Consequences of negative calories

thunderstruck134

As I was browsing the internet today, I stumbled across someone who said that you can't have positive net calories and lose weight. They gave an example of a person who ate 1200 calories but burned 2000, making it a deficit of -800. Now this has actually made me curious about the consquences of a negative caloric intake. What exactly happens to the body?

diannethegeek

It depends on how you define your burn, I suppose.

For instance, my TDEE is about 1900 calories. So my burn per day is 1900 calories. When I eat 1600 calories I am eating 300 less than I burn a day. This kind of deficit is required for weight loss.

If I were to eat 1600 calories and then do another 2000 calories worth of exercise, bringing my TDEE up to 3900, I'm at a deficit of 2300 calories and if I plugged this into MFP it would give me a net calorie total of -400. This is generally considered too large a deficit to be a healthy sustainable diet. What happens to the body really depends on how long you maintain such a deficit. A day might not be a problem. Early on I might see lethargy, moodiness, hunger, etc. A prolonged deficit at this rate and I would probably see muscle loss (including around the heart), trouble concentrating, hormonal imbalances, bone loss, hair loss, sagging skin, brittle nails, etc.

arrseegee

What you are describing is expending far more calories than are being consumed, rather than a normal weight loss goal of having your calorie intake at a slight loss to lose weight safely... right? if that's the case then read on...

I've tried to simplify this.

In a normal situation your body uses fat, muscle and glycogen stores to access energy. When you are in calorie deficit and not exercising glycogen (glucose stores) tend to get used up first, as well as about 45% of energy coming from fat stores and 15% from a process where muscle proteins are broken down into sugar. You cannot lose weight without a calorie deficit, because it is through this deficit that your fat cells are 'harvested' for energy.

A diet of 1200 per day offset by 2000 calories burned would quickly (eg in the course of days) become metabolically a starvation diet. Being in such an extreme calorie deficit will quickly deplete glycogen (sugar) stores. Glycogen is 'attached' to muscles by water, which is why rapid weight loss occurs and is often referred to as 'water weight'. When you start eating normally again your body replenishes these stores of glycogen, which also results in water being reaccumulated. This is why crash diets work in the short term but you rapidly gain weight back, most of which is water.

When glycogen runs out fatty acids (from your fat cells) are broken down to produce something that your body uses similarly to sugar, but protein from your muscles is also broken down. Since muscles are metabolically active, eg they use energy just existing, you reduce the energy your body expends when you have less muscle. This is part of the 'slowing down your metabolism' process, that you want to avoid.

If your body burns 800 calories a day more than it is getting from food you would see rapid weight loss but the majority of it will come from glycogen, then muscle and fat loss. Seeing as the point of losing weight is to lose fat, going at it this way is counter-productive - yeah you lose weight and you might even look a bit thinner, but most of what you lost were things that your body needs.

If you ate 2000 calories a day and exercised 800 calories then you have enough of an energy intake to provide calories for those parts of the body that need them (e.g. your brain, which uses a lot, and also your muscles! and calories used for breathing etc.), while still creating a deficit that will help to burn your fat stores. If you ensure you get enough protein and combine your weight loss with strength training exercise you will hopefully build muscle, or at the very least prevent the extreme muscle loss that I described above.

Hope that's helpful!