This is what happens to your body when you eat fast food

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 per cent of the time), but also enjoy the occasional less healthy food (less than 20 per cent of the time), if that's what you really want.

I've seen this approach work well with my clients who were previously chronic dieters yet hadn't been able to lose weight. Once I give them permission to have "forbidden foods", those foods lose their power and they're able to make healthier choices the bulk of the time.

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 in your overall health?

Junk food and fast food defined

What is "junk food"? Essentially any food that is highly processed, high in calories and low in nutrients. Junk food is also usually high in added sugars, salt and saturated or trans fats. Some evidence points to junk foods as being as addictive as alcohol and drugs.

"Fast food" is food that is prepared quickly and is eaten quickly or taken out. Although there are a growing number of healthier fast food options, most fast food can still be classified as junk food.

Long-term effects of eating junk food

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 on fast food and heart health 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.

Short-term effects of junk food

It's human nature to think about benefits and risks over the short term rather than considering the impact our choices have over the long term. So how does consumption of junk food affect your body over the short term?

A few days of junk food

Just a few days of junk food could change your metabolism. A small study of 12 healthy young men found eating junk food for just five days led to a reduced ability of their muscles to turn glucose into energy, even though they didn't eat more calories as part of the study. Over the long term, this change could lead to insulin resistance and eventually type 2 diabetes.

Another effect of just a couple of days of junk food is poor digestion. Because junk food lacks fiber, eating too much of it could lead to constipation.

One junk food meal

That 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.

The good news: Every healthy meal helps

The amount of inflammation and oxidative stress your body will experience after eating occasional junk food seems to be a function of the "big picture" of your choices over time.

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?

When you're making the choice between a healthier option and junk food, consider that just one healthy meal a day worked into the typical American diet could reduce overall stress and inflammation in your body. Every meal is an opportunity to positively impact your health.

Based on the current research, my advice to my client essentially remains the same: Once you're aware of all of the short-term and long-term impacts of junk food and you still really want some, have it less than once a week and really savor it. Then get right back to enjoying nourishing, nutritious foods.

• Brissette is a dietitian, foodie and president of 80TwentyNutrition.com. Follow her on Twitter @80twentyrule.