– Vitamin D is an unusual vitamin because it acts like a hormone in your body. It helps the bones absorb calcium, supports the immune system and may aid strength gain and fat loss.
– Vitamin D is made in the body when your skin absorbs ultraviolet light, but most people don’t get enough sunlight to reach adequate levels
– Deficiency of vitamin D is common, so consuming a D supplement may be beneficial
– The D3 version of the vitamin is absorbed better than D2, making it a wiser choice for supplementation
What Is Vitamin D?
While classified as a vitamin, D is unique among vitamins in a few different ways. For one thing, your body makes it itself when it’s exposed to sunlight. For another, it behaves like a steroid in the body (no, not that kind of steroid), meaning that it can turn genes on and off. This makes vitamin D an especially powerful nutrient with many potential influences on your health.
When your skin absorbs ultraviolet radiation, a cholesterol precursor in your body is converted to vitamin D3 (one type of D vitamin). However, it’s difficult for most people to get the amount of vitamin D they need from the sun alone. Fortunately, vitamin D is available in food, and many other foods are fortified with it. Nevertheless, vitamin D deficiency is common, and a serious threat to health (see Do I Have A Vitamin D Deficiency? below).
Vitamin D is fat soluble, meaning that it dissolves in fats and oils, and can be stored in the body for long periods. For years, scientists have known that Vitamin D promotes the absorption of calcium and phosphate from the food in your gut, making it a key supporter of bone health. More recently, D has also gotten credit for the role it plays in muscle strength and performance, nerve transmissions, and immune health.
How Do Vitamin D2 and D3 Differ?
There are two main kinds of vitamin D—D2 (ergocalciferol) and D3 (cholecalciferol). D2 is found in plant foods, including fungi like mushrooms and yeasts. D3 is only available in animal foods, and it’s the type of vitamin D your body makes on its own when it’s exposed to UV light.
D2 is inexpensive to produce, so it’s often added to foods—such as milk—to boost their vitamin D content. However, D2 is not as well absorbed by the body as D3. Some studies indicate that D3 may be almost twice as effective at raising levels of vitamin D in the blood as D2 (1, 2, 3). If you supplement with vitamin D to achieve optimal levels, nutrition experts generally recommend choosing vitamin D3 supplements.
According to an American Journal of Clinical Nutrition report, medical literature regarded D2 and D3 “as equivalent and interchangeable” for many years, “yet this presumption of equivalence is based on studies of rickets prevention in infants conducted 70 years ago…
Despite an emerging body of evidence suggesting several plausible explanations for the greater bioefficacy of vitamin D3, the form of vitamin D used in major preparations of prescriptions in North America is vitamin D2.”
In addition to failing to raise blood levels of the vitamin as well as D3, D2 also has a shorter shelf life. Therefore, the Journal concludes, D2 “should not be regarded as a nutrient suitable for supplementation or fortification.” However, some D2 supplements are oil-based, and may be more shelf-stable and bioavailable as a result. More research is needed to see how they compare to D3, but, in the meantime, D3 supps seem to be the smarter alternative.
What Does Vitamin D Do For the Body?
Due to vitamin D’s ability to act like a hormone in the body, it has the ability to support multiple aspects of health. While it’s long been known to help regulate bone health and growth, D has also been linked to the following.
A 2015 meta-analysis of seven studies found that vitamin D supplementation significantly aided gains in upper- and lower-limb strength. The subjects ranged from 18 to 40 years of age.
A 2014 study in The American Journal of Clinical Nutrition followed overweight and obese women on a diet and exercise routine for one year. Half the subjects received a vitamin D supplement, and the other half a placebo. Researchers found that the ones who got up to healthy vitamin D levels lost more weight than the placebo group—by an average of seven pounds.
Meanwhile, another study in Nutrition Journal showed that women who took vitamin D for 12 weeks didn’t lose weight, but their body fat percentages did go down, indicating that D may have helped with recomposition.
Innate immunity is the term used to describe the body’s general defense mechanisms—the ones that turn on when it senses that an unwelcome invader has entered your system. Barriers, such as the skin, and white blood cells—the body’s soldiers in the war against a pathogen—are examples of your innate immune defenses.
Adaptive immunity refers to the immune responses that are specific to a particular antigen (any foreign substance your body wants to get rid of). These include the antibodies you develop when you’re recovering from a cold, so that the same virus doesn’t make you sick again in the future.
