Here are the 14 most unhealthy fast food shakes. Can you guess No. 1?

Would a milkshake's "healthiness score" help you determine whether or not the sweet treat is worth it?

In a study published in December, online healthcare company PlushCare analyzed the most unhealthy vanilla milkshakes sold at American fast food chains based on a score determined by calories, sugars, saturated fats and sodium. The study pitted chains like McDonald's, Sonic, In-N-Out, Dairy Queen, Arby's and Fatburger against one another and the No. 1 rank may catch some by surprise.

Got a sweet tooth? Here's a look at the top most unhealthy fast food milkshakes in America.

What fast food milkshake is the most unhealthy?

Compared to 13 other fast food chain's Fatburger's 890-calorie vanilla milkshake is the most unhealthy, according to PlushCare's analysis. At the end of the spectrum, McDonald's ranks No. 14.

What are the top 10 most unhealthy fast food milkshakes?

PlushCare analyzed vanilla milkshakes from 14 different fast food chains. Here's a look at the top 10 most unhealthy:

How did PlushCare determine 'unhealthiness scores'?

PlushCare assessed the "relative unhealthiness" of the 14 milkshakes based on each drink's kilojoule (a metric unit used to measure energy based on calories), sugars, saturated fats and sodium, the study explains.

Despite being based in California, PlushCare utilized the United Kingdom's nutrient profiling method, a tool to determine the "healthiness" of foods, to assign points to each of the four categories. Points were then added together to determine a final score. The higher the score, the more unhealthy the item.

For example, Fatburger's vanilla milkshake "unhealthiness score" is 63. This is because the shake received 11 points for energy, 30 points for saturated fats, 19 points for sugar and three points for sodium.

Study also ranked burgers, nuggets and fries

In addition to milkshakes, PlushCare analyzed the "most unhealthy" fast food cheeseburgers, chicken burgers, chicken nuggets and fries. Here's a look at the rankings:

Cheeseburgers

Chicken burgers

Chicken nuggets

Fries

Greta Cross is a national trending reporter at USA TODAY. Story idea? Email her at [email protected].