Nutrition Without the BS (Part I — Carbs)

Recently I took an academic course in Nutrition Science. I thought I knew a lot about this stuff, but I’ve since learned that my understanding was distorted by popular opinions and press after all.

To evaluate new claims about nutrition, health or obesity, you need to know the basics of how the food is digested and absorbed and why our bodies need different nutrients.

The thing is when you leave junk science and personal convictions aside, and look at what we know for sure, you won’t find easy or simple solutions, and in many cases, you won’t find definitive answers.

So, short version of what you need to know is here:

GI Tract and Digestion

The purpose of the alimentary tract is digestion and absorption of food. Digestion is a mechanical and chemical breakdown of macronutrients (protein, carbs and fat) into their basic components:

• Protein into amino acids
• Fat into fatty acids
• Carbs into, mainly, glucose

Mechanical breakdown refers to chewing your food. Chemical breakdown is carried out by digestive enzymes in the:

• Mouth (starches undergo significant breakdown here),
• Stomach (important in protein digestion, a minor role in fat digestion),
• Small intestine (most important for chemical digestion by enzymes produced by the pancreas and intestinal cells).

When food is broken down into absorbable molecules, they pass through the intestinal wall into the bloodstream to be distributed to the rest of the body. Undigested food (such as fiber) ends up in the colon, where it’s broken down by the bacteria that lives in the colon.

Carbohydrate

Carbs can be simple or complex. Simple can be monosaccharides or disaccharides. Complex carbs are called polysaccharides.

Monosaccharides (glucose, fructose, and galactose) can be directly absorbed into the bloodstream.

Disaccharides are what we usually mean when we refer to “simple sugars”. They are formed from two monosaccharides. They can be:

• Sucrose -> glucose + fructose (common table sugar)
• Lactose -> glucose + galactose (found in milk)
• Maltose -> glucose + glucose (we rarely consume this)

The monosaccharides can be directly absorbed into our bloodstream so they don’t require any digestion. However the disaccharides need to be broken down to the individual monosaccharides, this process is part of normal digestion and is called hydrolysis.

Starch is a complex carb. It consists of glucose molecules, so consequently, what you end up with after digestion is pure glucose.

Fiber is a chemically diverse group of non-digestible carbohydrates. They are not subject to digestion by endogenous enzymes, but may be digested by bacteria in the colon. It can be soluble and insoluble.

Carbohydrate Digestion and Absorption

The process of carbohydrate digestion starts in the mouth. Enzyme amylase, present in saliva is able to break down starch.

As we swallow, food reaches the stomach and stomach acid inactivates amylase, so the digestion stops while the food is in the stomach.

The content of the stomach is gradually released into a small intestine, where it mixes with the bicarbonate-rich pancreatic juices that neutralize the acid. Pancreatic juice is also rich in pancreatic amylase, which is responsible for most of the starch digestion.

Starch is now broken down into disaccharide: maltose, lactose, and sucrose, that need to be broken down further (by enzymes maltase, lactase, and sucrase)

Finally, we end up with monosaccharides (predominantly glucose, galactose, and fructose). These are ready to be absorbed into the bloodstream. Once absorbed into the bloodstream, they are transported to the liver by the portal vein. Most of the fructose is then cleared in the liver, but most of the glucose is available to the body.

Fiber is not broken down by the enzymes. When it reaches the large intestine, it is subject to fermentation by the gut bacteria (microbiota).

Important byproducts of fermentation are short-chain fatty acids (acetate, propionate, butyrate). These short-chain fatty acids are considered at least partially responsible for the beneficial effects of fiber.

Currently, there is a lot of interest in microbiota and researchers are trying to figure out how the microbiota is changing in response to certain dietary changes and what the impact of the changes may be on human health.

Carbohydrate Metabolism

Now we’ll turn our attention to what happens to monosaccharides once they’re absorbed into the bloodstream. After a meal, blood glucose level goes up, and this triggers the pancreas to release insulin.

Insulin has two major functions; it stimulates the uptake of glucose into tissues and it stimulates the conversion of glucose into the storage form of glucose which is called glycogen.

As a consequence of those two actions, the blood glucose level goes down. This triggers the production of another hormone by the pancreas called glucagon, which acts as an opposite of insulin. It promotes the breakdown of glycogen and thereby it contributes glucose to the bloodstream. It helps maintain blood sugar levels during periods of fasting when no food is coming into the body.

Glycogen is stored in the liver and muscles. Glycogen in the liver helps maintain blood sugar levels since it’s broken down during fasting (process triggered by glucagon)

Glycogen in the muscle is used locally, inside the muscle where it is stored and it provides energy for that particular muscle (and cannot be distributed elsewhere)

When both of stores of glycogen are filled up, glucose converts into fat. Fat, on the other hand, can’t be converted back into glucose.

