What are low-carb diets? | BBC Good Food

Simple carbs (sugars)

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

These simple carbs are digested quickly and send immediate energy (in the form of glucose) into the blood stream.

Complex carbs (starches)

Made up of longer chains of sugar molecules, these are found in starchy foods including oats, bread and rice. They can be found in both wholegrains such as wholemeal flour, as well as in the refined equivalent, white flour. Complex carbs are slower to digest and, as a result, are said to supply a more steady source of energy.

Fibre

Fibre is an important form of carb – in its soluble form it’s found in oats, barley and root vegetables, as well as some fruit. Studies suggest this type of fibre may help lower blood pressure and manage cholesterol. Whereas the insoluble fibre that’s present in wholegrains, vegetable skins, nuts and seeds may help reduce the risk of heart disease and stroke. Dietary fibre also supports our gut health.

Selection of carbohydrates

How much carbohydrate do you need?

We need carbs because they provide the body with glucose, which fuels our muscles and organs, including our brain. The amount of carbs we need depends on our age, sex, gender and activity levels. As a guide, the Reference Intake, which is based on a moderately active adult female, is currently 260g of carbohydrates a day.

What is a low-carb diet?

There is no official definition for a low-carb diet, although some researchers suggest a diet supplying less than 130g of carbs per day would qualify. Other means of defining low-carb involves assessing the percentage of carbs in your total daily macronutrient intake. If this is less than 26% it’s considered low-carb.

There are numerous programmes that adopt this approach, including the Atkins diet and the Dukan diet.

How do low-carb diets work?

The theory behind a low-carb diet is that by restricting the carbs you eat you trigger the body to burn fat. This is because under normal circumstances our body uses glucose (from the carbs we eat) to fuel our activity, keep us warm and support essential organs. When we deprive the body of its preferred source of energy, it turns to an alternative source instead. After 3-4 days with limited carbs in your diet, your carb reserves (in the form of glycogen in muscle) become exhausted, and the body turns to stored fat. The process of burning fat for energy is called ‘ketosis’ and involves the liver converting your fat stores into compounds called ketones.

How quickly you reach ketosis depends on a number of factors including your body mass index (BMI), your body fat percentage and your resting metabolic rate. In order to trigger ketosis, the carbs you eat need to be heavily restricted – with some suggesting no more than 20-50g per day. To put this in perspective, an average banana contains 20g and a medium baked potato 41g, so clearly this is a diet that demands very careful planning and rigid compliance.

How to follow a low-carb diet

A low-carb diet focuses on foods that provide predominantly protein and fat. There are numerous low-carb programmes, including Atkins, Dukan and the Banting diet, each with varying limits on the type and amount of carbs permitted. The best way to understand and track the carbs you are eating is with a nutrition app that provides the carb values for different foods.

If you embark on a low-carb diet, think carefully about the fats and proteins you choose, and aim to limit foods high in trans fats. Be sure to include plenty of non-starchy vegetables, such as kale and watercress, which are very low in carbs but contribute valuable vitamins and minerals.

What are net carbs?

Anyone familiar with a low-carb diet will have come across net carbs – it refers to the amount of carbs that are absorbed by the body and contribute to calories. The figure is calculated by subtracting the amount of indigestible carbs in the form of insoluble fibre (and some sugar alcohols) from total carbs. It is worth bearing in mind that this calculation is rarely reliable or an exact science.

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What can I eat on a low-carb diet?

Broadly speaking, a low-carb diet focuses on protein, fat and non-starchy vegetables. Foods to enjoy include meat, fish, oils, butter, dairy such as cheese, and low-carb vegetables such as leafy greens, as well as specific fruits such as avocado and olives.

What foods to avoid on a low-carb diet

Unsurprisingly, you will need to restrict all bread, pasta, rice, cereals and most conventional baked goods. Depending on the diet you’re following, you may need to restrict legumes, beans, root vegetables, sweetcorn and some fruits as well as starchy vegetables, such as potatoes.