Vitamin D has been shown to help modulate both innate and adaptive immune responses (The Journal of Investigative Medicine), supporting a strong immune system. In 2017, a review published in the British Journal of Medicine analyzed 11,321 people from 14 different countries. It concluded that vitamin D supplementation helped promote immunity in both subjects who were deficient in the vitamin, as well as those who had healthy levels.
At the same time, a lack of D in the diet can wreak havoc on health.
One study published in Archives of Internal Medicine followed 19,000 subjects for six years. Those with lower vitamin D levels were more likely to report upper respiratory health issues than those who were getting sufficient D. Meanwhile, a review in the Journal of Investigative Medicine explains that a D deficiency is associated with increased susceptibility to health issues.
Do I Have A Vitamin D Deficiency?
Most people living in first-world countries don’t have many vitamin deficiencies. We can get most of the nutrients we need through food alone, and many foods have vitamins added to them to ensure that we get enough. Vitamin D deficiency, however, is still rampant, even among otherwise healthy people.
The main reason why is lack of sun exposure. Contrary to what you might think, just walking around outside on a sunny day usually isn’t enough, even if you live on a tropical island. Clouds and shade dramatically cut down on ultraviolet light—the stuff that your skin needs to produce vitamin D—and window glass blocks it completely. Furthermore, the National Institutes of Health explain that wearing sunscreen with an SPF of eight or more won’t allow your body to absorb enough light to make D, although it’s obviously important to use some anyway to avoid sunburn and long-term skin damage.
Ethnicity also plays a factor. Melanin is a pigment that darkens the skin, but it also reduces the skin’s ability to absorb vitamin D. African Americans and Hispanics typically have more melanin than white people, so they tend to be more deficient.
A Nutrition Research study concluded that more than 41% of Americans are deficient in vitamin D. African Americans and Hispanics are most at risk, as 82% and 69% are deficient, respectively. The study went on to say that, “deficiency was significantly more common among those who had no college education, were obese, with a poor health status, hypertension, low [HDL] cholesterol level, or not consuming milk daily.”
Looking more closely at obese populations, the National Institutes of Health observe that “obesity does not affect skin’s capacity to synthesize vitamin D, but greater amounts of subcutaneous fat sequester more of the vitamin and alter its release into the circulation.” Even if an obese person gets bypass surgery to aid weight loss, his/her vitamin D levels will still be suspect, as the part of the small intestine that absorbs D is bypassed.
Senior citizens are also in jeopardy. A trial in the American Journal of Geriatric Pharmacotherapy stated that elderly subjects had insufficient D levels, “despite vitamin D intake consistent with national recommendations.”
Just what the recommended D dose should be is a subject of great debate. The current Recommended Dietary Allowance (RDA) for vitamin D is 15 micrograms, or 600 IU, for people up to age 70. (Folks older than that need 20 micrograms, or 800 IU.) A serving or two of fatty fish, such as salmon or trout, should have it covered. However, these recs might be very optimistic. A review of vitamin D studies in Nutrients accuses the current RDA of being flat out inaccurate because of an error in math. It concludes that, “With the current recommendation of 600 IU, bone health objectives and disease and injury prevention targets will not be met.” The appropriate IU, the researchers assert, may be many times more than what is currently advised.
Evidence published in Advances in Experimental Medicine and Biology agrees, suggesting that significantly higher doses, such as a minimum of 25 micrograms/1000 IU of D is more appropriate. The Journal of Clinical Endocrinology and Metabolism also explains that at least 1,500–2,000 IU per day may be necessary for adults, and at least 1,000 IU for children and teens. The Linus Pauling Institute echoes these numbers as well.
If reading all this has sent you scrambling to the cabinet for your multivitamins, your next question may be, “how much vitamin D is too much?” The National Institutes of Health say you can probably get as much as 100 micrograms/4,000 IU before you see side effects, which may include nausea, poor appetite, weakness, and weight loss. Vitamin D toxicity is rare, however, and it mainly comes from overuse of vitamin supplements. (You can’t get too much D from the sun, as your body will shut down production before then.) One review showed that there were no health risks associated with consuming 1,800–4,000 IU of D daily.
Whatever the optimal D dose may be, it seems fair to assume that we should all get our levels tested and, if low, aim to take in more. An article in the Journal of Evidence-Based Complementary & Alternative Medicine calls vitamin D deficiency “pandemic,” noting that health organizations worldwide are refocusing on the importance of D due to the discovery that “vitamin D receptors are present in nearly every tissue and cell in the body and that adequate vitamin D status is essential for optimal functioning of these tissues and cells.” It concludes that it is “imperative that all individuals be encouraged to obtain vitamin D from either sunlight or supplementation.”
