How do I make sure I lose fat and not muscle?

Well you probably lost a bit of water weight. What are your stats?

If you want to save LBM, you need to be on a bodyweight or heavy lifting program with a progressive overload, meet your protein goal, and continue with your deficit to lose fat.

@Killerdolphin_9 you are off to a good start. It was 6 weeks before I lost a pound.

As to your question as how to make sure you lose fat and not muscle I do not have an answer in your case. I researched the same question for myself last year and decided to use nutritional ketosis at my age (63) when I started trying Nutritional Ketosis to manage my pain yet not lose what little muscle I had at the time.

Just Google the term Nutritional Ketosis and you will have a couple years worth of reading you can do as you learn about how it is muscle sparing in nature. It may not be of interest to you but it has been life changing for this old man.

I'm with misskarne - 7 lbs in two weeks is a lot - unhealthily a lot. And having days where you're eating less than 1000 calories isn't great for you either. In the short term it will help you drop weight, but in the long term it can mess up your metabolism. Cardio is great for dropping weight, but if you're concerned about muscle mass I would suggest a weight routine. Increasing your muscle mass will help burn fat around the clock. The best workout routines will have cardio and weight training.

It looks like you've lost over 12 lbs in three weeks. That is quite a lot! Even if some of it is water weight (glycogen stores etc.) I wouldn't continue at that rate if I wanted to retain my muscle mass or if I wanted to keep the weight off over the long term. You didn't gain weight eating under 1000 calories so it sounds like you are severely restricting your intake. You don't have to do that in order to lose weight. Just eat at moderate deficit and you will lose weight.

+1 to starting a progressive strength training program. The sooner you start, the sooner you will see actual changes in your body. I only started strength training a few months ago and I love how my arms are starting to look. I've lost weight before and never saw the kinds of changes I am seeing now. I never thought it mattered whether I kept my lean muscle mass. I thought I just wanted to weigh less, but I was wrong. Ten years ago I lost 30 pounds, but I could barely tell. I fit in somewhat smaller clothes but I still looked fat. I'm much happier going about it this way now. The number on the scale is one thing, but feeling and looking great is ever better!

You're ahead of the game because you have learned about this early on in your journey! Best of luck.

Everyone on here who has said to lift weights is 100% correct!!

I am 5'2" tall, currently 186lbs. I started at 228lbs and a tight size 18. I've been lifting weights for most of 3 years now, half of that heavy weights. I'm now in a 10/12. Most people guess that I weigh about 150-160lbs.

For me, there is something very satisfying about being able to squat 60lbs MORE than I weigh. To being able to see muscle definition in my arms and legs.

You could take some measurements with measuring tape and use online calculators to figure out your BF%. Then as you lose weight you could make sure you're not losing too much muscle. Or there's fancy clinics and gyms that will measure your BF% for you.

For instance, if you're 183 right now, and let's say 30% BF (just a random guess). That means you have about 55 lbs of fat. If in three months you re-measure and you're at 165 with 25% BF, that means you lost a total of 18 lbs, and 14 of those lbs were fat (so you lost 4 lbs of "lean mass". Lean mass is muscle, blood vessels, bone, etc.).

If instead, you lifted weights and in three months you weigh 165 with 22.5% BF, then that means you still lost 18 lbs, but it was all fat!

If you are in this for the long haul, eat more and exercise. You have a fair chunk to lose but you really don't need to be set to lose more than 1.5 pounds a week. The 7 pounds you have lost in the first two weeks is huge. That is .5 pounds a day. Probably mostly water loss which is not unusual when you first start but not something you want to have continue going forward.

Don't want your hair falling out do you? So eat enough to get adequate nutrition. Exercise to maintain muscle. And don't be in a big hurry to get to your goal.

Eat sufficient protein, and include full body strength training in your exercise plan. Both are muscle sparing, helping you retain mass while in a caloric deficit.

Unless you are on a medically supervised nutrition plan, you should not be eating under 1200 calories a day. It is not considered safe/healthy.

Minimum RDA for protein is 0.8g/kg, and highest beneficial recorded in studies is ~ 1.5g/kg. You should be looking to land within this range to gain the muscle sparing benefits. Where you fall will/should fall in the range is based on nutrition choices, amount of exercise, etc. which you will have to determine for yourself.

I have had success eating an average of ~ 1g/kg while on a Stronglifts 5x5 program, which is a 3x day/week strength program.