Dr Marshall (54), of the Otago biochemistry department, began taking part in the innovative Otago University weight reduction study last November.
And he has already noticed small but important differences in his weight and health, having shed between 2kg and 3kg.
He opted for the 5:2 diet, during which much less is eaten for two days a week.
And he is exercising more, with more regular walking and swimming, and monitors his progress via a smartphone app provided by study organisers.
''It's really easy. It makes a difference,'' he says.
Run through the Otago medicine department, the study is called SWIFT -Support strategies for Whole-food diets, Intermittent Fasting and Training.
Researchers are seeking men and women, aged 18 or over, with a body mass index of 27 or higher.
Dunedin people wanting to participate in the study can contact organisers via email at: https://swift.otago.ac.nz.
Associate Prof Rachael Taylor is the principal investigator and PhD students Michelle Jospe and Dr Melyssa Roy, a medical doctor, are also involved.
More than 90 people are already participants, but organisers want to increase this total to 250, and attract more overweight males.
The study aimed to ''test different ways'' of helping people make dietary and lifestyle changes they could ''actually stick to'', Prof Taylor said.
There was no single ''best'' weight-loss diet, but a range of suitable options that suited different people.
This was the first such study she knew of which allowed participants to choose their own diet and exercise plan, with different support strategies decided for participants randomly.
Diet options included a modified ''Paleo'' (high animal protein, fewer carbohydrates) diet, Mediterranean, and the 5:2 diet.
