10 Best Foods High in Vitamin A

Vitamin A | Retinol | Provitamin A | Benefits | Intake | Foods | Absorption

Vitamin A is vital for healthy skin, organs, bones, and fertility.

However, most people do not know that there are no active vitamin A plant foods containing retinol.

Find out which foods are the highest in vitamin A and how your body can adequately absorb the potent micronutrient.

What Are Foods High in Vitamin A?

Our bodies require several vitamins to function sustainably. The most critical essential vitamins are A, B, C, and D.

Ideally, you meet the needs for these vitamins by consuming whole foods.

Although it is involved in as many – if not more – vital functions of our body as the other micronutrients, few people are aware of vitamin A’s health benefits.

For example, vitamin A is necessary to ensure the following critical tasks of the human body:

• Immune function
• Maintenance of vision
• Growth of children

Therefore, if you do not consume enough foods with vitamin A as part of your diet, you may also experience the following deficiency symptoms:

• Increased risk of infections
• Hair loss
• Blemished skin
• Night blindness

Accordingly, vitamin A deficiency is one of the leading causes of blindness in developing countries in general.

In Western civilization, vitamin A deficiency contributes to age-related macular degeneration, the most common cause of vision loss in the elderly population (Beatty et al. 2000¹).

However, few people know that a severe distinction exists between plant and animal vitamin A sources.

While the pre-formed vitamin A is known as retinol and is limited to animal foods, plants contain only provitamin-A.

Best Vitamin A Foods Are High in Retinol

For this reason, sources of active vitamin A1 belong to the following food groups:

• Fish
• Meat
• Dairy
• Eggs

However, since nutrition facts do not distinguish between pro-vitamin A and true retinol, they are usually challenging to track.

Therefore, it is crucial to know the difference between retinoids and carotenoids. While animal sources provide retinol, plant sources only have beta-carotene.

Since the pre-formed vitamin A from animal sources has much higher bioavailability, the body can utilize it more efficiently.

In contrast, the body must first convert pro-vitamin A to retinol to benefit from it.

Foods High in Pro-Vitamin A

Most internet sources will always suggest the following foods to you as foods rich in vitamin A:

• Carrots
• Sweet potatoes
• Pumpkins
• Honeydew melons
• Bell peppers
• Spinach
• Broccoli

Although these plants are considered vitamin A foods, they only provide a precursor. However, converting pro-vitamin A into retinol is not always a sure thing.

On the one hand, the conversion often functions only to a limited extent in people with digestive problems, hormone disorders, or diabetes.

On the other hand, according to studies, women can absorb only about 3.3% and men about 2.25% of beta-carotene (Lin et al. 2000²; Hickenbottom et al. 2002³).

Afterward, the body needs to convert these absorbed carotenoids into retinol in a further step.

Finally, this leads to the fact that only tiny amounts of beta-carotene can be converted into vitamin A by our bodies.

For example, in this study, only 3% of absorbed beta-carotene was converted to retinol (Hickenbottom et al. 2002⁴).

Besides, about 45% of women and men can only convert such small amounts of beta-carotene to vitamin A that it is not even measurable (Lin et al. 2000⁵; Hickenbottom et al. 2002⁶).

For this reason, you should better be ready to eat several kilograms of sweet potatoes to meet the equivalent daily requirement of retinol.

Benefits of High Vitamin A Foods

Why vitamin A deficiency is a global problem that can lead not only to blindness but even to death becomes clear when we survey the wide range of nutrient tasks (Wiseman et al. 2017⁷).

Accordingly, the action of retinol always plays a supporting role in the growth, maintenance, and immunity of the human body:

1. Reproduction

Vitamin A is crucial for developing children already in the womb (Hadi et al. 2000⁸).

Therefore, vitamin A deficiency in the mother during pregnancy is associated with a child’s higher mortality rate.

Nonetheless, handle vitamin A supplements with care since they may cause malformations (Guillonneau et al. 1997⁹).

Moreover, retinol deficiency may limit fertility. While it affects sperm quality in men, it affects the quality of ovules in women (Clagett-Dame et al. 2011¹⁰).

2. Immune System

The intestinal wall relies on vitamin A to form and maintain cells (Huang et al. 2018¹¹).

