Spotlight on low-carbohydrate diets

Simple carbohydrates are made up of just one or two sugar molecules. They can be divided into ‘naturally occurring’ sugars, found in foods like fruit and milk, and ‘free sugars’ which are either added to foods such as cakes and pastries or found in fruit juice, honey and syrups.

What are complex carbohydrates?

Complex carbohydrates are made up of longer chains of sugar molecules, and are found in starchy foods including jumbo oats, rye bread and brown rice.

What is soluble and insoluble fibre?

Fibre is often forgotten, but nevertheless, it's a very important form of carbohydrate. There are two main types, soluble and insoluble – these are typically found in starchy carbs as well as in whole fruits and veg. Soluble fibre may be found in oats, barley and root vegetables as well as some fruits – studies have shown it may help lower blood pressure and cholesterol. Insoluble fibre is found in wholegrains, vegetable skins, nuts and seeds, and including it in your diet may help reduce the risk of heart disease and stroke. Dietary fibre also helps maintain a healthy environment in the gut.

During digestion, simple carbs break down quickly while complex carbs release their energy more slowly. How the food is processed can also affect how our bodies handle the carbs – refined foods such as white bread will be broken down more quickly, releasing their energy faster. Ideally, the bulk of the carbohydrates in your diet should be complex, unrefined carbs, because they are a rich source of nutrients and dietary fibre. Meanwhile, ‘free’ sugars should be kept to a minimum because they often supply little in the way of nutrition.

Do fruit and vegetables contain carbohydrates?

The natural sugar found in fruit and vegetables is called fructose. While fructose is considered to be a simple sugar, the high fibre content of whole fruit and veg means we digest it more slowly than a glass of juice, which contains the fructose but not much fibre. As a result, eating whole fruit and vegetables containing simple sugars is thought to have a less dramatic effect on blood sugar levels, while also supplying valuable nutrients and fibre.

What is a low-carb diet?

Currently, there is no official definition of a low-carb diet, although some researchers suggest a diet supplying less than 130g of carbs per day would qualify. There are numerous diets which incorporate some of the principes of low-carb eating, including the Atkins diet and the Dukan diet.

The amount of carbohydrates an individual needs will be unique to them and will depend on their age, sex, gender and activity levels. As a guide, the Reference Intake (RI) for a balanced diet is currently 260g of carbohydrates a day. Those following one of the low-carbohydrate weight loss programmes often aim for less than 50g a day.

Can low-carb diets promote weight loss?

Cutting carbs may be a useful strategy if weight loss is your goal and your doctor agrees that it will be safe for you. By reducing the carbs you eat you reduce insulin levels and increase a hormone called glucagon which triggers the body to burn fat. However, when you go into this form of fat burning, a process called ketosis occurs and compounds called ketones start to build up in the body. This process can cause side effects including nausea, headaches and fatigue, which can make low-carb diets difficult for some people to stick to.

Much of the weight loss that people experience on a low-carb diet is actually water loss as you deplete your glycogen stores, so some people find that they easily regain the weight when they resume eating carbs. Alternatively, the weight loss may be a result of an overall reduction in calories, rather than as a result of eating less carbohydrate.

Can low-carb diets help to manage diabetes?

There have been a number of news stories suggesting that a low-carb diet may help to manage, or even reverse, type 2 diabetes. Studies do support low-carb diets which are not high in saturated fats as a useful tool for managing type 2 diabetes. Furthermore, research supports a low-carb diet to be as effective as low-fat calorie restriction in terms of weight loss and significantly better for glucose control.

In their position statement on this issue, Diabetes UK cites that there is evidence to suggest low-carb diets may be safe and effective for people with type 2 diabetes. They believe that adopting such a diet may help weight loss and glucose management, and reduce the risk of cardiovascular disease. However, they also say it shouldn't be seen as the diet for everyone and may not be appropriate in the long term. For those with type 1 diabetes there is no strong evidence to say that a low-carb diet is safe or effective. Because of this, Diabetes UK does not recommend low-carb diets to people with type 1 diabetes.

How do low-carb diets affect heart disease?

A widely reported study in 2010 found that women who consumed higher amounts of carbohydrates, most notably simple and refined carbohydrates (those that cause a rapid rise in blood sugar), appeared to have an increased risk of heart disease. These high-GI foods – which included bread, pizza and rice, as well as sugar, honey and jam – appeared to influence inflammation, coagulation and vascular function.

The important factor here is the type of carbs, because other studies demonstrate that consuming complex carbs, including wholegrains and other low-GI foods, may provide significant benefits for the heart.

With specific reference to low-carb diets and heart health, the limited studies available suggest that we need to consider the diet as a whole – in particular the source of fat and protein. This is because when plant sources of fat and protein are chosen in preference to animal sources, low-carb diets may moderately reduce the risk of coronary heart disease.

What research has been done into low-carb diets and long-term health?

There has been plenty of debate over the long-term health implications of following a low carb diet. One 2017 prospective cohort study, covering participants from 18 countries, concluded that high carbohydrate intake was correlated with increased human mortality. A 2018 study concluded that both high- and low-carb diets, as opposed to more balanced diets, were correlated with increased mortality. More specifically, this study supports the findings that low-carb diets which favoured plant-derived protein and fat rather than animal sources, were more likely to be associated with lower mortality. This suggests that the source of protein and fat is an important modifier on long term health and mortality outcomes.

One significant downside to a low carb diet is that it is difficult to achieve guidelines on dietary fibre intake (30g per day). Increasing evidence suggests that the action of beneficial bacteria in our gut when we consume dietary fibre is important for overall health. This is because these bacteria act on the fibre to produce short-chain fatty acids (SCFAs) which prevent the growth of harmful bacteria, keep the gut healthy and may have wider implications for insulin management, weight and immune function.

Who shouldn't follow a low-carb diet?

Very restrictive diets are not proven to be conductive to long-term health and should be discussed with your GP or health professional before you start. In particular, if you are elderly, under 18 years of age, on medication, have a low body mass index (BMI) or have emotional or psychological issues around food, including any history of eating disorders, you should consult your GP before embarking on any radical change in eating patterns.

Likewise, if you are pregnant, breastfeeding, diabetic or have a condition that requires you to keep a close eye on your blood sugar levels, you should consult your GP or healthcare professional. It should be noted that there have been reports of adverse effects of low-carb diets adopted by children with diabetes including poor growth, a greater risk of cardiovascular disease, and psychological problems. This reinforces the fact that no child should follow a restrictive diet.

If your goal is weight loss and you have a lot of weight to lose, you should seek the advice and guidance of a dietitian to ensure that the diet you follow provides adequate fibre and all the necessary nutrients you need.

Enjoyed this? Now try...

What is the Dukan diet?
What is the Atkins diet?
What is a ketogenic diet?

This page was published on 26 September 2018.

Kerry Torrens is a qualified Nutritionist (MBANT) with a postgraduate diploma in Personalised Nutrition & Nutritional Therapy. She is a member of the British Association for Applied Nutrition and Nutritional Therapy (BANT) and a member of the Guild of Food Writers. Over the last 15 years she has been a contributing author to a number of nutritional and cookery publications including BBC Good Food.

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