Will fasting for 1-3 weeks cause my metabolism to slow down? My bmr is already only 1200kcal, and I don’t want it to slow down more.
In general, fasting does not cause your metabolism to slow because it activates things like growth hormone, adrenaline, and cortisol to prevent it (and presumably to stimulate you to have energy to go get food). However, if your body fat is already at very low levels then it certainly can slow down, because your body is assuming there is NO food and it needs to conserve everything it can to keep you alive until food returns. This is also where “starvation” and potential organ damage can occur. What counts as very low levels will vary between people, but a BMI below normal (<18.5) or a body fat of below 10-12% is likely to be in the danger zone.
Fasting for greater than a week at a time also increases your risk of electrolyte imbalance when you eat again, so definitely keep up on electrolytes during and break longer fasts gradually and without carbs.