Healthy habits: Can there be too much of a good thing?

Sedentary living and dietary caloric surplus have led to a myriad of chronic health conditions and contributed to countless deaths. Exercise, particularly activities that target cardiovascular health ("cardio"), is practically a panacea for both problems. Researchers have shown that cardio exercise can help maintain healthy weight, increase endurance, boost the immune system, and improve mood, among other benefits.

But anything can be taken too far, literally and figuratively. Extreme cardio exercise, which includes endurance and obstacle races that can stretch over 100 miles, can have a lasting impact on cardiovascular health due to the high demands it places on the body. In one study, investigators found that the risk of atrial fibrillation may be higher in endurance athletes compared with non-athletes. In another study, researchers noted that coronary artery disease and arrhythmias can be found in some of the most active veteran endurance athletes, and concluded that this population should be monitored to reduce the risk of adverse outcomes.

How much is harmful?

This depends on a person's baseline level of fitness. A fit person can handle more cardio than someone who never runs. But ask yourself this: Over the course of man's evolution, how frequently do you think we ran 100 or more miles at a time? The human body can only handle so much mileage.

Where would we be without ascorbic acid, better known as vitamin C? For one thing, we'd all probably resemble 18th-century pirates with open wounds and bleeding gums. We'd also be doing harm to our blood vessels, muscles, and bone cartilage and collagen, notes the Mayo Clinic.

While it's unlikely that you'll die if you take too much vitamin C, the experience won't be pleasant. According to the Mayo Clinic, you can expect diarrhea, nausea, vomiting, heartburn, abdominal cramps, headache, and insomnia.

How much is harmful?

The NIH recommends 90 mg daily of Vitamin C for adult males and 75 mg daily for adult females. Anything beyond that is excess.

Strong muscles are a cornerstone of healthy aging. And a healthy amount of strength training can also help you maintain a healthy weight, keep your bones strong, and potentially improve your cognitive health.

But keep in mind how strength training works. We use exercise to intentionally damage muscle groups so that muscle fibers grow back stronger and in higher quantity. Overdo it and you could end up with rhabdomyolysis, a condition that has popped up as of late among the CrossFit and extreme endurance communities. Rhabdomyolysis develops as a result of muscle damage that leads to the dissolution of muscle tissue, which in turn leads to kidney failure. Symptoms include extreme muscle pain and dark urine.

How much is harmful?

Like cardio, tolerance for strength training duration and intensity is contingent on fitness level. However, a good rule of thumb is that if it's causing you pain, it's time to put the weight down. Workouts should be challenging, not debilitating.

If you want strong bones, you're going to need vitamin D, which helps bones absorb calcium and phosphate. Although you can take oral vitamin D supplements, it's best derived from sunlight. Vitamin D deficiency can lead to rickets in children and osteomalacia and osteoporosis in adults.

But don't overdo it. Getting sunburned counteracts any benefit you might get from vitamin D production. And oral supplementation comes with its own risks. If you megadose vitamin D, you could face vitamin D toxicity. You'll see excess calcium in your blood, which could lead to nausea, vomiting, frequent urination, and weakness. If you don't stop, you might develop bone pain and kidney stones.

How much is harmful?

The Mayo Clinic recommends that adults get 600 IUs of vitamin D daily and up to 800 IUs daily for those over age 70. Push beyond that, and you could reach toxicity levels.

Yoga offers the life-enhancing combination of a mindfulness practice with physical exercise. According to the Mayo Clinic, practitioners of yoga enjoy reduced stress levels, improved fitness in the form of better balance and flexibility, and reduced risk factors for some chronic conditions such as high blood pressure.

One of the core tenets of modern yoga is listening to your body and not pushing beyond your limits. Anyone who has taken a yoga class will tell you that the natural tendency is to strive to achieve some of the deeper, more intense poses. Perhaps this is why researchers of one study found that those who do yoga experience more muscle pain. Of the 354 participants in the study, nearly 11% said they experienced increased muscular aches and pains after practicing.

How much is harmful?

Like strength training and cardio, this depends on the fitness and experience level of the yogi. If a pose is painful, it likely isn't worth doing. Also, if you're sore from your practice the next day, maybe it's time for an active recovery day. Do something low impact, like going for a leisurely walk.

These days, it seems like every celebrity is putting their name on some miracle diet involving a so-called juice cleanse. Fruits and vegetables certainly have their proven health benefits, but, according to the Mayo Clinic, it is no healthier to consume them in juice form than it is to eat them whole.

Caloric, macro-, and micro-nutrient needs are going to vary from person to person. Regardless, it's difficult, if not impossible, to obtain all the necessary macro- and micronutrients exclusively from fruit and vegetable juice alone. Furthermore, restrictive diets, such as juice diets, do come with a major health risk: hyponatremia, a condition in which concentrations of sodium and other electrolytes become dangerously low in the body. Increased fluid intake can inadvertently lead to hyponatremia. One Israeli woman, for example, may have suffered irreversible brain damage as a result of hyponatremia from a 3-week long juice cleanse.

How much is harmful?

Most juices contain high quantities of added sugars, which can be just as toxic as alcohol—especially when consumed in liquid form—if daily intake is higher than recommended amounts (37.5 grams, or 9 teaspoons for men; 25 grams, or 6 teaspoons for women).

Is sitting the new smoking? Well, it's not quite as dangerous as smoking, but it can still have serious adverse effects on your health, according to one study published in the American Journal of Public Health. Specifically, researchers found that excessive sitting (roughly > 8 hours per day) nearly doubles the risk of type 2 diabetes and increases the risk of premature death associated with other chronic diseases by as much as 20%. In other research, prolonged sitting has been linked to obesity, high blood pressure, high blood sugar, and elevated cholesterol levels.

While some office workers use standing desks to address the issue, this may have some unintended health consequences. In a study published in Ergonomics, experts evaluated the cognitive and physical effects of standing on 20 adults. After 2 hours, participants reported increased bodily discomfort and decreased sustained reaction time. Study participants also experienced increased swelling in their lower extremities and a decreased mental state.

How much is harmful?

More than 8 hours of sitting daily has been shown to be have a negative effect on human health. An appropriate amount of time standing is less clear. A good rule of thumb is to take a break and have a seat when you feel tired and before you feel pain.