People who ditch burgers go through the same sadness, tiredness and depression that drug users experience when they try to quit.
But if someone can last the first five days, quitting becomes easier and cravings and tiredness begin to fade away, scientists say.
The research suggests fatty foods are addictive to the brain and it could be harder for people to change their diet than experts previously thought.
University of Michigan researchers are thought to be the first to have measured how stopping eating processed foods affects people.
Trials have been done on the withdrawal symptoms from sugar in animals, but now the effects of quitting junk food have been confirmed in humans.
Sugar and salt are both known to produce cravings which drive people to impulsively eat food which makes them fat and unhealthy.
And ignoring the body's plea for that next can of soda, burger, or packet of crisps could make people feel miserable and exhausted.
These feelings of despair could then drag people into a relapse of eating badly and make it difficult for them to change to a healthier diet, said lead researcher Dr Ashley Gearhardt.
Some 231 adults were quizzed about how they felt after cutting back on processed foods during the year-long experiment.
Participants reported feeling sad, bad-tempered, tired and having cravings during the first two to five days after stopping eating junk food.
The side-effects begun to fade away after this first five days, the researchers found, which they say mirrors what happens in drug addicts.
Dr Gearhardt and her team have called for more research to measure the effects of withdrawal as they happen in real-time.
They admitted they did not take into account whether people quit 'cold turkey' or if they gradually phased out fatty foods.
Previous research by the same scientists revealed unhealthy foods like chocolate and pizza trigger the brain to produce 'reward' responses when they're eaten.
This system means eating the foods can be addictive, leaving the brain yearning for the next hit of junk food.
The new findings were published in the journal Appetite.
The research comes at a time when millions of adults are overweight and almost a quarter of the global population is expected to be obese by 2045, signalling a desperate need for many to switch to healthier diets.
More than half of Americans (55 per cent) are expected to be obese by 2045 – up from 39 per cent in 2017, and 48 per cent of Brits will be dangerously overweight.
And as a result of expanding waistlines, one in every eight people will be expected to develop type 2 diabetes during the same period, more than the one in 11 today.
This will likely lead to an increase in the number of people dying of cancer, stroke and heart disease, all of which become more likely as people gain weight.
The Daily Mail
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