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:

1. 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.

2. 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.

3. 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 inthe 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.

4. 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.

5. 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.

The proposed revised Canada Food Guide is a recipe for dramatic increases in premature death resulting from chronic diet-related disease, some critics say.

Figure. Health Canada's proposed food guide could prove less than fruitful, say critics. Photo by: Canapress

Bill Jeffery, the Canadian coordinator for the US-based Centre for Science in the Public Interest, says he can't fathom the rationale behind recommending lower consumption of fruits and vegetables (for most categories, a daily intake of 5–8 servings, as opposed to the 5–10 recommended in the 1992 edition) and more consumption of meat (an intake of 4 servings for men, instead of the 2–3 urged in 1992).

"It will be disastrous for the health of Canadians," Jeffery says, noting that over 25 000 Canadians die prematurely each year because of diet-related diseases and that the WHO estimates nearly 5 years is shaved off life expectancy because of diet-related risk factors such as low fruit and vegetable intake.

But Health Canada says the revised guide has undergone "comprehensive" scientific review and will promote weight loss while preventing the onset of chronic disease, contends Mary Bush, director-general of the office of nutrition policy and promotion. (The revised guide was taken off the Health Canada Web site during the federal election. "We're awaiting direction before resuming consultations," spokes-person Renee Bergeron says.)

The 1992 version of the guide was considered the Canadian "bible" of nutrition, with some 24 million copies distributed nationwide.

Health Canada's review of the Guide began in 2003 and included stakeholder meetings and surveys of professionals and consumers. In the fall of 2005, Health Canada proposed a fold-out, 8-page guide to replace the current 2-sided single sheet. It retains the concept of 4 core food groups — fruits and vegetables, grain products, milk products, and meats and alternatives — while adding 6 pages of advice on issues ranging from healthy eating and shopping tips to reading labels.

More substantively, the proposed guide tailors nutritional advice to age and gender, while providing more specificity on suitable serving sizes. Instead of advising everyone to, for example, eat 5–10 servings of fruits and vegetables daily, the new guide recommends different daily intakes for 9 demographic categories: preschoolers, and 4 age groups (4–13, 14–18, 19–50 and over 50), divided by gender.

Dr. Yoni Freedhoff, an Ottawa specialist in obesity medicine, identifies a number of deficiencies in the proposed guide: inadequate recognition of the value of consuming more polyunsaturated fatty acids or more whole-grain products; a complicated and confusing array of recommended serving sizes; a total lack of guidance on recommended daily caloric intake; and insufficient warnings about the dire consequences of eating high calorie foods in the so-called "others" category. Canadians now consume about 600–800 calories worth of fried foods and sugary desserts in this category.

"It's obesogenic," says Freedhoff. With help from a dietician, he calculated that the fewest calories anyone scrupulously following the new guide would consume daily is 1700 (females aged 19–50), assuming they only drank water, didn't use salad dressing or have dessert. In other demographic categories, the daily intake topped 3200 calories, again without extras.

"What it means is that should anybody who is of average height and size follow Canada's Food Guide, there is a very, very good chance it will lead to weight gain."

Both Freedhoff and Jeffery also chide Health Canada for failing to furnish the scientific rationale for the changes and allowing industry representatives to sit on its external Food Guide Advisory Committee. The 12-member committee includes representatives from Food and Consumer Product Manufacturers of Canada, the Vegetable Oil Industry Council and the BC Dairy Foundation.

"It's obscene that industry is involved," Freedhoff says. "I would not have Exxon developing Canada's energy policy."

Health Canada's Bush says such criticism is unfounded. Representatives of industry and other stakeholders were involved because "philosophically, we absolutely believe that if we're going to improve the nutritional health of Canadians, it takes all sectors to . . . understand their role."

As for the changes, they're rooted in "comprehensive" internal reviews of nutrient standards recently developed by the US Institute of Medicine, WHO's 2003 report on Diet, Nutrition and Prevention of Chronic Disease and a 2005 US Dietary Guidelines Advisory Committee report.

Bush also dismisses concerns the guide will contribute to obesity or encourage consumption of fewer fruits and vegetables. Rather, it offers a greater degree of specificity and precision by demographic group, she argues.

None of the changes are written in stone, Bush adds, as Health Canada will complete consultations and conduct focus group testing before releasing the finished product — probably later this year. Both exercises were suspended until after the federal election, which may give influential groups such as the Dietitians of Canada time to weigh in — provided its members can reach a consensus, says director of policy Lynda Corby.

Others are convinced Health Canada is on the right track. "Overall, the direction seems appropriate," says Elinor Wilson, CEO of the Canadian Public Health Association.

"Given the obesity epidemic, promoting healthy eating is an important part of public policy," says Dr. Arya Sharma, chair of the Canadian Obesity Network. The guide is not a treatment for obesity, or diabetes or any other medical condition, he adds. "This is for someone who is healthy and wants a healthy diet."

Sharma believes the proposed guide is based on the best available science, but acknowledges that contradictory evidence is common in the nutrition sciences.

But Jeffery is so persuaded the revisions aren't evidence-based that he recommends Canadians look to the "Healthy Eating Pyramid" developed by Dr. Walter Willett at the Harvard School of Public Health as a superior, independent alternative for nutrition advice.

Source: CMAJ