evidence, adults should aim to both improve their diet quality (eating more whole foods, vegetables, and healthy fats like those in a Mediterranean diet) and increase physical activity. The combination appears more effective than either alone. For people who are overweight or currently inactive, these changes may produce particularly noticeable results. Start with small, sustainable changes rather than dramatic overhauls, as the study tracked changes over years, suggesting lasting habits matter more than quick fixes. (Confidence: Moderate to High—this is a large, well-designed study, but it shows associations rather than definitive proof of cause and effect.)

These findings apply to most adults, especially those who are overweight, obese, or physically inactive. People with existing health conditions should consult their doctor before making major changes to diet or exercise. The study focused on adults aged 29-65, so results may not directly apply to teenagers or very elderly people. Athletes or people already exercising intensely may see different results than the general population studied.

Based on this study, meaningful changes in body composition typically take several years to develop. The study measured changes over 7 years, so don’t expect dramatic results in weeks or months. However, some benefits like improved energy and mood may appear sooner. Most people can expect to see measurable changes in weight and body composition within 6-12 months of consistent effort, though the full benefits shown in this study developed over years.

Want to Apply This Research?

  • Track both Mediterranean diet score (count servings of vegetables, whole grains, fish, and healthy oils daily) and physical activity energy expenditure (record minutes of moderate to vigorous activity daily). Monitor changes in these two metrics together monthly rather than focusing on weight alone, as the study shows body composition changes are more meaningful than scale weight.
  • Set a specific goal to improve one aspect of diet (like adding one extra vegetable serving daily) while simultaneously increasing one type of physical activity (like a 20-minute walk 3 times per week). Use the app to track both changes together and celebrate progress in both areas, since the research shows combining changes creates the strongest results.
  • Establish a quarterly check-in system where you review changes in diet quality and activity levels together, rather than separately. Track body measurements (waist circumference) and how clothes fit every 3 months, as these may show changes before scale weight does. Monitor energy levels and overall health markers as leading indicators of positive change, since the study suggests benefits develop gradually over months and years.

This research shows associations between diet and activity changes and body fat changes, but does not prove these changes directly cause weight loss. Individual results vary based on genetics, metabolism, and other health factors. Before making significant changes to your diet or exercise routine, especially if you have existing health conditions, take medications, or are pregnant or breastfeeding, consult with your healthcare provider or a registered dietitian. This information is for educational purposes and should not replace professional medical advice.