Lead researcher Robert Fletcher said while the study did not investigate the causes behind the rise, he felt there were three key factors.
He said adults in their 20s and 30s were ones who had grown up during a period when there was a boom in the unhealthy food market.
“They have been surrounded by unhealthy food in their formative years. On our high streets there has been a proliferation of takeaways and fast food outlets and unhealthy food has been heavily advertised as these age groups have been growing up.”
He also said the pandemic followed by the cost of living crisis was likely to have had a bigger impact on younger age groups.
“The stress of looking after children, while working from home and then being able to afford healthy food as inflation increased, has made healthier lifestyles more difficult,” he added.
Sarah Perman, from the Association of Directors of Public Health, agreed. She said unhealthier options now dominated “our supermarkets, corner shops and fast food outlets”.
“From infancy, children and young people are bombarded with advertising that encourages an unhealthy diet.
“It is also far cheaper and easier for individuals to consume foods and drinks high in fat, sugar, and salt than healthier alternatives.”
Research by the Food Foundation has found healthy foods are twice as expensive per calorie than unhealthy foods.
Katharine Jenner, of the Obesity Health Alliance, said the environment people in their 20s and 30s have grown up with has had a huge influence on “habit-forming behaviours”.
She said the digital environment was also likely to be playing a role with younger age groups more likely to be using food apps and spending time on social media, while the pandemic disrupted physical activity and other habits at a “crucial moment”.