The actual minimal number of calories that you can safely eat long term depends on your basal metabolic rate (BMR), which is the energy necessary for your body and its organs to maintain normal function while simply at rest. Although there are equations to calculate BMR, such as the Harris-Benedict equation, these are all theoretical estimations and your actual number could vary. There are other, more accurate methods to calculate your BMR through either direct or indirect calorimetry techniques that usually measure gas exchange between oxygen consumption and CO2 production. You could do an internet search to find test facilities near you for these methods if you’re genuinely curious, otherwise the BMR equations might give you a good enough estimate.
You’re correct in stating that 1,200 calories is typically the conventional lower threshold value given and I think that it’s fair to use as an estimate for most of the general population (again, your actual number will vary). By far, however, 800 calories per day would be considered a very low calorie diet (VLCD) because it would be very hard to meet your nutritional needs with so few calories. This type of diet would generally not be recommended for individuals who are not obese and who don’t have proper medical supervision. Your doctor will take your specific circumstances into consideration before helping you decide if you should go on a VLCD and you may need to work with a dietitian to develop a specific eating plan to help ensure that you meet your nutrient needs while following the diet.
It’s important to note that although VLCDs have been shown to lower weight and help with obesity-related conditions like hypertension, diabetes, or high cholesterol in obese individuals; they don’t seem to be any more effective than a more modest dietary eating plan long term. You will likely regain any weight loss after going off the VLCD unless you transition into a healthy eating plan while incorporating regular exercise, which is another aspect of why medical supervision from your doctor or a dietitian would be essential.
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