BackgroundSocial media platforms such as Twitter are used increasingly to consume and share information, including health-related information. Previous research has analysed the obesity conversation on Twitter, but no work to date has explored content related to specific contributing health behaviours, such as physical activity, diet and weight loss. The aim of this study was to identify the content and source of physical activity, diet and weight loss information on Twitter, in order to identify common content and sources of these tweets.Methods Exploratory thematic and source analysis was conducted on tweets containing the words “physical activity”, “diet”, or “weight loss”, collected over a 7-day period in March 2017. Sources were coded as professional, non-professional, or information intermediaries (non-protected titles and potentially misleading terms, such as ‘personal trainer’). Tweets were coded as educational (news and research, guidelines and recommendations), or non-educational (promotional, conversational, behavioural, or directing to another platform).ResultsA 1% sample of tweets (n=1,433) was analysed. The majority of tweets (69.2%) were related to diet, and most tweets were non-educational in nature. Among non-educational tweets, promotion of products or services was the most commonly identified theme (33.4%). Educational tweets comprised just under a third of the sample, and most of these (96.8%; n=455) came from non-professionals or information intermediaries. Most of the educational tweets from a non-professional source (74.0%; n=296) were providing advice in the form of recommendations or guidelines. Only 1.3% of all tweets came from a professional source. ConclusionsOnly 3% of the educational information shared on Twitter regarding physical activity, diet and weight loss came from a professional source. This suggests that Twitter users are at high risk of receiving information from non-professional sources, which may include non-evidence-based information or potentially harmful misinformation.