You will also need to watch what you drink – fruit juices, fizzy drinks, cordials and even milk-based drinks may need to be limited.

What’s the evidence for low-carb diets?

By limiting carbs and including more protein in your diet, you may help suppress your appetite hormones and better manage your hunger after you have eaten. Including more protein in the diet may also improve the rate at which you burn calories by as much as 20-35%.

For these reasons, research suggests that a low-carb diet may lead to weight loss. However, the initial loss is mostly water and you’ll have to stick to the plan long-term in order to see fat loss. Some studies also look at ketones and the role they play in reducing inflammations in the body.

Does a low-carb diet lead to weight loss?

In the short term, weight loss may be achieved at a relatively fast rate when carbs are heavily restricted. However, over the longer term (beyond 12 months), there is no evidence that low-carb diets are any more effective than a standard calorie-restricted weight loss plan.

Healthy wholegrain carbs on wooden background

Are low-carb diets healthy? Our nutritionist’s view…

Cutting carbs may be a useful weight-loss strategy in specific situations. For example, if you need to achieve weight loss prior to surgery and as long as your doctor has agreed that it will be safe for you to adopt this style of eating.

Minimising carbs also helps to reduce insulin levels and increase a hormone called glucagon which triggers the body to burn fat. However, with this comes side effects including nausea, headache and fatigue as well as possible electrolyte imbalance, which makes this form of diet difficult to stick to and potentially dangerous.

It’s also worth remembering that much of the initial weight loss is actually water as the body uses it to store glucose. As the glucose becomes depleted, it releases the water. Hence why some people regain the weight once they resume a more balanced way of eating.

It’s important for followers of a low-carb diet to understand which fruits and vegetables are low in carbs. Without this knowledge, it may be difficult to achieve your 5-a-day. Eating a wide range of fruit and veg not only allows us to get plenty of vitamins and fibre but also means we benefit from protective plant compounds including flavonoids and carotenoids which may help fight heart disease, prevent certain cancers and may slow the signs of aging.

Another downside to low-carb eating is that it’s very difficult to reach your dietary fibre target (30g per day). Increasing evidence suggests that fibre is important not just for digestion but for other reasons, too. For example, beneficial gut bacteria break down fibre in the gut to produce short-chain fatty acids (SCFAs) which keep your gut healthy and have wider implications for insulin management, weight and immune function.

Currently, we don’t know the risks of following a low-carb diet over a longer period of time because most studies have been over short periods. For this reason, more long-term clinical trials are needed to assess changes in nutritional status and body composition, and to evaluate cardiovascular risk factors as well as the impact on the gut microbiota.

What about a low-carb diet for those with diabetes?

There have been a number of media headlines suggesting that a low-carb diet may help to manage, or even reverse, type 2 diabetes. Studies support low-carb diets, which are also not high in saturated fats, as a useful tool for managing type 2 diabetes.

Diabetes UK cites 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, as well as reducing risk of cardiovascular disease. However, they also say it may not be appropriate in the long term or for those with type 1 diabetes.

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.

Who shouldn’t follow a low-carb diet?

All diabetics and anyone with a blood sugar management issue should discuss the potential implications of a low carb diet with their GP and healthcare team before embarking on such a regime. Similarly, anyone who meets one or more of these criteria:

  • is under 18 years old or elderly
  • have a low body mass index (BMI)
  • has a pre-existing medical condition
  • are pregnant or breast-feeding
  • has an emotional or psychological issue around food, including has or are recovering from an eating disorder
    or are on prescribed medication

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 all the necessary nutrients you need, including adequate fibre.

Very restrictive diets are not proven to be conducive for long-term health and should be discussed with your GP or health professional before you start.

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This page was reviewed on 6 November 2023 by Kerry Torrens.

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.

All health content on bbcgoodfood.com is provided for general information only, and should not be treated as a substitute for the medical advice of your own doctor or any other healthcare professional. If you have any concerns about your general health, you should contact your local healthcare provider. See our website terms and conditions for more information.

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