When you're gaining weight on 800 calories a day it's often a result of your body going into starvation mode in which the body stores more calories due to the restrictive diet.
Also not accurately tracking your intake or not meeting your body's specific needs can also lead to weight gain when eating only 800 calories a day.
Even medical conditions like hormonal imbalances and even lifestyle factors can cause unintended weight gain.
If you're not eating enough, then your body goes into starvation mode and it will conserve energy by slowing down metabolism and store more calories as fat.
This can make the weight loss more difficult, even if you're eating less calories than you burn.
It's also easy to underestimate how many calories that you're actually consuming, especially with snacks and drinks or even small amounts of foods which add up over time.
So make sure you're tracking everything you consume, which includes sauces, condiments and even small amounts of food.
Being on a very low diet can also lead to muscle loss, which can also affect your metabolism and body composition.
If you're losing muscle mass, then you might not see a decrease in your weight, even if you're in a calorie deficit.
Having certain medical conditions such as PCOS or hypothyroidism can also impact your weight management and can lead to weight gain even when eating fewer calories.
These conditions such as PCOS and hypothyroidism can affect your body's metabolism and hormone regulation, which makes losing weight more challenging.
Hormonal imbalances and insulin resistance can lead to weight gain as well as other hormonal imbalances in the body.
Even fluid retention can cause weight gain temporarily and even factors like certain medications, stress and medical conditions could lead to fluid retention in the body and cause weight gain even when you're eating fewer calories.