It’s when you consume less energy (expressed in calories) by eating than you burn by living normally.
So for example: if you burn around 2000 calories in a day you would be in calorie deficit if you ate fewer than 2000 calories in that same day. That way your body will have to make up the difference by taking from its own energy reserve, which results in weight loss.
Since other people have covered what it is, I’m going to try to go into a little more depth on how to do it. The first and easiest step is to stop drinking calories. The biggest struggle for most people eating in a calorie deficit is that they feel hungry. A can of soda has more calories than an apple or like 5-6 cups of spinach and will do nothing to help you feel full.
The low calorie things that make you feel full are protein and fiber. So essentially eat lean meats and vegetables and you’ll be fine.
Fish is also very good. A salmon patty on bread with lower calorie spreads like Dijon mustard, hummus, etc. Will taste great and not take too many calories.
Latest Answers