The National Obesity Forum and International Chair on Cardiometabolic Risk said it was important to be upfront because of the health risks associated with being overweight.
It can increase the risk of diabetes, heart disease and stroke.
The groups conducted a poll of over 2,000 people, which found 42 per cent of 18-24-year-olds wouldn’t tell a loved one they need to lose weight, because they would be scared of hurting their feelings.
For those aged 22-44, it was over a third and for older people 1 in 4 people would not tell someone.
Men find it harder to tell their partners but women feared telling a friend they need to lose weight.
Professor David Haslam, chair of the National Obesity Forum, said: “Suggesting to someone that they should consider losing a few pounds may not be a comfortable conversation to have.
“But if someone close to you has a large waistline then as long as you do it sensitively, discussing it with them now could help them avoid critical health risks later down the line and could even save their life.”
A sedentary lifestyle and fast food are some of the causes of obesity.
But it’s not all bad news – researchers have said that even a walk after Christmas dinner helps to clear fat molecules from the bloodstream.