Published on December 21, 2020

Studies and Statistics Part 4: Cohort Studies

There are many different types of nutrient research studies, including randomized controlled trials (covered in a recent post), cohort studies, case-control studies, and cross-sectional studies. In this blog series we have been exploring key concepts including study types and statistics to help you better understand research results. In previous posts we discussed how the amount of disease is measured within a population and dove into study significance and the p-value. In this post we will focus on study types, specifically cohort studies.

What is a cohort study?

A cohort study is where a group of people, or cohort, is followed prospectively over time and collects information about risk factors or exposures, such as nutrient intake, and outcomes, such as a cancer diagnosis. There is no specific intervention or treatment introduced, such as in a randomized controlled trial (RCT). Remember, the primary analysis from an RCT is intent-to-treat (ITT), to assess the effect of a treatment or protocol on the proportion who develop a disease, according to if they were in a treatment group or a control group.

On the other hand, a cohort study looks at a single large population over time to determine the difference in a particular outcome based on certain variables shared within that group. Analyses of cohort studies seek to determine the association between a particular exposure (something people are doing, eating or being exposed to as part of their daily lives), and an outcome (such as developing cancer), while adjusting for other risk factors that could bias the results (such as age or BMI). However, there still could be unknown or unmeasured risk factors could influence the results. Since exposures and outcomes are collected in chronological order, cohort studies, like RCTs, can distinguish between cause and effect.

Showing the Data – Kaplan Meier Curves

The Kaplan-Meier curve is a visual representation of events over time, such as those occurring throughout a cohort study. The curve shows what the probability of an event (for example, survival or being diagnosed with a disease) is at a certain time interval. For example, we may follow a cohort over time to find out if an exposure such as smoking influences whether they are diagnosed with cancer, or whether a treatment reduced the number of deaths from a heart attack in comparison with controls.

If the sample size is large enough, the curve should approach the “true” survival for the population under investigation. It is usually used to compare two or more groups in a study (such as in a cohort study comparing whether they had a certain exposure compared to those who did not, or in a RCT the group that got treatment A vs a group that got placebo).

Click to Enlarge & Print

The Kaplan-Meier curves above illustrate the rates of incidence of dementia and Alzheimer’s Disease risk by vitamin D levels for a population consisting of 916 participants, aged 65 and older, whose measurements for vitamin D status and cognitive decline were tracked over 12 years. A total of 177 cases of dementia were validated, of which 124 were confirmed Alzheimer’s disease. The lines on the charts represent the percent of participants who remained disease free (were not diagnosed with dementia or Alzheimer’s Disease) – as you can see, those with higher vitamin D levels (greater than 20 ng/ml or 50 nmol/L) were more likely to remain free of dementia and Alzheimer’s compared to participants with lower vitamin D levels (10-20 ng/ml and less than 10 ng/ml or 25 nmol/L). The risk of disease also went up as vitamin D level declined.

What is the GrassrootsHealth study?

The GrassrootsHealth approach is similar to a cohort study, but unlike other studies it focuses on participant education and involvement. Armed with information about nutrients and their test results, participants can take ownership over their own health. Further, GrassrootsHealth focuses on physiological measures of nutrient status, such as vitamin D levels, which captures the effect of multiple input sources (e.g. food, supplements, sun) and makes provision for the variability in dose-response between individuals unlike analyses of dose amounts.

For example, GrassrootsHealth has compared the amounts of colds and flus that participants report based on their intakes of vitamin C and D and their Omega-3 index.

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We found that the percent of participants who experienced the flu in the prior 6 months was 72% lower for those with a vitamin D level at or above 40 ng/ml who also had an Omega-3 Index at or above 8% and also reported that they usually took supplemental vitamin C compared to those with a vitamin D level less than 40 ng/ml and an Omega-3 Index below 8% who did not report usually taking supplemental vitamin C (P=0.005). For colds, it was 21% lower (P=0.17).

Take Charge of Your Own Health!

