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Dietary advice explained: carbohydrates

Posted on Saturday 2 November 2019

Last month we looked at fat as one of the three macronutrients of our diet. This month we turn our attention to carbohydrates.

What is a carbohydrate? As the name implies, in chemical terms it is a molecule constructed of carbon, hydrogen and also oxygen. We group carbohydrates into two categories - "simple" carbohydrates, made of just one or two molecules, and "complex" carbohydrates, which are constructed of many molecules. Simple carbohydrates are also referred to as sugars. A carbohydrate composed of just one molecule is known as a "monosaccharide".  The most well-known natural monosaccharides are glucose, found in all foods; fructose, mainly found in fruits and vegetables; and galactose, mainly found in milk products. Carbohydrates composed of two molecules are known as "disaccharides". These include sucrose (glucose + fructose), what we know of as table sugar; maltose (glucose + glucose), mostly found in grains; and lactose (galactose + glucose), which is found in dairy products. Whilst glucose and galactose (and disaccharides made of them) taste slightly sweet to humans, fructose is extremely sweet. 

Complex carbohydrates are also known as polysaccharides. They are constructed of a minimum of three simple carboyhydrates but are usually made of many more. Plants store most of their energy as polysaccharides, usually in the form of starch. The other vital components in addition to starch are cellulose and lignins, which are types of fibre that provide structural support to the organism. As humans, we have the capacity to store about a day's worth of energy in our liver and muscles as carbohydrate, which is in a special form known as glycogen. The body breaks down the carbohydrates we eat into simple sugars. Having high blood sugar is toxic to our cells, so our pancreas releases the hormone insulin to convert the sugars to glycogen. When we need the energy for fuel, the glycogen is broken down again, stimulated by a hormone called glucagon, into simple sugars to provide fuel for our cells. 

It would seem logical that if we eat carbohydrate it is stored in the body as carbohydrate, and fat and protein are stored in the body as fat and protein. Sadly, this is rather simplistic. All excess calories will be stored as body fat once glycogen stores are full, and the body can convert proteins and fats into carbohydrate when needed to stop our blood sugar levels dropping too low. This is known as gluconeogenesis. However, when the diet is very low in calories or carbohydrate and there are no glycogen reserves, the body starts burning body fat as its main source of energy. This process is known as being in a state of ketosis.

The apparent benefits and disadvantages of being in a state of ketosis have been discussed for years, with proponents claiming that it can aid weight loss due to improved satiety levels. Low-carb "keto" or Atkins-style diets are all about inducing the state of ketosis to burn fat stores and reduce overall calorie intake without feeling hungry. There is also evidence that ketogenic  diets have benefited diabetics and some children with epilepsy. Furthermore, our body needs certain fats and proteins to survive, but it is quite possible to stay alive without eating carbohydrates. There are also issues surrounding phytic acid, a compound found in many carbohydrate-rich foods such as pulses, grains, seeds and nuts, which can reduce the absorption of certain minerals - most notably calcium, zinc and iron.

However, critics have pointed out that being in a state of ketosis causes problems with sluggish thinking, dehydration, loss of muscle strength and endurance, bad breath and constipation, as denying our body of carbohydrates - a fuel that our body metabolises very easily - comes with many side-effects. They also observe that dietary fibre, which is a form of carbohydrate, is essential for our digestion and provides food for our gut bacteria. Phytic acid has been shown to act as an anti-oxidant and there is no convincing evidence that ingestion of phytic acid is causing widespread mineral deficiencies in otherwise healthy people, and indeed it is possible to reduce the amount of phytic acid in foods by soaking and sprouting or fermenting them, as many cultures have done for thousands of years. 

It is largely because of the influence of low-carb diets that carbohydrate as a macronutrient has become increasingly vilified in recent years. Low-carb diets have gone in and out of fashion for well over a century, and the debate over whether they are good or bad for us is unlikely to end any time soon. Diets high in carbohydrates have been blamed as the cause of obesity and diabetes, and there is a public perception that carbohydrates are something bad that should be avoided.

However, it's important to note what foods people think of when they consider avoiding carbohydrates. Unless someone is following a very low-carbohydrate diet, they are generally referring to the avoidance of carbohydrate-rich foods such as bread, pasta and cakes - all based on heavily processed and usually refined grains and often sugar. They are not referring to sources of carbohydrates such as fruit, vegetables, pulses, nuts and seeds or even whole grains such as brown rice, barley and oats.

This distinction is the key point. Whether a carbohydrate-rich food is a healthy choice is largely to do with the degree of processing. Refined flour - that which has been stripped of its bran coating, and refined sugar - which has been extracted from fibre-rich sugar beet or cane - are broken down and absorbed very rapidly into the blood stream, causing our blood sugar levels to peak and then trough as the body releases too much insulin in response to the sudden influx of carbohydrate. In the long-term this can contribute to diabetes and obesity but in the short-term it can cause energy and mood highs and lows. Once blood sugar has dipped too low, the temptation is to reach for another refined carbohydrate "quick fix" to get our energy levels back up again. And so the cycle continues.

The other problem with highly processed foods rich in fast-releasing carbohydrates is that they are often combined with large amounts of salt and fat, which we are genetically programmed to crave. During the course of evolution, these types of food would have been a rare treat, but now they are ubiquitous. The food industry makes the most of our genetic heritage and many processed foods contain these three ingredients in the form of refined carbohydrate, excessive salt and refined vegetable oils. They are known as the "holy trinity" of processed food production, as they are popular and highly profitable.

It would be easy to assume that if a product is marked "whole grain" that all would be well;, however, highly processed foods made of the whole grain - i.e. with the bran of the grain used as well as the inner part, have been pulverised to such a high degree that the molecules are often tiny and are also quickly broken down by the body. This is true of most whole grain processed cereals and bread. It is a preferable option in so far as they are richer in vitamins and minerals that come in the outer layers of the grain, but from a blood sugar level are still not ideal.

There has been interesting research in recent years in the area of resistant starch - that which resists digestion until it reaches our gut bacteria. Some foods naturally contain a lot of resistant starch such as pulses and some vegetables, but it is also possible to create higher levels of resistant starch within a carbohydrate-rich food (e.g. pasta or potatoes) by cooling it after cooking, and then reheating it to eat. It is not yet clear why the composition of the carbohydrate changes in this way when it is cooled and reheated, but it can be used to our advantage when trying to manage our blood sugar levels and we will help our gut bacteria to thrive in the process.

So in conclusion, when considering carbohydrates, perhaps we need to look beyond them being inherently good or bad. They have many health-giving properties when found in whole, natural foods, where they are accompanied by valuable vitamins, minerals, phytochemicals and fibre. The key factor is their degree of processing. An orange is a fibre-rich food, but our body reacts to orange juice very differently. Whole grain rice is absorbed by the body in a very different way to a rice cake, even if it is still whole grain. Different cultures have varying degrees of carbohydrate as a proportion of their diet. While some diets (such as that of the Mediterranean region) are relatively low in carbohydrates, others (such as the Japanese diet) are very high in carbohydrate, and when based on traditional foods, both diets are healthy.  Regardless of how much carbohydrate we eat, if we base our diet on natural unprocessed foods, we won't go too far wrong.


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