Glycemic Index

The glycemic index is defined as “the incremental area under the blood glucose response curve of a specific portion of a test food expressed as the percentage of the response to the same amount of carbohydrate from a standard food taken by the same subject”. Standard food is glucose or white bread, and a portion is usually 50g of digestible carbohydrate.

Therefore, glycemic index describes the relative increase in blood glucose after consuming a particular food in comparison with a standard food. Foods that have a high GI cause a more pronounced increase in blood glucose, whereas foods with a low GI cause a shallow increase in blood glucose.

GI is not constant for a particular food. Factors that may affect the GI of a food include the cooking method, processing, and ripeness of produce, the presence of fat, protein and fiber in the meal (they all slow down food absorption). The GI also varies between individuals and within an individual depending on the time of the day, prior food consumption etc.

Foods that have a high GI include white bread, white rice, glucose, and potatoes. Food with low GI include beans, whole grain cereals (oats, brown rice, wheat, rye), and most fruits and vegetables. Interestingly, fructose has a low GI.

The major criticism of the GI is that it is defined for isolated foods (which we rarely eat in isolation) and that it fails to take into account the total amount of carbohydrate consumed. In response, the glycemic load was introduced which represents the multiplication of the glycemic index of the food in question by the carbohydrate content of the actual serving.

Fiber

Eating dietary fiber is one of the keys of a healthy diet. Thousands of studies have examined effects of fiber, and the results are solid.

Soluble dietary fiber help:

• Lower blood cholesterol and reduces the risk of cardiovascular disease (Several dietary fibers including pectin, psyllium, beta glucans and possibly inulin, reduce blood cholesterol levels. It is not clear precisely why this happens, but it seems to be related to their fermentation to short-chain fatty acids.)
• Slow glucose absorption
• Protect against type 2 diabetes (possibly due to slowing down glucose absorption)
• Prevent constipation (through water binding capacity)

Insoluble dietary fiber:

• speeds up fecal passage through the colon
• Helps us feel full for longer

Fiber and colorectal cancer:

A number of ecological and control studies have shown a correlation between high fiber intake and reduced incidence of colorectal cancer. But a series of cohort studies failed to show this relationship. People with low fiber intake did not have a higher chance of developing colon cancer. The current view is that the evidence leans slightly towards the beneficial effect of fiber.

High Fructose Corn Syrup (HFCS)

To get HFCS, starch from corn is converted into glucose, followed by the partial conversion of glucose into fructose.

So high-fructose corn syrup (HFCS) is a mixture of glucose and fructose. Corn starch is converted into glucose, followed by the partial conversion of glucose into fructose. About 1/4 of HFCS is water, and the ratio of glucose to fructose can differ. HFCS 55 means the variety is 55% fructose. Another common variety is HFCS 42.

Health Effects of Sugar

Studies in mice have convincingly shown that elevated fructose intake promotes the development of a fatty liver, but studies with humans are much less clear. With few exceptions, existing studies do not support a benefit of replacing fructose with glucose.

The view of most nutrition scientists is that high sugar consumption is partly to blame for the obesity epidemic, but by fixating singularly on the sugar and fructose, we should not be distracted from the importance of avoiding caloric overconsumption, which ultimately is responsible for weight gain

Sugar also provides the substrate for bacteria that grow in the mouth and produce acids. Acid damages enamel surface covering the teeth, so sugar is connected to tooth decay.

As previously mentioned, high fructose corn syrup (HFCS) is a mixture of glucose and fructose (in approximately equal amounts), and as such it is indistinguishable from sucrose (table sugar) after the absorption in the body. Therefore, there is no scientific basis for the assertion that HCFS is somehow worse for you than regular sugar.

Likewise, glucose and fructose in “natural sugar” like honey, maple syrup, and fruit juices is chemically identical to glucose and fructose in table sugar. WHO argues that sugar in honey, syrups, fruit juices and fruit concentrates, should also be labeled as added sugar.

Scientific evidence also doesn’t support the notion of artificial sweeteners being harmful at normal doses. The European Food Safety Authority, which regulates claims in the European Union, issued the following statement: “Following a thorough review of evidence provided both by animal and human studies, experts have ruled out a potential risk of aspartame causing damage to genes and inducing cancer”.

However, a new research found that artificial sweeteners may be triggering higher blood-sugar levels in some people. Although the precise reasons behind the blood-sugar changes remain uncertain, researchers suspect that artificial sweeteners could be disrupting the microbiome.

Researchers themselves “insist that their findings are preliminary and shouldn’t be taken as a recommendation on whether people should reconsider using artificial sweeteners.” The linked article also mentions past studies showing that people are more likely to gain weight when drinking sugar-sweetened beverages than when using artificial sweeteners.

To summarize, overarching message is that it is good to reduce sugar intake, since sugar doesn’t provide any micro nutritionist (“empty calories”), and likely contributes to the obesity epidemics, but there are no definite scientific evidence that sugar is bad for health on its own.

Next up: Fats.