Carbs, Sugars, and More Sweet Things

6: Carbs, Sugars, and More Sweet Things, Brain Food


As described in the previous chapter, the brain is a particularly talkative organ. Its activity requires a continued fueling of the electrical impulses neurons use to produce the neurotransmitters and communicate with one another. This awe-inspiring process requires a tremendous amount of energy.

One of the very first questions that nutritionists sought to answer in their quest of a healthy diet was “What keeps us running?” Carbohydrates figured prominently in the answer.

There are many different forms of carbohydrates, classified according to their chemical composition as well as their capacity to provide energy. There are carbs that provide quick energy, such as simple sugars like honey, and there are complex carbs that require more digestion for their components to be fully absorbed by the body and consequently provide time-released energy, such as whole wheat and brown rice.

It was the quick-energy type of carbohydrates, the simple sugars, that prompted early nutritionists to recognize these nutrients as the main energy source and fuel for the body as a whole. But of all organs in the body, the brain is the one that needs them above all.

This is yet another major difference between the brain and the body. While the body can use both fat and sugar for energy, the brain relies exclusively on a sugar called glucose. In other words, all the energy our hungry brains need—every ounce of it—comes exclusively from glucose. Before sounding the alarm (“Sugar!”), consider that there is nothing exceptional about this. Normally, human bodies are sugar-driven machines: glucose is the go-to nutrient and the quickest way for the entire body to obtain energy. Whenever you eat foods that are naturally rich in carbohydrates, these are ultimately broken down into glucose, which is quickly absorbed in the bloodstream and transported all over the body to be immediately used for energy through the process of metabolism. Glucose readily crosses the blood-brain barrier to feed all those voracious billions of cells that populate your brain.

So don’t let the statistics mislead you: while it is true that carbohydrates only account for a very small percentage of the brain’s physical composition, there is a never-ending 24/7 influx of glucose inside the brain. Since its work is so demanding, glucose is expended at such a rapid-fire rate it simply doesn’t have time to slow down and accumulate in tissues.

Where is all this glucose coming from? Our diet, of course.

From a neuro-nutrition perspective, carbohydrates like glucose are far from being the enemy, as they are essential for proper brain activity and cognitive performance. The human brain is so critically dependent on glucose that it even developed sophisticated mechanisms to convert other sugars into glucose. For example, fructose, a sugar found in most fruit and honey, as well as lactose, found in milk and dairy, can both be transformed into glucose as a quick fix whenever our glucose runs too low.

However, if you are about to reach out for a sugary snack, hold on. When we talk about carbs, we are not talking about cupcakes. Also, we are not talking about stuffing ourselves silly. Although glucose is one of the few nutrients to be granted immediate entry into the brain, its entry is tightly regulated. In accordance with a strict supply-and-demand model, specific “sugar gates” are present in the blood-brain barrier that open when the brain needs glucose and close once enough glucose has been supplied. If your brain is active and needs glucose to work, it will take up as much glucose as it needs from the bloodstream. But if your brain is feeling satisfied and doesn’t need more glucose than it has already absorbed, that last bite of pasta or spoonful of gelato won’t make your brain work any harder or better—it would only be facing a closed door. It might make you gain a few pounds in other parts of your body, though.

Once inside the brain, whatever little glucose isn’t immediately used to make energy is converted into a substance called glycogen and stored away for future use. This is an efficient way of saving useful calories and providing your brain with an energy reserve that will keep you going in between meals. However, glycogen stores are minimal. Our reserves would last no more than a day, if needed.

When carbohydrate supply is limited (usually below 50 g/day, the equivalent of three slices of bread), glycogen stores are quickly depleted, leaving our working brains in potential jeopardy. But ingenious as ever, our brains have a Plan B. If carbohydrate supply runs too low, Plan B goes into effect, and the brain turns to the liver to start burning adipose fat and produces a new type of molecule called a ketone body. Ketone bodies are the only backup energy source for our brains.