What Are The Best Sources of Vitamin D?
It’s hard to get enough vitamin D from sunshine, but it’s even harder to get it from food—at least the way most people eat. The best source of dietary D is from fish livers, such as cod liver oil, but now ask yourself… when was the last time you ate cod liver oil?
Mackerel, salmon, sardines, swordfish, trout, and tuna all offer D, as do mushrooms and eggs. If you eat them regularly, you’ll meet the government-recommended requirement, but if you’re in the camp that thinks 600 IU is too low, you’ll need to be more aggressive to hit your D goals. Dairy products and cereals are fortified with vitamin D, which helps, but one review, and Harvard University, determined that supplementation with a multivitamin or concentrated vitamin D capsule provides a better insurance policy.
Of course, you shouldn’t completely avoid the sun. According to a report in Alternative Medicine Review, “the health benefits accruing from moderate UV irradiation, without erythema [reddening of the skin] or excess tanning, greatly outweigh the health risks, with skin pigmentation (melanin) providing much of the protection.”
The National Institutes of Health note that getting five to 30 minutes of sun on the face, arms, back, or legs—without sunscreen and between 10 a.m. and 3 p.m. at least twice a week—is usually enough to promote sufficient vitamin D synthesis in the skin. However, sun exposure, especially at these hours, can be difficult to get, and particularly during winter time, or during work weeks with a busy schedule. This is why scientists frequently recommend supplementation.
Why Take Vitamin D and K?
Vitamin K has similarities with vitamin D. It’s fat-soluble, found in egg yolks and liver, as well as some plant foods, and, like vitamin D, it assists calcium in promoting strong bones. K works with D to make sure calcium gets where it needs to go without causing a problem.
Here’s what we mean: your blood levels of calcium need to stay at a certain level. When you don’t get enough calcium from your diet, one of vitamin D’s functions is to take it from your bones and move it into your bloodstream. Obviously, this isn’t ideal, but if you generally get enough calcium in your diet, it isn’t cause for concern.
While D takes calcium from your bones, it doesn’t control where it ends up in the body. Vitamin K steps in as a protective measure, seeing that the calcium doesn’t accumulate in places that could be dangerous, such as the blood vessels or kidneys. For this reason, some people believe that any vitamin D you take should be supplemented with vitamin K. In cases where vitamin D intake was too high, some subjects ended up with too much calcium in their blood, suggesting that, if taken, vitamin K might have helped to regulate the buildup and prevent the problem.
But to date, there’s no compelling evidence to show that any of the aforementioned recommended doses of vitamin D are harmful with or without vitamin K in tow. If it turns out that the two should be taken together, however, chances are that you’re already covered. Unlike vitamin D, vitamin K is available in large amounts in many commonly eaten foods, such as spinach, parsley, kale, and soft cheeses. And because K is fat-soluble, it will last in your body a while after each serving.
Ever feel hangry when you miss a meal? Imagine waiting 16 or 18 hours before eating again. Or an entire day without breakfast, lunch, or dinner. That’s what proponents of intermittent fasting do on a regular basis.
At its simplest, intermittent fasting (IF) means cycling through periods of voluntary abstinence of food (or significant calorie reduction), interspersed with intervals of normal food intake.
Whenever we eat, the body releases insulin to help cells convert sugars (in particular glucose) from food into energy. If the glucose isn’t used immediately, insulin helps makes sure the excess is stored in fat cells. But when we go without food for extended periods, as people do in IF, insulin is not released. The body then turns to breaking down fat cells for energy, leading to weight loss.
That’s why we have fat stores, and if we’re not using them, all kinds of bad things happen.”
Monique Tello, MD, MPH, a Harvard Medical School professor and internal medicine physician at Massachusetts General Hospital, explains to Inverse what happens when people never get hungry enough to use up those fat stores. “A, we get fatter, and B, it’s all the things that go along with that,” she says.
“Your blood sugars are constantly high. Your insulin levels are constantly high. You get type 2 diabetes, which is a huge epidemic. Those high blood sugars cause damage to the insides of our arteries. The kidneys are getting clogged up. People go into dialysis. It’s a disaster, basically.”
A century of IF research in humans and animals illustrates her claims. Studies show IF can lead to weight loss, stabilized blood sugar, reduced inflammation, improvements in memory and stress resistance, slowed aging, and longer lifespan — all promising health benefits in return for considerable lifestyle changes.
What Is Intermittent Fasting?Unlike some complicated nutritional plans, IF requires minimal effort: You simply don’t eat, or eat very limited quantities, for hours at a time. No meal prep, counting calories, or restaurant guesswork.
