“You’re just a busy mom,” the doctor told me after returning with test results that showed no indication of an underlying medical condition that would account for my chronic fatigue: no anemia, no hyperthyroidism, no urinary tract infection or heart issues—all possible causes of extreme exhaustion.

“Try to give yourself a break,” he recommended. “Take time to exercise every day, and make sure you get enough sleep.”

But I did exercise, and I was getting enough sleep.

It wasn’t until I made my way to a nutritionist, on the advice of a friend, that I got the wake up call that I needed. The nutritionist had me keep a food diary for a week, writing down everything that I ate and drank each day. The results were eye-opening.

My breakfast consisted of a double soy mocha (I am lactose intolerant), followed by a muffin or piece of fruit at mid-morning. My lunch was a sandwich on a baguette with tuna salad or turkey. My dinner consisted of 2 glasses of wine, lots of pasta (I was always starving by dinner time), bread with butter, and some type of meat or fish. I’d make a green salad but wouldn’t eat much of it. After dinner I craved something sweet– soy ice cream and a handful of M&Ms usually did the trick.

“Of course you feel dull and drowsy most of the day,” my nutritionist informed me. “You are living on carbs, sugar, caffeine and alcohol.”

We’ve all heard the cliché “you are what you eat.”  But many of us fail to apply the adage to our own diet.  Your body, like your car, won’t perform well when it doesn’t have the proper fuel.  How you choose to fuel your body will impact your energy level and your body’s ability to function.

Although I wasn’t overweight, the foods I was eating were leaving me feeling sluggish and worn out through the day. Thanks to a total makeover in my diet, I have regained my energy and zest for life. I am also less moody and don’t experience the 4 p.m. “crash” that required a double espresso or nap to get me through the rest of the day.

Here is what I have learned about the food/energy connection and how what you eat can be making you tired.

4 Ways Food Could Be Making You Tired

1. Not enough protein for breakfast

Most of what we eat for breakfast—bagels, muffins, cereal—are high in carbohydrates, which send you into a downward energy spiral. The result? You are hungry an hour later and end up reaching for short-term snacks that only do more of the same.

Your body needs protein for sustained energy and to prevent blood sugar crashes from carbohydrate consumption (yes, fruits are carbs). A healthy protein shake in the morning will control your blood sugar levels all day and help keep your energy intact.

A high-quality protein shake is a great way to start your day. I use pea protein, which has 25 grams of protein per serving, a real mega-dose! You can also use whey, egg white or soy protein if you have no food sensitivities to these foods (more on that later). I add almond milk, some frozen blueberries or raspberries, a scoop of almond butter, and lots of ice. This keeps my hunger at bay for hours, and it’s delicious. If you don’t like protein shakes, you can have some turkey sausage and eggs, or another high-protein food (at least 20 grams) before heading out the door.

2. Eating too infrequently

Skipping meals, and going too many hours without eating, can cause your energy levels to plummet. After three to four hours, your body’s blood sugar levels start to drop. Your body sees that as a crisis and starts to slow down. To deal with the blood sugar crisis, your adrenals produce more cortisol to raise your blood sugar (since you have none in your body from food).

Cortisol, the stress hormone, is “public enemy number one” according to Psychology Today. Short term, elevated cortisol will make you feel edgy and exhausted. Long term, scientists have found that elevated cortisol interferes with learning and memory, lowers your immune function and bone density, and leads to weight gain, high blood pressure, and heart disease.

Lesson? Rather than three big meals a day, aim for three smaller meals and two snacks each day.

3. Undiagnosed food sensitivities

Food sensitivities are different than food allergies. Just because your lips don’t blow up from eating a nut or you haven’t been diagnosed with an official disease or allergy, it doesn’t mean its safe to eat any food.

When you eat something you’re sensitive to, you might have an immediate reaction like bloating, gas or diarrhea—or you may have no reaction in your stomach at all. You could experience things like acne, joint pain, weight gain or fatigue. Since these reactions may happen slowly over time, you may never make the connection between them and food.

The two most common food sensitivities are to dairy and gluten

You’ve probably heard a lot about gluten sensitivity. It’s well known that gluten can trigger symptoms such as bloating, migraines, fatigue and brain fog. It’s also been known to contribute to the onset of autoimmune diseases for some people. Dairy can cause many of the same gastrointestinal and energy problems. Yet even if you stop drinking milk and eating yogurt, there are dairy-based additives such as casein and whey snuck into foods like dips, mayonnaise and salad dressings (thick sauces typically contain gluten), and even canned chicken broth. Read labels carefully before you buy! You can try eliminating gluten and dairy, each for one week, to see how you feel without them in your diet.

If you suspect that other foods might be causing you to feel tired, you can try cutting out them out for a few days at a time and see if you notice a difference. As long as you keep a sensible, balanced diet while doing this, there should be no harm in experimenting.

4. Too much sugar in your diet

You already know that sugar is bad for your waistline. Sugar is stored as fat in the body, so if you’re looking to lose weight, you need to cut out the sweet stuff. But that’s not the only reason to go easy on the sugar. The average American consumes 22-25 teaspoons of sugar each day without even knowing it. Besides the obvious culprits like soda and candy, sugar is hiding everywhere, in every packaged food you can imagine: ketchup, salad dressing, and even in “healthy” foods like yogurt and granola bars. Many “organic” packaged foods, while appearing healthy on the outside, are loaded with sugar in the form of honey, molasses or dates.

Besides dramatically boosting your chances of becoming diabetic, sugar in any form causes your blood sugar to spike and crash, which leads to a major drop in energy. Make a habit of checking food labels on everything you buy and ditch foods that have more than 6 grams of sugar per serving. When you have a craving for something sweet, it’s usually a sign of low blood sugar. Opt for an apple with almond butter or a handful of almonds instead. Protein will satisfy your sugar craving and leave you feeling satiated.

Final word: Feeding your body the right foods at the right times can dramatically increase your energy levels—making you feel better and more ready to respond to the many demands of your busy day. If you can make the decision to shift some old habits, you have the ability to feel more focused and alert than you have in a long time.

Originally published in Forbes.