People who never manage to stick to a diet could blame poor memory rather than lack of willpower, according to a new study.
Researchers from Aberdeen University in Scotland say people who have poor "executive function" are more likely to struggle to maintain a healthy routine.
Executive function refers to our brain's capacity to prioritise, multi-task, plan ahead and weigh up options. It's the type of memory that we use to stick to plans.
Gallery: 51 fat-loss secrets
People with poor prospective memory forget things they've planned, such as going to an appointment or posting a letter –– and they could forget they are on a diet.
"Prospective memory keeps you on track. Every time you are offered something to eat, you have to bring to mind that you are on a diet," said researcher Dr Julia Allan, a health psychologist at Aberdeen University.
The researchers tested some volunteers' executive function then asked them to write down what they ate for three days. They found those with poor executive function ate more sweet snacks and less fruit and vegetables than they intended.
When the researchers gave the dieters the opportunity to have chocolate, those with poor executive function were more likely to eat it.
"A person with less efficient executive function is less likely to resist temptation and stick with what they had planned on any given day, than someone with excellent executive function," said Dr Allan, who presented her findings at the British Science Festival.
"We know that there has been a lot of time, effort and money directed at health information campaigns telling people what they should be eating, why they should be eating more fruit and vegetables, why they should be eating less fat … the problem is more that people don't act on these intentions."
Dr Allan has been investigating ways to help people with poor executive function stick to a diet.
One method she trialed involved putting signs on two coffee shop counters that pointed out the calorie content of different snacks.
After three months they found sales of low calorie food increased while sales of fattening food decreased.
Dr Allan found positioning the products with fewest calories on the left made customers' inbuilt bias to look that way help them make a healthier choice.
Gallery: Ten ways to break bad habits
"From our research, it is clear that sticking to a diet is not simply a case of making a decision to eat more healthily," Dr Allan said.
"Dietary control involves lots of different psychological skills and resources and so will be much easier for some people than others."