Expert: Laurie Beebe, MS, RD, LD - 11/6/2009

Question
QUESTION: How do I know how many calories to eat per day? I weight 135 pounds and I am 5'4. I have already lost 13, so I have hit a plateau. I am 18 years old. I run 3-4 times a week for 20 minutes with about 10 minutes of walking before and after. I would like to lose 5-10 pounds more. I have lost it from my arms and legs, yet I still have a stomach pooch.

Also, why does my weight go up and down by a pound almost every day? I weigh myself every morning at 8am after going to the bathroom before eating.

ANSWER: Hi Claire,

Congratulations on your successful weight loss and your new healthy lifestyle!

First of all, it doesn't necessarily matter if you know how many calories you need per day.  You see, the problem is that you will then have to count all the calories you eat and that is a very difficult way to live!  That said, you can find out a recommended calorie level on numerous websites which allow you to track your intake by logging the food you eat and the exercise you complete.  A few that come to mind are TheDailyPlate.com, FitDay.com, and SparkPeople.com, just to name a few.
Another way is to go to www.mypyramid.gov and design your own diet based on healthy food choices; that site will tell you how many calories you need based on your height, weight, age, and desired weight just as the others will.

Probably most helpful is just logging your food intake.  In this way you will become more aware of what you eat and as a result, tend to cut back on "empty calories" like sweets and snacks.

Weight fluctuation occurs because so much is at work in determining your actual body weight.  For example, if you stand on the scale holding a pint of water, the scale will say you weigh one extra pound.  Drink the water and you weigh the same extra pound!  Every day our weight fluctuates with fluid shifts and perspiration and fat cell mobilization and forming new cells.  Starting an exercise program may lead to the number on the scale going up temporarily due to increase in muscle development and in the retaining of water in muscle cells.
You might find it helpful to graph your weight for an excellent illustration of how your weight trend decreases over time.

---------- FOLLOW-UP ----------

QUESTION: Also, how does fat physically leave the body? What happens to it?

Answer
Hi Claire,

I love that question!
Well, it's a matter of biochemistry.  Each fat molecule is made up of atoms of Carbon, Hydrogen, and Oxygen.  When your body needs energy (like when you are exercising, or if you haven't eaten enough to support your weight) the fat molecules get broken down: they go through a series of reactions which generate energy (calories) which has been stored up until now. When you burn a lot of energy (like when running) your body temperature actually goes up and you begin to sweat.
The final breakdown products (and these same ones are present in protein and carbohydrate as well--which go through a variation of the biochemical reactions to yield calories)  are Hydrogen Oxygen, and Carbon.  The Hydrogen and Oxygen leave your body as water through the kidneys, the intestine, your breath, and a small amount through your skin (as in sweat).  The Carbon atoms combine with the oxygen you breathed in, and are excreted as carbon dioxide. Ha! Remember in 6th grade when you learned "you breathe in oxygen and you breath out carbon dioxide"?  Now you know where the carbon comes from!

(If you are very interested in the actual reactions, see more at http://en.wikipedia.org/wiki/Citric_acid_cycle
It's an entire year's worth of biochemistry in one page, so there's no way to comprehend it all at one glance, but it'll give you an idea of the magnificent workings of the body that are going on every second of the day, without us even noticing!)