At this time of year the media is full of new diets and weight-loss hints, supposedly because we have all eaten far too much over Christmas. ‘Low-carb’ has been a supposed miracle weight-loss diet since the 1970s – but what does it mean, and does the science stack up?

In the affluent developed world, where the problem is too much food rather than too little, there is always an eager market for the next miracle diet. The obvious advice from the health agencies to eat healthily and move more does not sell books or magazines, and requires commitment, sense and effort. It is human nature to look for the easy solution, and so there are always those willing to sell those solutions.

‘Low-carb’ diet advice has been around for a surprisingly long time, and has many vociferous advocates and attackers. It is difficult to find an unbiased opinion, and so the best way to assess the truth is to take a look at the science of nutrition.

Firstly – what is a carbohydrate? In scientific terms, a carbohydrate is a chemical compound of carbon, hydrogen and oxygen. In food science, a carbohydrate is a compound of sugar molecules. There are two types of food carbohydrate: simple and complex.

Simple carbohydrates fall into two groups.  Glucose, fructose and galactose all have the same chemical formula (but different structures) and consist of one sugar molecule. Lactose, sucrose (common sugar) and maltose contain two sugar molecules, and again have the same formula but different structures. These compounds are quick to digest and easily absorbed by the human body. Simple carbohydrates are found in fruits as well as in the obvious sugary treats (biscuits, cakes, sweets and so on). These simple sugars are also added to many foods – often to ‘low-fat’ food to improve the taste.

Complex carbohydrates are a chain of three or more sugar molecules. The chemistry of these compounds is more elaborate, but from a nutritional point of view the important difference is that they take much longer to be broken down in the human body. Hence these are also known as ‘slow release’ carbohydrates. Complex carbohydrates also include the items usually known as ‘starchy food’, and are found in items such as bread, pasta, vegetables and legumes (including peas, beans and lentils).

So what is the nutritional difference between the two forms? Simple carbohydrates provide a quick energy boost, because the glucose that goes into the blood stream and provides energy is easily available. However the ‘boost’ does not last long and is quickly followed by a ‘crash’. Anyone who has tried to combat the mid-afternoon energy dip with a bar of chocolate will be familiar with this phenomenon. Eating fruit does provide additional fibre, whereas fruit juice, smoothies and sugary foods are ‘empty calories’ with limited nutrition.  In short – simple carbohydrates should be limited in a healthy diet.

Complex carbohydrates do not provide the instant ‘boost’, as they take longer to be broken down in the digestive system. Hence it takes more time for the food to be converted to glucose and to enter the blood stream, and so more time for the body to be able to make use of the energy provided.

Most medical authorities (i.e. those without a diet book to sell!) advocate that a normal daily diet should include between 40% and 60% complex carbohydrates. A typical meal should contain twice as much carbohydrate as protein.

As weight loss or gain is primarily controlled by the balance of calories consumed versus calories expended, complex carbohydrates are good news for those watching their weight. This is because they contain fewer calories, gram for gram, than protein or fat. This means that you can eat more carbs for the same calories. Complex carbohydrates are digested slowly, so hunger is held at bay for longer. The effect is that less food will be eaten if the proportion of complex carbs is increased.

Studies of the extreme low-carb regimes show that they are no better for weight loss than a higher-carb, lower-fat plan. Extreme low-carb also results in bad-tempered dieters due to the inconsistent blood sugar levels, and high-fat diets are repeatedly proven to be bad for the circulation. The attempts to put the body into ketosis (‘starvation mode’ when fat is used, not glucose) are also considered bad for health by many authorities.

It is also interesting to read the case studies of those on extreme low-carb diets. Most lose a lot of weight rapidly, because they have greatly restricted their food intake. However, as soon as they come off the diet and return to what they perceive as normal eating, the weight goes back on. This is because the extreme regime is simply not sustainable. This ‘yo-yo’ dieting is very bad for health and usually results in a gradual weight increase.

Perhaps recognising this, the low-carb diet regimes have changed in the forty years since they were first proposed, and now take a less extreme approach. In fact, if you now look on the website of one of the most popular ‘low-carb’ diets, you may be surprised to find what is included. The diet now advocates eating ‘good’ (i.e. complex) carbohydrates, restricting high-sugar (i.e. simple carbohydrate) foods and eating a reasonable proportion of ‘good’ (i.e. non-hydrogenated) fats.  The diet plan also now recommends exercise – the original versions made no mention of this.

So it appears that even low-carb now means good-carb, and everyone is recognising that there is really only one effective and healthy way to lose weight and to keep it off. ‘Eat healthily, move more’ may yet win the day.