If you put $1,000 in a bank account and keep making withdraws without adding deposits, you’ll eventually wipe out your account. It’s just simple math, right? Well, our bodies aren’t quite that simple. It would be awesome if all we had to do to slim down was stop “making deposits” (e.g. stop eating) and withdraw fat from our energy reserves, but it just doesn’t work that way.

Every day, your body needs a wide range of nutrients to help it function, including not just vitamins and minerals, but also calories, from carbohydrate (the preferred source of fuel for your brain and muscles), as well as protein and fat (which are used to repair and heal your body’s cells). Unfortunately stored fat alone can’t take the place of these essential nutrients, so if you stop eating, or stop eating enough, the jobs these nutrients do don’t get done, and the side effects are serious.

In order to lose weight, you do need to cut calories, and that will allow your body to pull some fat out of storage (you fat cells) and burn it off. But you still need to eat enough food, in the right balance, to support the other parts of your body you want to keep strong and healthy, namely your organs, muscle, bone, immune system, hormones, etc. Undereating essentially means that you starve these systems in your body and they’ll become run down, damaged or stop working properly.

When I first became a nutritionist, I worked at a university and the campus doctors referred a lot of college students to me because their bodies were showing the signs of too little nutrition, such as missed periods, anemia, injuries that didn’t heal, a weak immune system (e.g. catching every cold and flu bug that comes around), thinning hair and dry skin. I still frequently see clients who  chronically undereat, usually because they’re trying to lose weight, and they often panic at the thought of eating more. But the truth is, eating less than it takes to support your body’s healthy tissue can actually cause you to hang onto body fat for two key reasons. First, healthy tissue (muscle, bone, etc.) burns calories by just being on your body. Every bit you lose causes your metabolism to slow down, even if you work out more. Second, too little nutrition triggers your body to go into conservation mode and you guessed it, burn fewer calories. Historically this is how we survived times of famine – when a smaller amount food was available, we adapted by expending less.

So, how do you know if you’ve cut your calories too low? I have three tell tale signs:

Use a “quick and dirty” formula. Without any activity, your body needs at least 10 calories per pound of your ideal weight. For example, let’s say you weigh 150 but your weight goal is 125. You shouldn’t eat less than 1,250 calories for an extended period of time. But remember, that’s a sedentary formula (e.g. sitting at a desk or on the couch all day and night). If you have an active job or work out, you need extra calories to fuel your activity.

Tune into your body. How do you feel? You can certainly be well-nourished while you’re losing weight. If you feel lethargic, have trouble concentrating, need caffeine in order to function or exercise, feel irritable, moody, or have intense food cravings, you’re not eating enough. Short-term strict plans or “cleanses” are OK for jump-starting a new healthy eating plan, but long-term (more than a week), eating enough to nurture your body is essential for both health and weight loss.  

Heed the warnings. If you follow an inadequate diet for too long, you’ll start to see the ramifications. I’ve mentioned a few, such as hair loss, missed periods and getting sick often. I hope you won’t have to experience any kind of unusual physical side effects, but if you do, please know that your diet can be the culprit. I’ve counseling many people who’ve attributed such side effects to genetics or stress when in reality, undereating was the offender. 

As a nutritionist and registered dietitian, I want to help you lose weight (or keep it off) safely, healthfully, in a way that allows you to feel great in mind, body and spirit. Losing weight at the expense of your health is never a worthwhile trade off!