Generally when people talk about “starvation mode” in the context of dieting, they are referring to your body’s natural response to slow metabolism after long-term reduced calorie intake. This is also sometimes called “metabolic damage”, which is also a misnomer, since nothing is damaged.
This is a natural physiological response, and is more accurately (and less dramatically) known as “adaptive thermogenesis”.
Although the body goes into adaptive thermogenesis with any sort of calorie restriction (which doesn’t prevent weight loss), the body’s response is far more pronounced with severe calorie restriction (i.e. very low calorie diets).
Why Crash Diet are Bad
Your body’s main response to very low calorie diets or crash diets, is hormonal, with a change in hormones including leptin, thyroid hormone and norepinephrine, which affect body composition, energy intake, and energy expenditure by:
- Increasing appetite, which is more likely to result in cravings and unhealthy eating patterns.
- Reducing how many calories you burn a day, which includes a decrease in in Basal Metabolic Rate (calories your body uses for essential functions), Thermic Effect of Food (calories used for digestion), Thermic Effect of Exercise (calories used for deliberate physical activity), and Non-Exercise Activity Thermogenesis (calories used for subconscious activity e.g. fidgeting)
- Loss of muscle mass.
Basically your body has implemented physiological changes to make your body more efficient in using energy (i.e. you burn less calories) and increases calorie intake. As a result weight loss slows down or stops.
These changes in hormone levels may continue even after you stop trying to lose weight, which means you’re highly likely to regain the weight you lost after you stop dieting. That means that crash diets not only come with potential health risks, but also simply do not work.
To avoid this slow down in metabolism you need to lose weight slowly and steadily. That means implementing a moderate calorie reduction and eating sufficient protein, as well as incorporating exercise, including strength training, into your routine to stave off the loss of muscle.