But new research suggests they should also be careful about when they eat.
Researchers have pinpointed the best times to eat each meal if you are slimming.
They found the optimum time to have breakfast is shortly after 7am, with 7.11am being the best.
Lunch is best enjoyed earlier rather than later, with the key time between 12.30pm and 1pm – 12.38pm was the ideal.
Leaving dinner later than 7pm can be ruinous for diets – the best time is between 6pm and 6.30pm, with 6.14pm the best.
The timings come from a new survey by the diet company Forza Supplements, which asked 1,000 slimmers when was the best time to eat to maximise weight loss.
The research revealed 84 per cent of dieters think sticking to defined mealtimes is crucial for people who want to lose weight.
A further 76 per cent said breakfast was the most important meal of the day – and eating it daily had helped them cut calories.
Three-quarters of those polled said they had benefited by having a light lunch – and never skipping a midday meal.
Dinner must be eaten early to maximise weight loss because people are less active in the evening – meaning extra calories are more likely to turn to fat.
Two-thirds of slimmers recommended eating dinner before 7pm.
Most dieters – 72 per cent – said the key was not to exceed their lunchtime calorie intake at dinner – so they have even calorie intake throughout the day.
Six out of ten respondents said that skipping any meal would make losing weight more difficult.
The research proves the truth of the dieting mantra: Eat breakfast like a king, lunch like a prince and dinner like pauper.
The findings showed that the crucial four hours between 6pm and 10pm was when most diets went wrong.
The majority of respondents – 56 per cent – said they consumed the most calories during this time.
A further 54 per cent said they ate more than half their daily calorie intake during this period.
What scuppered many diets was snacking around 8pm in front of the TV – a lapse suffered by 62 per cent of those polled.
Forza Supplements managing director Lee Smith said: ‘The key for many dieters is not how much they eat but when they do it.
‘We found that the optimum times were 7.11am for breakfast, 12.38pm for lunch and 6.14pm for dinner but obviously no one is going to stick to those times rigidly.
‘Most dieters recommended these meals be supplemented with low-fat snacks when you get hunger pangs in later morning or mid-afternoon.’
Mr Smith said the key to successful weight loss was never to skip meals.
He added: ‘The results show that breakfast really is the most important meal of the day for successful dieters.
‘Skipping it just makes you hungrier and more likely to over-indulge in later meals – causing a surge in blood sugar.
‘What you need to do is ease your body into a more consistent blood-sugar pattern by eating three meals a day of between 400 and 600 calories.
‘The most important message is not to skip meals and not to leave dinner too late.
‘Calories get burned up no matter when you eat them – but if you eat dinner late, you’re not as likely to get rid of those calories before going to bed.
‘What you don’t burn off is more likely to be stored as fat, as you become less active towards the end of the day.
‘Eating too close to bedtime increases your blood sugar and insulin, which makes it harder to sleep.
‘Your last meal should be lightest of the day and should be eaten at least three hours before you go to sleep.’