If you’ve followed me for a while, you already know the importance of retaining muscle as you lose fat.

Among the more obvious reasons (like helping you look better and preserving your metabolic rate) a study (link below) published in The American Journal of Clinical Nutrition investigated how changes in body composition affect appetite and weight outcomes.

Their results suggest that composition of weight loss may have functional importance for energy balance regulation, with greater losses of fat-free mass potentially being associated with increased weight regain and appetite.

Why this happens isn’t yet fully understood, but one theory is the Protein Lever Hypothesis (PLH). The PLH states the body will increase appetite until its protein requirements are met.

Why? Because lean mass plays a crucial role in our survival, so when we begin losing lean mass our body will do everything it can to recoup lost mass to keep us alive.

This is why strength training and adequate protein are so important during a deficit.

Although metabolism can’t be damaged, during fat loss there are some metabolic adaptations that occur that are totally healthy and to be expected - if you’re losing fat properly you have nothing to worry about.

But what about those who try to lose weight the wrong way? For example, aggressive detox diets that are very low in protein and have no, or improperly programmed, weight training.

In this situation, there could be a larger than expected reduction in metabolic rate because lean mass is directly associated with Basal Metabolic Rate.

Now imagine fluctuating between ‘crash’ diets and normal eating behaviour - aka ‘yo-yo dieting.’

Every time weight is lost, there would also be a loss of lean mass and BMR would drop a bit. Sticking to the crash diet wouldn’t be sustainable, so a return to usual eating habits (most likely a surplus + insufficient protein) would happen. Now, not only would muscle be lost, but body fat would be regained.

Do this for a long enough period of time, and body fat will be gained despite eating at what was once maintenance calorie intake.

However, if sufficient protein is eaten while engaging in resistance training, lost lean mass would rapidly be gained back, appetite would stabilise, and dropping fat would actually start again.

More research is needed here, but the study makes for an interesting read https://academic.oup.com/ajcn/article/111/3/536/5707675