Huge deficit

After awhile they would probably start to feel extremely fatigued and have a hard time keeping to the routine. They also might be more irritable than normal and lash out at family/friends/coworkers (hangry). The rapid loss could also cause them to get gallstones. They could also end up losing lots of lean body mass and potentially causing long term damage to the body.

shadow2soul wrote: »

After awhile they would probably start to feel extremely fatigued and have a hard time keeping to the routine. They also might be more irritable than normal and lash out at family/friends/coworkers (hangry). The rapid loss could also cause them to get gallstones. They could also end up losing lots of lean body mass and potentially causing long term damage to the body.

To add further even in the short term it can impact hormones, which can cause hair loss/brittle nails/sallow skin/interference with menstrual cycles in women.

I will tell a story of a day where I tried something similar. A couple months ago I did my first crossfit competition. It contained 4 intense workouts starting at 9 am and ending around 6 pm. I ate a light breakfast of a banana and a can of pre-workout. I then completed the first 2 events. I attempted eating a meal at lunch, but my nerves did not let me and I only ate a piece of bread (the baguette side from panera bread to be exact). I then performed the 3rd workout. I had a few more light snacks prior to the final workout. Throughout the competition, I probably had maybe 600-800 calories. By the last workout, I felt very fatigued and struggled making it through. I could absolutely feel the lack of properly fueling my workout and it 100% effected my performance during that final workout.

Moral of the story, it won't kill you, but you will feel like crap. And you will likely not have the energy throughout the day to support that.

What Are the Risks of Rapid Weight Loss?

Rapid weight loss creates physical demands on the body. Possible serious risks include:

• Gallstones, which occur in 12% to 25% of people losing large amounts of weight over several months
• Dehydration, which can be avoided by drinking plenty of fluids
• Malnutrition, usually from not eating enough protein for weeks at a time
• Electrolyte imbalances, which rarely can be life threatening

Other side effects of rapid weight loss include:

• Headaches
• Irritability
• Fatigue
• Dizziness
• Constipation
• Menstrual irregularities
• Hair loss
• Muscle loss

andysport1 wrote: »

So the NHS in England are promoting these 800 calorie diets.

I'm not in England, but in previous threads we've had about the NHS policy, my understanding is they aren't "promoting" an 800 cal diet. They published data showing that for some obese people, health improvements were seen when eating an 800 cal diet for a short period of time under a doctor's careful monitoring. In rare occasions, the dangers of staying obese are worse than the dangers of a VLCD, and a doctor might decide it's worth the risk. They are by no means saying anyone should just decide to go that low on their own.