After years thinking a low-fat diet was the best, suddenly we're being bombarded with low-carb diets. So which of these two food-group demons is worse if you're on a mission to lose weight?

Any products in this article have been selected editorially however if you buy something we mention, we may earn commission

If you are trying to lose weight or just stick to a well-balanced diet, whether carbohydrates and fats are friend or foe can be confusing, with diet gurus firing at each other across a great divide. Until lately, high levels of cardiovascular disease - dodgy hearts and clogged blood vessels - has led to doctors to recommend we cut back on our intake of fats. However, now it’s obesity that hits the headlines, carbs are in the firing line because high intakes are thought to increase fat - particularly around the middle and in our vital organs. The latest diet fads swing between the two - low fat one minute, low carb the next. No wonder we’re confused.

What’s wrong with fats

It’s true that fats are higher in calories than the same weight of carbohydrate or protein. And saturated fats, found mainly in animal produce like the fat on meat and poultry and in dairy foods, are associated with the high cholesterol levels that can be a risk factor for heart problems and strokes. Liver disease, now on the rise and the fifth largest cause of death in the UK, is also thought to be linked to eating fatty foods.

That’s why there are so many low fat and “light” products around - once the health risks of high-fat diets emerged, the food industry spotted a good opportunity and went for it. The trouble is, once they took tasty fats out of juicy burgers and bangers, succulent sausages, creamy sauces and puds and made them “low fat” or “lite”, they compensated by making them taste nicer with carbs. More of that later.

What's good about fats

The fact is, our body needs fats - they are essential for promoting cell growth, producing hormones and absorbing certain vitamins. Some, like the Omega 3s found in oily fish such as salmon, trout and fresh tuna, can actually protect against heart disease. They are a type of polyunsaturated fat, which is also found in grains, vegetables, seeds and the oils made from them. People who use olive oil, containing monounsaturated fat, seem to have fewer strokes than those who don’t. Dairy produce, meanwhile, is a useful source of protein and calcium - about 25 per cent of teenagers in the UK are thought to be lacking in this vital nutrient.

Our bodies digest fats more slowly than some foods and taste nice, so they leave us feeling pleasantly full. It’s why the Atkins Diet, which included them, was so popular.

What about carbs?

Carbohydrate is the umbrella term for different types of starch and sugar found in a huge range of foods from hearty wholegrain bread, pasta and rice, to potatoes, pulses like chick peas and cannellini beans, fruit, cakes, chocolate, sweets, alcohol, fizzy drinks and the sugars added to processed food. So they are a big part of our diet and, once converted into blood sugar, a major source of energy. Children need lots of carbs because they tend to be more active and are growing fast - and that burns energy.

So what’s wrong with eating carbs?

Like some fats, some carbs get the nutritional thumbs-up. The NHS eatwell plate , for example, suggests that while a tiny amount of our diet should be based on foods high in sugar, one third of calories should come from starchy foods - preferably potatoes and wholegrain varieties of bread, cereal, rice and pasta. These contain complex carbohydrates, are digested more slowly so fill us up for longer and are less likely to be converted into fat.  Unlike their refined relatives such as white flour, or grains with the husks and kernel removed, they retain all their nutrients, like fibre for a healthy gut and B vitamins in wholegrains. Simple carbohydrates include fruit - full of sugar, of course, but vitamins and lots of other nutrients too - and the sugar we spoon in tea or stock up on  when inspired by The Great British Bake Off.

The secret life of sugar

So you don’t need a degree in dietetics to work out that some carbs come with built-in benefits, and if you cut them out completely your risk losing out on some healthy-eating essentials. The trouble starts with the refined kind, and the sugar and its substitutes in junk food -  some of which, apart from the energy boost, are literally good for nothing.

Many obesity experts argue that we are consuming more refined and simple carbs than ever, particularly in junk and processed food, fizzy drinks and alcohol (whose calories all come from sugar), and that’s what’s making us fatter. What is more, although scientists don’t yet fully understand the process, this kind of simple sugar may get converted more quickly into fat around our middles if we eat more than our bodies need for energy. Muffin tops and beer bellies are associated with increased risk of Type 2 diabetes, which accounts for 90 per cent of the 2.5m cases overall in the UK. Liver disease is also associated with high sugar intakes.

We are probably consuming more refined carbs and sugar because they come cheap for the food industry. Sweetening cheap, processed food makes it taste better  - especially when it’s had the fat removed for low and light versions.

The consumer organisation Which? released a survey in September 2012 that found that six in ten consumers thought these were healthier than standard products when they often contained more sugar and the same number of calories. And sugar is very clever at hiding so that many of us may think we are eating a lot less than we really are. Look out for maltodextrin, different types of syrup and things that end in -ose on food labels. The higher up the ingredients list they are, the more the product contains.

So is it better to go low-fat or low-carb?

If your goal is healthy eating, it’s a no-brainer. Put refined carbs on the casualty list and set your sights on more complex ones to get their full nutritional monty, stick with fruit for the vitamins and keep an eye on the saturated fat in animal products.

The diet front demands a little more strategy. Cut out carbs and you will shrink fast because they help the body to retain fluid. Once that’s flushed out (and boy, you will be heading for the loo on the first few days of a carb-exclusion menu) you’ll find that the weight loss slows down. Cut out fats and you might find yourself getting hungry sooner, cheating and then beating yourself up for it, and missing out on some vital nutrients.

Cutting a whole food group like carbs or fat out of your diet stacks the cards against you - like it or not, we live in a world rich in ready meals, fast food outlets, and much loved staples like fish and chips, spaghetti bolognese, chicken tikka masala... Do you really want to turn into the sort of irritating guest who makes everyone else around the table feel guilty by saying no to a potato, a glass of wine or dessert, or spend your lunch break foraging for a fat- or carb-free meal? This approach may work for a while, but it's hard to sustain.

The key to long term weight loss is making changes that are easy to stick to and you are happy with - permanently - and nutritionally balanced.

So cut out carbs or fat? We think not. Cut back on both? Go ahead.