Dunedin researcher Wathsala Nanayakkara, who undertook the study with University of Otago researchers, said such residents were also missing out on essential vitamins and nutrients, despite being offered a variety of food.
Ms Nanayakkara said the study was based on the diets of 54 people, most over 70, at a Christchurch aged care facility.
However, some other studies suggested that the findings were also likely to apply in Dunedin and Otago, and elsewhere in the country.
“We hope our study will raise awareness among the general public that over half of aged care residents do not consume adequate calories or energy and protein to maintain their body weight and muscle mass,” she said.
"A good balanced diet can let you have more energy and and can fight infections."
Providing more "protein-rich snacks", such as sandwiches with egg or meat, could provide a helpful "top-up", and fruit-rich smoothies were another good option.
She also favoured "a bit more encouragement" from staff over the health benefits of consuming vitamin C.
A second helping could also be offered to people who had eaten a meal but might be ready for more.
The study found key nutrients such as protein, fibre, water, vitamins A, E, B12, folate, calcium, iron and zinc were suboptimal even in male residents, who had the least nutrient decay rates.
On average, 30% of planned nutrients were not consumed by residents.
The study, From Menu to Mouth, was published recently in the British Medical Journal Open.
Ms Nanayakkara, who has an Otago MSc degree in human nutrition and is the study lead author, said studies examining nutritional intakes of aged care residents were scarce, despite a high prevalence of malnutrition in nursing homes, and a rapidly ageing population worldwide.
“We found on average 90% of the nutrients planned on the menu were served, and 70% were actually consumed.”
The majority of study participants were aged over 70, and consisted of 12 male residents eating standard size meals, and 42 female residents of whom 64% ate smaller-sized meals.
More than 70% of residents did not consume enough protein, which was important for bone health, and helped preserve muscle mass to support mobility, and combat infections.