Many people struggle with the ‘yo-yo’ weight phenomenon: they are able to lose weight, but because we are built by evolution to defend our body weight, weight regain – ‘and then some’ – often ensues.
The 1940s Minnesota Starvation Experiment (MSE), which replicated famine diets that prevailed in occupied Europe during World War II, gives some insight into why excess weight gain after weight loss often happens. A fascinating article recently published in Obesity Reviews discusses learnings from this dated, but very informative, study.
In this rigorously conducted experiment, 32 healthy young male volunteers were subjected to semistarvation (average loss of 24% body weight) over 24 weeks, followed by 12 weeks of refeeding at four different caloric levels. After the controlled refeeding phase, 12 of the 32 men (three from each of the four refeeding groups) continued for an additional 8 weeks of unrestricted refeeding.
During semistarvation, basal metabolic rate (essentially, energy burned at rest) was reduced by 25% after 24 weeks of semistarvation. After 3 months of refeeding, basal metabolism was still reduced by 10% (adjusted for body weight and composition).
The greater the fat depletion, the lower the basal metabolism, suggesting that our bodies turn down our metabolism further when there are less fat stores around, in an effort to ‘survive the famine’ (from an evolutionary perspective). (this relationship does not exist for lean body mass (muscle)).
They found that during weight regain:
- Fat stores recovered before lean mass (muscle).
- After body fat was completely recovered, there was still a deficit in lean mass, and the higher calorie intake continued (driven by appetite) until the lean mass was completely recovered.
- Thus, more body fat was regained than was lost, and weight ended up an average of 3.3kg (about 7lb) higher than when they started.
- Not only is more weight regained than was lost, but proportionally more fat than muscle was regained.
Thus, it appears that we have a ‘fat store memory’, whose function is to spare energy to accelerate specific recovery of fat stores rather than muscle. This system was built by evolution with survival in mind – after famine, regain fat (energy stores) first, then muscle, and try as hard as possible to keep on as much energy storage as possible so as to survive the next famine. However, in modern times, where the majority of adults in most countries struggle with excess weight, this does not work to our advantage.
So what does this mean to modern-day weight management? Well, we must remember that this study included only fit, healthy men without obesity, and that this was an extreme semistarvation scenario. However, these data suggest that:
- When we regain weight after weight loss, fat recovers before muscle. Our bodies replenish fat fully before we replenish muscle.
- Once fat stores are completely recovered, we are still lower in muscle mass, and we continue to gain weight (due to continued increase in appetite) until the muscle is recovered as well. Thus, weight ends up higher than when it started.
- Because fat is much less metabolically active than muscle, our resting metabolism thus ends up lower (per kg body weight) than before weight loss.
So, with each ‘yo-yo’ weight cycle (repeated cycles of weight loss and regain), proportionally more fat is likely to accumulate, and metabolism will be lowered – a vicious cycle that is likely to worsen obesity with each ‘yo’.
These data speak to the importance of helping each person who struggles with excess weight to find a long term, sustainable plan that works to lose weight and keep it off over the long term.
Note: I have focused on some key points from this 19 page review article – interested readers are encouraged to deep dive into the whole thing! (it’s a free download)
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