We all need fat. Indeed, even slimmers need fat.

Doctors now know that the popular very low-fat diets of the 1980s and ’00s were not as healthy as we liked to think, and certainly not the best way to lose belly fat, because they tend to lower ‘good’ cholesterol by at least as much as they did ‘bad’ cholesterol. And they tended to restrict foods that contain the essential fats – nuts, seeds, oily fish for example.

Fat is a great tool for helping satiety and, like protein, helps reduce the sharp blood-sugar spurts associated with high-GI carbs. So a high Glycemic Index baked potato topped with a knob of butter or some grated cheese will be a low-Gl meal. Its only downside is that it appears to have little thermogenic (DIT) effect when you eat it.

Having said that, there have been several small, but scientific, trials to show that unsaturated fats (such as those found in olive oil and walnuts) have a much higher DIT effect than saturated fats (such as cream), and the conclusion is that – especially for obese people and those with fat around their middle – unsaturated oils are worth swapping to, for their DIT bonus. One 2009 trial found DIT around 25 per cent higher with olive oil and walnuts than with high-fat dairy produce.

This could be why people who eat a diet high in items such as butter, cream, ice cream and fatty pastries are more likely to be obese and/or have a high waist circumference than people who eat a high-oil Mediterranean style diet.

Saturated fat (that found in largest quantities in meats and dairy produce) is still regarded by most professionals as the ‘baddy’, along with hydrogenated fats – which are used much less widely in processed foods now as their negative effects on health have been documented.

Fat also offers protective and wellbeing benefits. Omega 3s, found in oily fish and some plant foods, help protect your immune system, memory and brain power, and protect against major diseases, including diabetes and cardiovascular disease. Mono-unsaturated fats found in olive oil and avocados, for example, help boost HDL ‘good’ cholesterol and contain plant chemicals that can help minimize inflammation. The oleic acid they contain also helps prevent hunger.

Remember: plant and fish fats are good and you should include them in your diet. Even if they are listed as saturated, some saturated fat is essential.