Table 5.

Facilitating factor that improved recruitment and retention of males

Facilitating factor or approach	Study details (for either sex)	Study details for male specific approaches
Advertising through mainstream media, recurrent attention from medical press and general media	[42, 43, 55, 57]	[61]
Annual Cohort Symposium	[42]	-
Clinical coordinator referrals	[37]	-
Continuity of care with the same research staff	-	[38]
Delivery of intervention using friendly, relaxed, non-directive style with easily understood information	-	[41]
Delivery of GIFS after assessment were beneficial providing humour and personalisation	[49]	-
Development of loyal, close relationships in person or one on one check-ins over the phone	In person: [38, 51] Telephone: [38, 53]
Discussion of reasons for refusal, drop out, or non-attendance	[40, 44, 51, 58]	[61]
Enrolment of spouse or all family members	[39, 51]	[36]
Free medical screening/laboratory test results and clinical report	[43, 46, 48, 57]	[36]
Incentives/Reimbursement	Monetary: [46–49, 56, 57] Paid parking: [48, 51] Small gifts: [43, 48, 51, 52, 56] Snacks during assessment: [48]	-
Invitation to press conferences	[43]	-
Maintaining regular contact using one or several methods of contact	[44, 48, 51, 57]	[41]
Membership card to study and certificate of completion at each follow-up	[43]	-
Participant choice in amount, timing and frequency of participant intervention or location of interview/ Convenience of time/location	Timing and frequency: [44], Time and location of interview: [40] Convenient timing and/or location: [58] Convenient location including home assessment, options for less burdensome data collection or combined with clinical visit: [48]	Convenient timing and/or location: [36, 41]
	Multiple geographical locations, low frequency of the procedures and flexible appointments: [38]
Participant newsletters	Yearly newsletter written from PI: [42] Annual holiday letter: [51] Monthly email with scientific information about health/nutrition: [43] Study result newsletters, birthday, and holiday cards: [48]	Birthday and Christmas cards: [61] Study Newsletter: [36]
Participant perceptions	Aiding research particularly publicly funded research: [43], helping family and friends with COVID-19 and advancing research: [37], knowledge gain and treatment benefit: [38]	Perceived health benefits: [36, 41] Satisfaction at being part of ‘health elite’ (Hawthorne Effect): [36]
Participant website	[48]
Payment of wages to employers to attend study visit without loss of income or work penalty	[52]	[36]
Personal notes to participants	[56]	-
Prompting participants to update changes in address or phone number by SMS, post or prior to surgery	Pre-surgery [48]	SMS: [41] Post: [61]
Referrals by family and friends to the study	[37]	-
Reminders for questionnaires/ appointments or follow up after non-attendance/non-responders	[40, 42, 46, 48, 51, 52, 56–58]	[61]
Recruitment via “mass mailing”	[55]	-
Resources specific to intervention	[40, 57, 58]	[41, 61]
Sensitive and informative communication by research staff/ participants or partners of participants to meet individually	Sensitive communication style: [38]	Individual (participant or partner of participant instead of group settings: [50]
Screening participants from geographical stable workplaces	-	[36]
Simple consent process	Online: [46] Verbal and Written: [38]	Via SMS: [41]
Study data collected completely online	[43, 57]	-
Vested personal interest	Desire to contribute to chronic disease risk: [43], Desire to help country and learning how to protect health: [37], Desire to help others: [38], No other medical options available: [38]	Desire to contribute their own weight loss: [41], Desire to help others with disease and for self: [41], Desire for education about chronic disease: [38, 41]
Written Questionnaires	[38]