Scientists studied people who ate 25% fewer calories for two years and found something surprising. Even after accounting for their smaller body size and less muscle, these people burned fewer calories than expected when sleeping. This suggests our bodies have a built-in survival mechanism that slows down our metabolism when we eat less food. The researchers used advanced body scans to get a clearer picture of how different body parts contribute to this metabolic slowdown, which could help explain why maintaining weight loss is so challenging.

The Quick Take

  • What they studied: How eating 25% fewer calories for two years affects metabolism and body composition in healthy adults
  • Who participated: Normal-weight adults who followed either a reduced-calorie diet or ate normally for 24 months
  • Key finding: People who ate less burned fewer calories than predicted, even after accounting for their smaller body size and muscle loss
  • What it means for you: If you’re trying to lose weight, expect your metabolism to slow down more than you might think, making weight maintenance more challenging

The Research Details

This was a carefully controlled study where researchers randomly assigned healthy, normal-weight adults to either eat 25% fewer calories or continue eating normally for two years. The calorie-restricted group lost about 13% of their body weight in the first year, then maintained that weight loss for another year. Throughout the study, researchers used advanced MRI scans to measure changes in different body tissues like fat, muscle, and organs.

This type of randomized controlled trial is considered the gold standard for research because it can show cause and effect. By using advanced imaging techniques, researchers could see exactly how different parts of the body changed and contributed to metabolic changes, rather than just looking at overall weight loss.

This study was published in a respected scientific journal and used the most rigorous study design possible. The researchers followed participants for a full two years and used multiple methods to measure metabolism and body composition, making the results more reliable than shorter or less detailed studies.

What the Results Show

The main discovery was that people eating fewer calories experienced ‘metabolic adaptation’ - their bodies burned fewer calories than expected based on their new, smaller size. This happened even when researchers accounted for the loss of muscle and fat tissue. The effect was strongest at 12 months and was still present at 24 months. The advanced MRI scans showed that different body tissues contributed to this metabolic slowdown in ways that simpler measurements couldn’t detect.

The calorie-restricted group lost both fat tissue and muscle mass compared to the control group. Interestingly, the MRI-based measurements were better at predicting and detecting metabolic changes than simpler methods like just weighing people or using basic body composition scans.

This study confirms what researchers have suspected but couldn’t prove definitively in healthy people - that our bodies actively slow down metabolism during calorie restriction as a survival mechanism. Previous studies were often shorter or done in people who were already overweight.

The study doesn’t specify exactly how many people participated, and the results may not apply to everyone since participants were healthy, normal-weight adults. The metabolic adaptation might be different in people who are overweight or have health conditions.

The Bottom Line

If you’re planning significant calorie reduction for weight loss, be prepared for your metabolism to slow down more than expected. This isn’t a reason to avoid healthy weight loss, but it explains why maintaining weight loss requires ongoing effort and potentially fewer calories than you might calculate.

Anyone trying to lose weight or maintain weight loss should understand this research. It’s particularly relevant for people who have hit weight loss plateaus or found it harder to maintain their goal weight than expected.

The metabolic slowdown appeared within 12 months and persisted through 24 months, suggesting this is a long-term adaptation rather than a temporary response.

Want to Apply This Research?

  • Track your resting metabolic rate or morning body temperature alongside weight and food intake to monitor metabolic changes
  • Gradually reduce calories rather than making dramatic cuts, and incorporate strength training to preserve muscle mass during weight loss
  • Monitor energy levels, sleep quality, and hunger signals as indicators of metabolic adaptation, adjusting calorie targets as needed over time

This research is for educational purposes only and should not replace professional medical advice. Consult with a healthcare provider before making significant changes to your diet or exercise routine, especially if you have underlying health conditions.