A client recently asked me, "How often can I get away with eating junk food?" She knows that my nutrition philosophy is the "80:20 rule": Eat healthy foods as often as possible (at least 80 percent of the time), but also enjoy the occasional less healthy food (less than 20 percent of the time), if that's what you really want. 

There is some evidence that "cheat meals" (although I hate that term) can help boost fat loss and mental health among dieters. Yet I wanted to give my client a more quantifiable answer. Could a few days of junk food or even a single fast food meal make a difference to overall health?

Eating a poor-quality diet high in junk food is linked to a higher risk of obesity, depression, digestive issues, heart disease and stroke, type 2 diabetes, cancer, and early death. And as you might expect, frequency matters when it comes to the impact of junk food on your health. 

A review of studies found having fast food more than once a week was linked to a higher risk of obesity, while eating fast food more than twice a week was associated with a higher risk of metabolic syndrome, type 2 diabetes and death from coronary heart disease. 

This is disturbing, considering nearly half of American adults eat fast food at least once a week.

Another effect of just a couple of days of junk food is poor digestion, because junk food lacks fiber. 

A single fast food meal can narrow your arteries, leading to an increase in blood pressure. And the quick spike in your blood sugar from eating junk foods high in refined carbohydrates and added sugars can cause a surge in insulin, leading to a quick drop in blood sugar. That leaves you feeling tired, cranky and hungry for more.

Just one serving of junk food can increase inflammation throughout your body. Further, an Australian study suggests that in people with asthma, a fast food meal high in saturated fat can increase inflammation in the airway, potentially making an asthma attack more likely. So it seems the quick hit of junk food, while fleetingly rewarding, does carry short-term risks.

If you want to enjoy junk food once in a while but are concerned about the impact on your health, take a look at your overall health habits. Do you smoke or overdo it on alcohol? Are you exercising regularly and eating plenty of nutritious foods such as vegetables, fruit, legumes, fish, nuts and seeds, and whole grains? When it comes to your health, it seems you can "get away with" the occasional junk food more easily when you follow a healthy lifestyle most of the time. So think about your ratio of healthy to less healthy foods. Are you achieving 80:20, or is there room for some improvement?