But there are several ways to go about the diet. Some IF proponents maintain time-restricted eating, squeezing all their meals into an eight to 10-hour period, followed by a 14- to 16-hour overnight fast. Others rave about the 5:2 diet, which involves eating normally for five days of the week followed by two days of eating 25% of their daily caloric intake (that’s around 500 to 600 calories for most people). Others simply restrict food intake completely on certain days of the week, relying on water, black coffee, and tea to stave off hunger cues.
There are no guidelines or nutritional suggestions for “on days” when eating is unrestricted. But physicians and dieticians suggest eating a nutrient-dense diet full of plants and protein to tide you over through fasting periods.
What Happens During Intermittent Fasting?To understand IF, you have to first understand what happens when you eat.
“Insulin is a hormone that’s released when we eat, but it isn’t meant to be released all the time,” Tello says. “Intermittent fasting is simply letting your insulin level go down to basically normal so that you unlock your fat stores. So nobody’s going to lose any weight unless they get that insulin level down. Which is why eating very small meals throughout the day doesn’t really help with weight loss.”
Waiting a bit longer than usual between meals is totally normal, she says, despite what societal cues tell us.
“Historically, we didn’t have access to things like snacks even up until a hundred years ago,” Tello says. “People did not have KIND bars. People, nowadays, keep food in their glove compartments, for God’s sake. It’s like people can’t stand to be even a little bit hungry for a second. That’s not normal, that’s not healthy.”
Mark Mattson, Ph.D., a professor of neuroscience at the Johns Hopkins University School of Medicine and former chief of the Laboratory of Neurosciences at the National Institute on Aging, compares how fasting affects the body to the way exercise does.
“During exercise, there’s a stress on the cells. They don’t grow and get stronger and bigger during the exercise but during the resting period. So we think that with intermittent fasting — during the fasting period, the cells go in kind of a stress-resistance mode. And then when you eat, they’ve prepared themselves to quickly take up nutrients, proteins, and grow.”
How to Get StartedWhen first trying IF, the transition can be challenging, Mattson admits.
Many fasters report uncomfortable, even painful hunger pangs as they abstain from food. Mattson suggests people take it slow, trying different kinds of IF to see what works with their goals and routine. He advises first narrowing the “feeding window” from an initial unrestricted period down to 12 hours, then 10 hours, then eight hours, before finding a feeding window that is sustainable.
During the first week or two of IF, fasters may notice their stomachs grumbling or feel a sense of hollowness. Some people are exhausted, lightheaded, or shaky from diminished blood sugar on fasting days. Others experience poor concentration, irritability, mood swings, and even dizziness. In particular, people who work in long shifts or with heavy machinery should take more care when fasting to stay hydrated, rested, and alert.
After about a month, people typically feel better and less hungry, says Mattson.
Tello agrees: “The longer you can go between meals, the less hungry you are. I think a lot of people’s hunger levels actually decrease as opposed to the very low calorie diet where you’re constantly feeding yourself small, tiny bits of food. That’s like a tease all day long.”
Mattson encourages fasters to be patient and wait for those side effects to pass.
“A lot of people will say, ‘Well, I tried it. I’m too hungry, I’m irritable, and cannot concentrate,’ but they have to stick with it,” he says. “It is kind of like exercise. If you’ve been out of shape, when you start, it takes a while for your body and brain to adjust. But after you get used to it, you actually feel better. And if you stop doing it, you feel worse.”
To make fasting manageable, Tello suggests an overnight fast, which can more easily fit into people’s routines. Research shows that restricting eating to daytime hours, an approach that aligns eating patterns with circadian rhythms, has been shown to have metabolic and weight loss benefits.
“Nighttime eating is horrible,” Tello explains. “Nighttime snacking is like the worst. Eating at night and then going to bed, that all goes right to fat. That is the surest way to gain weight, and it’s so bad for your blood sugars.”
Can You Lose Weight With Intermittent Fasting?A major reason many people start and stick to IF is to lose weight. A Reddit group centered on the practice, with over 380,000 members, brims with dramatic before and after photos of fasters, some who have dropped over 150 pounds using the weight loss approach. Most rave about their results: slimmer bodies, more energy, and happier outlooks.
A 2015 systematic review of 40 studies, published in Molecular and Cellular Endocrinology, showed that IF was effective for weight loss, with a typical loss of seven to 11 pounds over 10 weeks.