You might have heard of ketone bodies if you’ve ever been on certain low-carb diets. One in particular is called the ketogenic or “keto” diet—and it is a neuro-nutritionist’s nightmare. It is high in saturated fat and very low in carbohydrates and fiber, which forces the liver to burn all available sugars before turning to fat to stabilize blood sugar levels. At the same time, burning fat can promote weight loss—and according to some, even mental well-being. We’ll talk more about the keto diet in chapter 9. For now, keep in mind that, while it is true that the brain can use ketones in place of glucose, this is the exception, not the rule. Burning ketones instead of glucose is the body’s providential mechanism reserved for extremes such as starvation. Were the brain able to ask you for nourishment, it would not be asking for ketones. More important, the brain cannot run solely on these molecules. It still requires no less than 30 percent of its energy from glucose.

All in all, the brain runs best on glucose. In fact, it is so vulnerable to sugar deficiencies that any interruption in glucose supply causes a near-immediate failure of brain function, as can be seen from the rapid loss of consciousness caused by severe hypoglycemia (very low blood sugar). Especially as we age, we need to ensure that our brains have access to all the glucose needed to maintain functionality and mental sharpness on a daily basis.


Carbohydrates are controversial when it comes to dieting. But from the brain’s perspective, what differentiates “good carbs” from “bad carbs” is actually the food’s specific glucose supply.

Glucose has been the main subject of my research since college. Over the years, I’ve looked at glucose in every possible way, from blood tests to brain scans. Your brain needs it. No matter how many dietologists, doctors, or journalists tell you that carbohydrates are bad for you, the brain runs on glucose, and glucose is a carbohydrate.

The problem is, when most people say carbs, they think of white food: sugar, bread, pasta, and baked goods. As sugary as these foods might taste, they are not good sources of glucose.

So where can we find this precious sugar?

As you can see in Table 5, foods that we wouldn’t have necessarily thought of as sugary, such as onions, turnips, red beets, and rutabaga, turn out to be the best natural sources of glucose. The red beet in particular is “Nature’s candy.” A small red beet alone contains 31 percent of all the glucose you need for the day. Fruits like kiwi, grapes, raisins, and dates are also excellent, as are honey and maple syrup. Whether we’re speaking generally or looking for a brain pick-me-up, these foods are much better natural sources than others because they provide us with our precious glucose while minimizing the total amount of sugar ingested.

Instead, sugary foods like candy, cookies, and even orange juice contain plenty of other sugars but hardly any glucose. For comparison, white table sugar is 100 percent sucrose, a different type of sugar.

Food itemGlucose (g/100 g product)Total sugar (g/100 g product)% Glucose
Spring onion1.41.688%
Apricots, dried20.338.952%
Whole-wheat bread 1.43.936%
Red beets4.01331%

Table 5. Top ten glucose-rich natural foods, ranked by % glucose content.

This brings us to the next question. How much glucose do we need?

Believe it or not, you won’t find the answer on the Internet. In fact, as of today, there are no dietary requirements for glucose (or for carbohydrates, for that matter). We need to turn to science to find the answer.

The best way to look at brain metabolism is through PET scans. For many years, I have been using PET to study the way the brain burns glucose to produce energy (glucose metabolism) and its relationship to cognitive functions such as memory, attention, and reasoning.

While everybody is more or less familiar with MRI scans, not many people know what a PET scan actually is. Have you ever seen one of those pictures of the brain painted in bright red and yellow or the darker hues of blue and green? Those are PET scans. The brightly colored areas are the more active of the brain, while the darker ones show a lower brain activity. Since the brain uses exclusively glucose from our diet to stay active and produce energy, we’re actually looking at the brain burning glucose from our foods.

This procedure involves injecting a small amount of glucose into the bloodstream. The glucose then rapidly enters the brain and flows straight to the most active regions in the brain, since that’s where fuel is needed most. But we’ve done something special with this glucose. It is attached to a unique, radioactive ingredient called fluorine 18, which glows as it’s deposited into the brain. We then use a brain scanner to detect its light, which, thanks to its varying intensity, demonstrates the degree and location of metabolic activity taking place inside your head.

Scientists have used this method to discover the exact amount of glucose a healthy brain consumes on a daily basis. In technical terms, the brain burns an average of 32 micromoles of glucose per 100 grams of brain tissue per minute. In plain English, this means that to stay active and healthy, a vital adult brain needs about 62 grams of glucose over a twenty-four-hour period. Some brains need a bit more; some need a bit less to function at their optimal level.

