There are two things your body wants when that 2 p.m. sleepiness hits, but those two things aren't sugar and caffeine.

What you really need are oxygen and certain amino acids. In the right combinations, foods that help deliver them can boost your mood and energy in a more sustained way than a cup of coffee or a candy bar.

Nutritional 'Tinker Toys' Produce Natural High

"Amino acids are the building blocks of protein," said Alegent Health dietitian Toni Kuehneman. "Our bodies mix and match them like Tinker Toys, but two in particular help neurotransmitters."

The amino acid tyrosine boosts dopamine and norepinephrine levels -- and both make us feel alert. Tyrosine is found in lean meat.

On the other hand, tryptophan is an amino acid that produces serotonin.

"Diets high in fat and sugar (deliver) tryptophan, and the brain produces serotonin from that, which is body’s tranquilizer," Kuehneman said. "When you drink sugar-sweetened beverages or have a candy bar, you start a vicious circle where tryptophan produces serotonin and blood sugar spikes then falls, so then you’re sleepy and go back to candy bar, and round and round."

Ultimately, your body breaks food down into glucose as its main fuel source. Straight sugar goes immediately into your blood stream, producing an instant energy boost -- but the effect has no staying power.

Kuehneman said the perfect snack -- such as whole-wheat crackers with cheese -- combines some carbohydrates, protein and fat. The science backs it up.

"Your stomach doesn’t have the ability to compartmentalize food, and protein and fat take longer to digest, so they hold the sugar back," said Beth Naylor, of Creighton University Medical Center’s Center for Digestive Disorders. "So if you eat something that is straight sugar, it directly goes to your intestine, which dumps it into your blood stream."

The higher-fiber choice of whole-wheat crackers also figures into the digestion equation.

"Fiber is an energy stabilizer," according to WebMD.com. "It slows digestion, providing a more steady supply of energy throughout the day. Boost your fiber intake by eating beans, whole fruits, vegetables, whole-grain breads and whole-grain cereals."

Oxygen Lifts Afternoon Fog

The other part of the mood and energy equation -- oxygen -- is delivered in a couple of ways. Iron helps the body move oxygen, so it is an essential nutrient.

"Iron is found in meats -- especially red meats -- eggs, and there are minor amounts in some fruits and vegetables," said Hy-Vee grocery dietitian Carrie Nielsen. "Vitamin C helps absorb iron, so have a glass of OJ with your eggs."

Iron-fortified cereals are another option.

Naylor said there's nothing better for moving oxygen than exercise, so a walk around the office can also boost your brain out of its mid-afternoon fog.

"You've heard of the runner's high. People that exercise continually say they have more energy," Naylor said.

Other nutrients specifically work to boost mood.

"Folic acid can counter depression, clinical studies show," Nielsen said. "A lack of selenium can cause bad moods, but Brazil nuts or tuna or sunflower seeds contain a lot of selenium."

Other considerations you should think about if your energy is flagging -- both diabetes and anemia have fatigue as symptoms, Naylor said, so if you're regularly run down, consider getting tested.

Also, in addition to targeting foods that can help boost energy, make sure you aren't making your body go too long between meals.

"Eat like babies -- every four hours," Kueheman said. "We never outgrow it."

"Your body wasn’t designed to go 12 hours without fuel," Naylor said. "More than 6 hours between meals and you’re asking it for too much."

Finally, it shouldn't just go without saying: If you're tired at mid-afternoon, look to fix your nights. Aim for eight hours of sleep and don't drink caffeinated beverages too close to bedtime.