Can you segment by the demographic markers for the prediabetic users and then co-relate to similar groups in the prediabetes iPhone population?

This 2012 study from Deloitte may give you some insights for extrapolating an estimate: http://www.deloitte.com/assets/Dcom-UnitedStates/Local%20Assets/Documents/us_chs_2012_mhealth_HowMobileTechnologyIsTransformingHealthCare_032213.pdf

For example, 247 million people have downloaded a healthcare app. 60% were for weight loss and exercise. Of the 13, 600 downloaded for iPhone, 5.3% were for chronic conditions. "Mobile technology supports increasing consumer engagement in wellness, prevention, and treatment programs, as well as chronic disease management. Pilot studies in diabetes and hypertension management have shown improved outcomes through remote monitoring."

If you main objective is to know if there's a market for your app, then I would suggest to log on to your iphone and search for "diabetes". Then log on to appannie.com and try to see if some of these apps are successful (check their ranking, you might need to create a free account for that). You won't be able to know how many downloads they are getting... but if I see it I might give you a rough estimation. If no apps are successful, then there is no demand

It may be necessary for example, for management to know, not that a market is worth $85m annually, but simply that it is worth over $50m. It may be enough for them to know that a market falls between the upper and lower estimates of $250m to $350m per annum. Equally, there will be occasion, when high levels of precision are needed, perhaps because the investment is large within the context of the total market. In industrial market research it is seldom possible to state the precise accuracy of a market assessment, because methods of arriving at market size are generally subjective and not derived from statistically valid samples. Naturally, it is every researcher's aim to obtain the greatest accuracy at the lowest cost. Assuming it is necessary to assess a market size, here are some steps to consider.
Step One – A Top-Down Approach
Using the Central Statistical Office's Guide to official statistics, the search should begin for government publications which contain the market size data. A search through the Directory of British Associations will identify any trade associations which could be a repository for sales or production statistics. Finding a published source of data from a multi-client study is like striking gold for the researcher. The publication Market search from Arlington Press is a comprehensive listing of published market research reports. The published data which the researcher finds need not necessarily be in a convenient form. There are, for example, no published figures on the market for high quality labels used on bottles of gin and whisky but the sales of spirits are so well documented they could very easily be derived. If still unsuccessful in assessing the market size, we move on to step two.

Step Two – Looking At The Supply-Side
Build up a picture of the supply side of the market, by adding together the sales of the companies who sell within it. The researcher is likely to need to consult a variety of sources to obtain suppliers' sales. The researcher needs to strip out those sales which are derived from other products. With all the guesswork which is applied to this method of market size assessment, it should be clear that accuracy levels may well be hovering around the 20 to 30 per cent mark. An option which provides a quick and often more accurate means of assessing market size than juggling turnover figures is to find someone with an overview of the market. Insights from these market gurus help sharpen the estimates and add to the researcher's wider appreciation of the market. In the case of a narrow market sector with just a few large and medium sized customers, the researcher may well attempt a virtual census. Where there are many buyers of varying sizes the researcher may sample most of the large ones and a proportion of the medium and small ones to arrive at a picture. For example, a researcher may gross up by taking the average consumption per company and multiply by the number of companies in the market.

Market size formula = Total purchases by sample firms X Total people employed in sector
Of course, the above is just one example of a market size calculation that could be used to determine the likely volumes bought or sold in a given sector. In any market size calculation, there are likely to be a series of key variables that when combined give a sense of market size. Market assessment can take place within existing markets or new industry sectors or new geographical markets. For many companies, the first place to look for more sales is among existing customers.
Market research is a must in making all these decisions.
Besides if you do have any questions give me a call: https://clarity.fm/joy-brotonath