It's common to resolve to lose weight, but any sane person dreads a diet's dulling effect on the brain.

In fact, many studies have shown that counting calories, carbs or fat grams, is truly distracting — to the point that it taxes short-term memory. But how we eat can affect our minds at more fundamental levels, too.

Here are five things you should know about feeding your brain:
Fuel it up
The brain, which accounts for 2 percent of our body weight, sucks down roughly 20 percent of our daily calories. A picky eater, it demands a constant supply of glucose — primarily obtained from recently eaten carbohydrates (fruits, vegetables, grains etc.). Only in extreme instances of deprivation will the brain use other substances for fuel.

More recently evolved areas of the brain, such as the frontal cortex (it's like the CEO of the brain), are particularly sensitive to falling glucose levels, while brain areas regulating vital functions are more hardy, said Leigh Gibson of Roehampton University in England. "When your glucose level drops, the symptom is confused thinking, not a change in breathing pattern," he said.

This is not to suggest that we should constantly slurp soda to keep our brains functioning optimally. On the contrary, high glucose levels slowly but surely damage cells everywhere in the body, including those in the brain, said Marc Montminy of the Salk Institute for Biological Studies in California.

And according to a recent study published in the Oct. 3 issue of the journal Cell, by Dongsheng Cai and colleagues at the University of Wisconsin, the brain may react to excess food as if it were a pathogen. The resulting immune response, which occurs irrespective of weight gain, may cause cognitive deficits such as those associated with Alzheimer's.

Similarly, high blood sugar, coupled with a cognitive task, is associated with elevated cortisol — a hormone known to impair memory in high doses, Gibson said. In other words, don't get out the flash cards after that second (or third) piece of cake.

Become a grazer
The brain needs Goldilocks portions of energy: not too much, not too little.

To optimize brain power, Michael Green of Aston University in England suggests one tactic would be "more frequent but smaller meals." The brain works best with about 25 grams of glucose circulating in the blood stream — about the amount found in a banana, said Gibson.

If trading three-meals-a-day for an all-day nibble seems unappealing, unpractical or simply anti-social, read on.

Eat lower on the glycemic index (GI)
The glycemic index ranks foods according to how they affect blood glucose levels. Pretzels are high on the index, because they cause blood sugar to rise very quickly. Raw carrots, by comparison, have a low glycemic ranking.

Carbs in lower glycemic food are broken into glucose molecules more slowly, thereby providing a steadier supply of energy to the brain. Low GI meals, gratefully, also best satiate hunger, writes J.M. Bourre of the French National Medicine Academy in the September 2006 issue of The Journal of Nutrition, Health and Aging.

High fiber carbohydrates are relatively low glycemic but combining them with fat or protein can slow absorption even more. For example, the traditional white Wonder Bread is high glycemic; it is digested quickly, causing a stressful, and brief, spike in glucose levels. Dark fiber-rich whole wheat bread is lower on the index; its spike is slightly less sharp. But add some meat or other protein to the bread and the glucose absorption rate becomes a gentle curve. Top it off with a little olive oil and presto: brain-friendly fuel masquerading as a tasty lunch.

The key is a balanced diet, where all macronutrients — carbohydrates, fats and proteins — are given their due, Green said.

Know your fats
Despite fat's ability to lower the GI of a meal, not all fats are equal. Trans fats, common in fast food, are the worst. Saturated fats are not great. Unsaturated fat is the healthiest.

"People who eat diets high in saturated fat are more susceptible to cognitive deficits," said Gibson. The increased likelihood of strokes is just one acute example. Rats that gorged on saturated fat for several weeks had obvious damage to the hippocampus — a brain area critical to memory formation, he said.

Still, "the brain is 60 percent fat," Green said, and very low levels of cholesterol have been associated with depression, aggression and anti-social behavior. While most people in developed countries need to limit their fat intake, "zero fat is definitely not the way to go," he said.

Essential fatty acids, such as Omega-3s, are proving valuable in treating depression and other psychiatric disorders, such as schizophrenia, as well as benefiting infant brain development, Green said. However, he added, the effect of supplements on a healthy adult brain is controversial. It may be best to stick to natural sources, such as cold-water fish, seeds and nuts.

Know yourself
Despite broad similarities, food affects everyone's brain a little differently. For example, Gibson explained, extroverts are more likely to succumb to the "post-lunch dip" – that desire to nap, or chug coffee, mid-afternoon. And size matters: Children and the very thin may feel faint or grumpy due to low blood glucose faster than an average-sized adult, explained Montminy.

Nutritional habits are also important. People who chronically under-eat, over-exercise or regularly skip meals can become fuzzy-headed even after a minor dip in glucose. They become sensitized to not getting enough, Gibson said.

But with the Goldilocks approach, there is no need to diet to distraction. "Every single fad diet is total rubbish," Green said, but there is merit to eating low glycemically.