11/13/2013 04:54 pm ET Updated Jan 23, 2014

When to Exercise: Timing Is Everything

Exercise intensity, duration, frequency and progression have long been recognized as the essential elements of any training regimen. But the question of when to exercise has only recently sparked scientific inquiry.

This relatively new interest reflects the gradual evolution of exercise from fitness activity to disease intervention. A sedentary lifestyle now represents the greatest cause of chronic diseases, including diabetes, obesity and cardiovascular disease. It is estimated that inactivity causes at least 300,000 premature deaths and over $90 billion in direct health care costs annually in the United States.

As you might imagine, this situation has forced the question, "How does sitting around make us so damn sick?"

The answer covers a wide range of causal pathways. I will focus on just one because it is perhaps the most fundamental, the most fixable, and informs when we most need to be active.

All life requires energy. However, more energy does not provide more life. In fact, it's just the opposite. The combination of inactivity and energy excess creates a perfect storm.

Every cell and organ in our body requires glucose in order to fuel normal function. But the body is exquisitely sensitive to the deleterious effects of high glucose levels. Much like oxygen, glucose quickly passes from life-sustaining to toxic.

We now know that fasting glucose levels well below diabetes cutoffs are a cardiovascular disease risk factor. A progressive relationship between glucose and cardiovascular risk ranges from normal glucose levels up to diabetic readings. In fact, there does not appear to be a clear lower threshold for cardiovascular disease.

Our bodies rely on insulin to protect us from the potential damage of toxic glucose levels. Insulin allows glucose to enter cells and be metabolized as an energy source. This clears glucose from the blood stream. Muscle is the largest insulin-sensitive organ in the body. It accounts for 80 percent of insulin-stimulated glucose disposal.

Here's the catch. When muscle is not used (sedentary lifestyle), insulin doesn't work very well. Even one day of sitting considerably decreases insulin action.

So a sedentary lifestyle provides a one-two punch. It dramatically decreases our energy requirement and disables insulin function. When we continue to consume highly caloric meals this causes blood glucose levels to spike after eating.

These spikes set off a cascade of reactions that is a who's who of contemporary pathology: oxidative stress, inflammation, vascular dysfunction and sympathetic hyperactivity (the fight or flight response in overdrive).

Now, "When should I exercise?"

Exercise after the largest meal of the day will give the most bang for the buck. Light physical activity for 30 minutes after eating has been shown to have an acute blood glucose reducing effect similar in magnitude to diabetes medications.

It is important to make clear however that this will not suffice. Exercise, as we know it, (even a daily one hour bout of a mix of aerobic and strength training) does not prevent prolonged sitting from causing disease. Unfortunately, it's what we do the other 23 hours of the day that make the difference.

Therefore, regular brief activity breaks throughout the day are necessary to maintain insulin sensitivity and prevent hyperglycemic damage. Walking for as little as one to two minutes every half hour has been shown to achieve this.

This should come as good news. You don't need money. You don't need a gym. You don't need equipment. You don't need another person. For a remarkably small investment of time you can dramatically lower your risk for the most common diseases of our time.