Do 62 grams of glucose sound like a lot of sugar?

It isn’t. In fact, it is less than 250 calories a day. And more important, it can’t be just any sugar. It has to be glucose. For example, 3 tablespoons of raw honey will give your brain all the glucose it needs for the day. As a comparison, you’d need to eat 16 pounds of chocolate chip cookies to achieve the same goal.


In addition to focusing on the glucose content of the food we eat, we also need to pay attention to our total sugar intake over the course of the day. A major downside of the brain’s reliance on glucose is that our mental sharpness is highly vulnerable to any drops in blood sugar levels. Therefore, we need to provide adequate amounts of glucose while keeping our blood sugar levels stable, an essential for proper brain function.

Additionally, you don’t want your blood sugar to be too high, either. As any diabetic patient knows only too well, blood sugar levels can easily run amok. But for the vast majority of the population, high blood sugar doesn’t occur because of a medical condition or because your DNA has a sweet tooth. It all comes down to what you’re eating.

This is how it works. Blood sugar levels are regulated by a hormone called insulin. The pancreas secretes insulin when there are large amounts of sugar in a meal. Insulin helps cells and tissues take up that sugar for energy, while at the same time removing the sugar from our bloodstream.

People who consume high quantities of sugar, especially refined white sugar, on a regular basis end up overworking their pancreas and exhausting their insulin sensitivity. Once the pancreas is burnt out, blood sugar levels will remain high regardless of the amount of insulin produced. This leads to a medical condition called insulin resistance, in which the body produces insulin but isn’t able to use it efficiently. When people have insulin resistance, glucose builds up in the blood, altering the body’s metabolism to its great detriment.

Insulin resistance has become a frighteningly common condition in the United States. In 2012, the U.S. Department of Health and Human Services estimates that at least 86 million American adults, ages twenty or older, had insulin resistance or pre-diabetes. The prevalence is estimated to rise even more in the future. People with insulin resistance are at high risk for type 2 diabetes, obesity, and heart disease, which have long been recognized as risk factors for dementia. As many as 6 to 8 percent of all dementia cases are attributed to type 2 diabetes, while heart disease and stroke account for another 25 percent of patients. As an additional side effect, insulin resistance is coupled with increased fat accumulation, which only worsens one’s metabolic imbalance, with terrible repercussions for the health of our brains.

Moreover, the hippocampus itself (the brain region specialized in memory) can actually experience insulin resistance. Without going into too much detail, insulin resistance can lead to brain inflammation and accelerated free radical production, making it very hard to remember anything at all.


Have you ever experienced the highs and lows caused when your blood sugar levels jump too high and then come crashing down? Have you noticed how weak and fatigued you feel afterward? That’s “the candy bar effect” and it doesn’t do you, or your brain, any good.

As mentioned above, high blood sugar levels cause inflammation, insulin resistance, metabolic disorders, and type 2 diabetes—which, in turn, raise one’s risk of dementia.

But you don’t even have to go as far as actually having diabetes to be in trouble. Research shows that high blood sugar levels can have a damaging effect on the brain, especially as we age. For example, a study tracked blood sugar levels of over two thousand elderly people for seven years, to compare high blood sugar with the possibility of poor cognitive outcomes later in life. While none of the participants had dementia when the study began, many of those with high blood sugar levels developed dementia over the course of the study. The higher the amount of sugar in the blood, the higher the risk of dementia—even at glucose levels considered normal in standard glucose tests (<120 mg/dL). In other words, sugar levels that are “tolerable” for the body as a whole might in fact be too high for our delicate brains.

Even more disconcerting evidence comes from brain imaging studies showing that dementia-free elderly with high sugar levels exhibit not only decreased memory performance, but also accelerated brain shrinkage, as shown on MRIs. This correlation was also found in participants without a trace of diabetes, and was particularly dramatic in the memory regions of the brain.

As a society, if we want to preserve our memories and lower our risk of mental deterioration (and diabetes), we urgently need to limit our sugar intake to just the amount and sort the brain really needs. This means focusing on healthy sources of glucose while reducing our intake of those insulin-crashing bad sugars to keep our metabolism strong and steady.