By joining the GrassrootsHealth projects, you are not only contributing valuable information to our study, but you are also gaining knowledge about how you could improve your own health through measuring and tracking your nutrient status, and educating yourself on how to improve it. Do you know what your status of vitamin D, omega-3s, and other essential nutrients is? Could your levels be improved? Test now to find out!

What does the Research Say about Vitamin D & COVID-19?

It’s TIME to start saving lives! If you can help PREVENT the majority of the death, it’s time! What’s it costing you/us not to take action NOW?

There is much published research that supports a clear link between vitamin D and COVID-19 showing that higher vitamin D levels are related to:

a decreased risk of testing positive for COVID-19

increased viral SARS-CoV-2 RNA clearance

better clinical outcomes among patients with COVID-19

less severe COVID-19 disease

decreased risk of death due to COVID-19

Be sure to educate yourself on the benefits and importance of vitamin D for immune health, and take steps to ensure you and your loved ones are getting enough.

You can review all of the COVID-19 and immune health information we have shared on this page.

What Does it Take YOU to Get Your D to 40 ng/ml (100 nmol/L)?

Did you know your health could be greatly affected by making sure you have a vitamin D level of at least 40 ng/ml (100 nmol/L)? Help us help you.

STEP 1 - Do you know what your vitamin D level is? If not, be sure to test today to find out.

STEP 2 – Determine your target level. Are you at your target level? Experts recommend a level of at least 40-60 ng/ml (100-150 nmol/L).

STEP 3 – Need to boost your level? Use the D*calculator to see how much vitamin D it may take to reach your target. Opt for the Loading Dose for a quicker boost.

STEP 4 – Optimize how your body absorbs and utilizes vitamin D with co-nutrients and these simple steps.

STEP 5 – Re-Test! This is an important step to make sure you have reached your target level, and to ensure you are not taking too much! Re-testing after 3-4 months is recommended.

STEP 6 – Adjust, Repeat…

Give your immune system the nutrients it needs to support a healthy you and protect yourself from unnecessary diseases, especially COVID-19.


The first Randomized Controlled Trial on vitamin D and COVID-19 has shown a 96% lower risk of ICU admission for those receiving vitamin D (as 25(OH)D to quickly boost vitamin D blood levels) along with the standard treatment, compared to those receiving standard treatment alone.

These results support many previous observational studies showing a relationship between vitamin D levels and intake and COVID-19 severity.

Review the Latest Nutrient Research for COVID-19

GrassrootsHealth Nutrient Research Institute has launched the new Immune Boost project with the use of our myData-myAnswers nutrient health system that nearly 15,000 people are already using for their health. Specific markers that influence immune health are suggested for testing as part of this project including:

  • Vitamin D
  • Omega-3 Index
  • Essential elements magnesium, selenium, and zinc
  • hsCRP

Our goal is to demonstrate how one can use the Nutrient Research Model established by Dr. Robert Heaney to show the effect of vitamin D serum levels of at least 40 ng/ml (100 nmol/L) on risk reduction for all ethnicities in the population. Status and intake of other nutrients will also be analyzed for any type of relationship to immune status and symptom severity. Join the project today!

Please let us know if you're interested in helping sponsor this project.

CLICK HERE for updates and new information about the project.

Through GrassrootsHealth Nutrient Research Institute, you can also test your essential elements magnesium, copper, zinc and selenium, toxins such as lead, mercury and cadmium, as well as your omega-3 levels, inflammation levels and thyroid stimulating hormone (TSH) level. Find out your levels today! Log on to the test selection page (click the link below) to get your tests and see for yourself if your levels can be improved.

Make sure you track your results before and after, about every 6 months!

Click Here to Access the Test Page

How can I track my nutrient intake and levels over time?

To help you track your supplement use and nutrient levels, GrassrootsHealth has created the Personal Health Nutrient Decision System called

For each specific supplement, you can track what days you take it, how much, and many other details.  This will help you know your true supplemental intake and what patterns of use work for you to reach and maintain optimum nutrient levels. Check it out today!