But IF isn’t necessarily more effective for weight loss than other energy restriction regimens, like simply counting calories or cutting out carbs. A year-long study of obese males, published in JAMA Internal Medicine in 2017, showed that alternate-day fasting did not produce superior adherence, weight loss, weight maintenance, or cardioprotection over daily calorie restriction.
How to Eat Healthy While Intermittent FastingIf losing weight in a healthy way is the goal, Tello suggests maintaining a plant-based diet alongside fasting and not using IF as an excuse to overeat or consume unhealthy foods during non-fasting windows. In a plant-based diet, plants make up the lion’s share of food consumption. Think vegetarian, vegan, or Mediterranean diets.
People do not have to be completely vegan but they do have to be, for the most part, eating plants.”
“We have so much evidence, overwhelming mountains of evidence on millions and millions and millions of people’s data that a plant-based diet is the healthiest diet,” Tello says. “And people do not have to be completely vegan, but they do have to be, for the most part, eating plants.”
Tello suggests aiming to make plants 80% of your diet. You can still have steak, cake, and whole grains on occasion, but the bulk of your diet should be coming from the ground or growing in a tree or bush.
“You will be a healthier person. It’s anti-inflammatory, it’s fiber full. It’s going to be decreasing your cholesterol levels. Your digestion is going to improve,” Tello says.
Decades of research, show that a plant-based diet is effective for controlling or losing weight, improved physical and emotional health, and a multitude of health benefits: lower mortality, better cardiovascular health, prevention of type 2 diabetes, and lower blood pressure and cholesterol.
Will Intermittent Fasting Make You Live Longer?In 2012, the BBC broadcast journalist Michael Mosley brought IF into the diet landscape with the documentary Eat Fast, Live Longer, which drew on research by Michelle Harvie, Ph.D., at the University of Manchester. It was followed shortly by journalist Kate Harrison’s book The 5:2 Diet in 2013 and Dr. Jason Fung’s The Obesity Code in 2016.
Research on IF’s effects on aging date back a century. “Nutrition and aging are very tightly linked,” John Newman, MD, Ph.D., a geriatrician at the University of California, San Francisco and researcher with the Buck Institute for Research on Aging, tells Inverse.
“One of the great early experiments almost a hundred years ago showed that if you feed rats less food, they live longer up until the point when you start to starve them. Fasting, dietary restriction, and the ketogenic diet turn on repair pathways and help animals to live longer.”
A century of laboratory research on animals and humans, collated by Harvie and Anthony Howell, Ph.D., both at the University of Manchester, links calorie restriction with the prevention of age-related disease, which includes tumors, cardiovascular disease, diabetes, and dementia. The research also suggests that it increases lifespan.
What Are the Risks of Intermittent Fasting?Some dieticians warn that ignoring hunger cues can have unforeseen consequences. Evelyn Tribole, registered dietitian and author of The Intuitive Eating Workbook, encourages her clients to listen to their hunger and satiety cues when choosing to eat rather than adhering to strict dietary rules. Tribole thinks ignoring these primal signals is a dangerous practice.
“I have a problem when someone is actually feeling biological hunger and you’re going to disregard that,” Tribole says. “I think that’s problematic, especially with anyone who has a dieting history or an eating disorder; they’re more likely to get engaged in binge eating and emotional eating.”
Anyone with a history of disordered eating patterns should consult a health professional to confirm that IF is right for them.
One systematic review published in the journal Stress in 2016, found that IF may initially increase stress levels of fasters. The increase may subside after a few weeks of fasting. Other research says IF could cause greater metabolic fluctuations and increased appetite on non-fasting days relative to intermittent energy restriction, a diet that allows some food.
The Future of FastingMattson, Tello, and Newman note that the research on IF has its limitations. Notably, a bulk of the studies have been done on rats and animal models, or obese or overweight human subjects.
IF’s impact on other populations — like growing adolescents, pregnant women, diabetic people, those with existing medical conditions, people using medication that requires food intake, or people over 65 years of age — remains to be determined.
More randomized controlled studies on a wider range of adult populations would help confirm some of the potential health benefits. But for now, it appears IF is here to stay.
“Intermittent fasting is well beyond a fad,” says Mattson, who is critical of its trendiness because he worries it can lead people to discredit the value of reputable evidence.
“Luckily, there’s plenty of research in human beings and intermittent fasting,” Tello says. “There are all these different studies calling it different words, but they are looking at the same thing, which is extended fasting.”
“At this point,” she concludes, “I think it’s really fair to say that any kind of modified fasting regimen promotes weight loss and improves metabolic health. I think we’re really safe to say that.”