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Everyone wants to be healthy. But sometimes, in our pursuit of fitness, we take things a little too far.

And that can ultimately mean a healthy behavior becomes an unhealthy one, and maybe even one that kills you. Take dieting, for example. Being overweight can increase your likelihood of certain diseases and even death, so you might decide to go on a diet to be healthier.

But dieting isn’t always so great, either. Lots of diets are based around cutting out specific foods, and axing things wholesale can mean you don’t get enough of key vitamins and minerals collectively called micronutrients. And, in the most ironic way, having micronutrient deficiencies actually makes you more likely to gain weight or develop diabetes, increasing your risk of mortality.

Trendy diets in particular are notorious for failing to include everything you need. For instance, a study published in 2017 showed how the Paleolithic diet, which excludes all processed foods, resulted in an iodine deficiency in obese, postmenopausal women. Iodine is especially important for thyroid function, that hormone-secreting gland in your neck which helps control things like your energy level.

It turned out that when the subjects went Paleo, they cut out good sources of iodine like table salt and dairy, among other things. Similarly, a 2018 study analyzed the nutrient content of the 3 most popular diet plans available through Amazon. They found that one week of commercial low carb diet didn’t deliver enough Vitamins.

B1, D, and E, as well as calcium and a few electrolytes. And the vegan weight loss plan they analyzed failed to provide enough of certain B vitamins, calcium, and Vitamin D; and it was low in protein, too. Those deficiencies, especially in calcium and B vitamins, are frequently seen when researchers compare vegans to omnivores, and they can compromise bone and muscle health as well as your immune system, making you more susceptible to infection.

That’s not to say that you can’t be vegan and healthy, or Paleo and healthy for that matter, you just have to make sure you’re actually getting everything your body needs. To avoid this whole micronutrient nightmare, you could diet by cutting the amount you eat rather than specific foods. Eating less is arguably the most effective way to lose weight.

But if you feel like you’re always fighting with the scale, losing 5 kg, then gaining it back, then losing it again, you could be doing more harm than just staying overweight. While obesity is generally not great for you, weight fluctuation, also called weight cycling or weight yo-yoing, may actually be worse, and carry a higher mortality risk. While there’s no consensus on how much your weight has to fluctuate to count, one study out of Finland quantified severe weight cycling as losing and gaining at least 5 kilograms at 3 different times.

And this ends up being a pretty widespread problem, since the majority of people who do manage to lose a bunch of body fat gain it back. Then, if they diet again, the cycle repeats. Bouts of weight loss aren’t usually all that harmful unless you lose a lot of weight quickly, but repeated periods of weight gain are harmful, and they stack up over time.

When your fat cells grow, your body experiences oxidative stress, the same stuff involved in sun and age-related damage. Adding fat can also trigger inflammation, an immune response that can harm your cells. And these take a toll on the body, leading to higher rates of cardiovascular disease and type 2 diabetes.

For example, studies in 2003 and 2005 suggested that weight fluctuation can increase the likelihood of hyperinsulinemia, too much insulin in the blood, and eventually lead to metabolic syndrome, a collection of conditions that predispose you to cardiovascular disease. Other research has shown that weight cycling increases other cardiovascular risk factors like unhealthy blood lipids and high blood pressure. That, along with the inflammation from weight gain, increases the development of plaques in the arteries and around the heart which can lead to heart attacks.

Given all of this, instead of dieting, you might try losing weight by ramping up your exercise regimen. But that’s another healthy behavior that can be overdone. Take running.

Running's great! After all, adding as little as 75 minutes of running to your weekly routine can reduce your risk of mortality. But if you start trading those 5Ks for marathons, you may lose those health benefits.

Studies have found that more than four or five hours per week of vigorous exercise, the kind where your heart is pounding and you’re drenched with sweat, carries the same mortality risk as no exercise at all. And some studies suggest it actually increases your chance of dying. That’s probably because extreme endurance athletes put a lot of stress on their hearts, which can make them undergo physical remodeling: a change in the structure of the heart muscles that makes heart conditions more likely.

And it’s not just excessive cardio that can be a problem; you can overdo strength training, too. Enter rhabdomyolysis, a condition where damaged muscles release protein into the bloodstream, ultimately causing damage to the kidneys. There are a couple ways to get it, including over-exerting your muscles by lifting more than you should or doing hundreds of sit-ups, push-ups, or squats in a single session.

And anywhere from 10-50% of rhabdomyolysis patients develop acute renal failure, which can be fatal. Of course, literally working your muscles to death is the extreme end of things. Dieting and exercise can be good for you, if you ensure you’re eating nutritiously, making lasting changes, and not pushing your body too far.

Things tend to go wrong when people assume that if it’s something’s healthy, then more is even healthier. But good health just isn’t that simple. And if you watch SciShow regularly, hopefully, you’re starting to imagine the human body a bit more complexly.

Thanks for watching this episode of SciShow! And thanks especially to our patrons on Patreon. If you want to learn more about your body and what’s really good for it, you might like our sister channel, Healthcare Triage.

On this channel, Dr. Aaron Carroll explains healthcare policy, dives deep into medical research, and answers all sorts of questions about medicine, health, and healthcare. So check it out at YouTube.com/HealthcareTriage! [♪ OUTRO].