Unfortunately, the typical American plate is filled with processed food and refined grains, and is often accompanied by large (or by the rest of the world’s standards, gigantic) cups of soda on the side, each one brimming with concentrated sugar. It’s just as common to see people snacking on candy bars, fluffy white-flour products, and extra-large, sugar-enriched fancy coffee drinks. According to the nutrient data provided by Starbucks, a tall (which is actually their smallest portion offered) Caramel Chocolate Frappuccino Blended Crème provides 300 calories of goodness. But hidden behind its inviting appearance, you’ll find a whopping 48 grams of refined white sugar. If that weren’t enough, you have the option to top it all off with an additional hearty dollop of whipped cream. And if you’re looking for fiber, look elsewhere.

The bottom line is that we eat far too much of the sort of sugar that makes us fat and not enough of the kind of sugar that makes us smart. For our own good, we owe it to ourselves and our children to correct this.

A good way to keep an eye on sugar intake is to pay attention to a food’s glycemic index. The glycemic index is a nutrition rating system that ranks foods based on their ability to affect blood sugar levels. If a food metabolizes quickly into sugar, and the sugar is absorbed quickly into the bloodstream, the food gets a high rating because of its ability to cause an unhealthy insulin spike in your body. If a food barely raises your blood sugar, it is assigned a low score instead. Additionally, it’s important to look at the glycemic load of your food. This is a similar system that ranks foods not only in terms of how quickly they produce a sugar response, but also by the amount of fiber a food contains. The more fiber, the lower the overall food’s effects on insulin.

In terms of brain activity, foods that metabolize quickly into sugar and contain very little fiber are the worst you can eat. These include sugary beverages, sweetened fruit juices, baked goods, and candy, as well as white-flour foods such as pasta and pizza.* Instead, complex carbs and starches are more fibrous and difficult for your body to break down, and the slower breakdown results in less of a blood sugar spike. Sweet potatoes or yams (especially eaten with the skin on), fiber-rich fruits like berries and grapefruit, and vegetables such as pumpkin, butternut squash, and carrots are all excellent lower glycemic foods. Other choices such as legumes (lentils, garbanzo beans, and black beans) and whole grains (with their husks still on) also provide you with more stable sugar levels while at the same time being a good source of brain-essential glucose. In other words, if you have a sweet tooth, the trick is to load up on fiber.

From a nutritional perspective, fibers are divided into soluble and insoluble fiber. Soluble fiber, like that found in oatmeal, blueberries, and Brussels sprouts, is the kind of fiber that turns into a gel-like texture as you eat, slowing down your digestion and making you feel fuller longer. Insoluble fiber, like that found in wheat bran and dark leafy greens, does not dissolve during digestion at all, adding bulk to your stool. This in turn helps your digestive tract eliminate waste more quickly. Many whole foods, especially fruits and vegetables, naturally contain both soluble and insoluble fiber.

Besides offsetting your insulin response, the amount of fiber in your diet plays a major role in supporting your gut and immune health. Diets that are low in fiber are typically associated with constipation, gastrointestinal (GI) disorders, inflammation, and an increased risk of some cancers like colon cancer. Those countries whose diets are dependent upon frequent fast-food consumption and little or no fresh produce (such as the United States) top the list of low-fiber eaters, consuming as little as 10 to 15 grams of fiber a day, which also correlates with high rates of GI issues in our country. As we’ll see in chapter 8, if your gut isn’t healthy, your brain will suffer as well, which further underlines the importance of eating fiber for optimal brain health.

In conclusion, to make your brain happy, focus on low-glycemic/high-fiber foods as the main source of carbs in your diet, and indulge in high-glycemic foods only in small amounts and infrequently.

If you, like me, can’t go without an occasional treat, do not despair. Some foods that qualify as “treats” still possess an overall low glycemic load, making a better food for you than originally thought. For example, a square of organic dark chocolate (70 percent or higher) has a low glycemic load, which makes it satisfying without the sugar rush. Same for popcorn. I invite you to check my website ( for several glucose-rich, lip-smacking treats that you can indulge in that won’t compromise your blood sugar levels nearly as much as a candy bar would