Therefore, sufficient consumption of bioavailable retinol helps stop the transport of pathogens from the gut to vital organs.

Also, even mild vitamin A deficiency can increase the risk of respiratory illness and diarrhea (Sommer et al. 1984¹²).

3. Skin

Vitamin A is an all-around active substance when it comes to our skin. It can slow its aging process, promote its renewal, smooth it, and counteract acne (Park 2015¹³).

For this reason, recent studies recommend natural foods high in zinc, and vitamins A and E to fight acne (Ozuguz et al. 2014¹⁴).

Fortunately, these same nutrients are found together in natural vitamin A foods.

4. Eyes

Damage to the retina from oxidative stress-so-called age-related macular degeneration, is the most common cause of blindness in the elderly population (Beatty et al. 2000¹⁵).

While natural foods rich in vitamin A can counteract this disease, studies have shown that beta-carotene supplements alone cannot halt its progression (Evans et al. 2017¹⁶).

Furthermore, vitamin A distribution programs in developing countries save the eyesight and lives of nearly half a million children annually (Sommer 2014¹⁷).

5. Inflammation

In addition to infectious diseases, insufficient intake of vitamin A primarily promotes chronic inflammation in the body.

Therefore, the anti-inflammatory effects of retinol from the skin to the lungs are indispensable (Reifen 2002¹⁸).

6. Bones

While people with lower levels of vitamin A in their blood have a higher risk of bone fractures, those with the highest levels have a lower risk (Zhang et al. 2017¹⁹).

To strengthen bones and minimize fractures’ risk, collaborative consumption of vitamin D is crucial, such as in the natural form of eggs (Joo et al. 2015²⁰).

7. Muscles

Scientists found decreased muscle protein synthesis in rats that consumed too little vitamin A through diet (Narbonne et al. 1978²¹).

In addition to muscle development and maintenance, retinol also impairs weight gain, as vitamin A deficiency in the diet can lead to obesity (Zulet et al. 2008²²).

8. Genes

Most people are not aware that your diet can have a direct impact on your genetic makeup. For example, the bioactive components of retinol can directly regulate genes (McGrane 2007²³).

Therefore, how you eat can eventually change your genes and those of your offspring over time.

9. Cancer

Animal studies suggest retinoids can prevent and suppress cancer (Sankaranarayanan et al. 1996²⁴).

Similarly, other studies state that retinoids can inhibit tumor development and prevent second primary tumors’ development (Sun et al. 2002²⁵).

In contrast to foods containing vitamin A, beta-carotene preparations do not show the same anti-cancer effects (Lee et al. 1999²⁶).

10. Blood

Researchers found that retinol supports stem cells’ self-renewal, including embryonic stem cells (Khillan 2014²⁷).

Moreover, another study suggests that retinoids regulate those stem cells instrumental in forming new blood cells (Purton 2007²⁸).

Vitamin A deficiency can also cause anemia (Jafari et al. 2013²⁹).

How Much Vitamin A per Day?

Health authorities recommend that men consume 900 mcg, women 700 mcg (770 mcg during pregnancy), and children and adolescents 300 to 600 mcg of vitamin A per day (ODS 2020³⁰).

Although vitamin A overdose is possible and harmful, it is doubtful when consumed through whole foods.

Accordingly, vitamin A toxicity may occur when people consume synthetic supplements in high amounts daily over months.

Ultimately, our bodies cannot always use artificial vitamins as we wish (Dibley et al. 1996³¹; Murphy et al. 1992³²).

However, this is only one of many reasons I prefer a piece of the liver over a multivitamin pill daily.

With this in mind, even a single consumption of 66 grams of liver per month could help South African preschoolers avoid the need for vitamin A supplementation (van Stuijvenberg et al. 2020³³).

Best Foods High in Vitamin A

Micro and macronutrients improving vitamin A absorption were decisive in creating the following list of the best foods rich in retinol.

To compare the nutrition facts, I always used a 100-gram serving. Moreover, the list states vitamin values as a percentage of the recommended daily allowance.

1. Calf’s Liver

Beef liver is a natural multivitamin bomb. Who would have thought it?

But because of its intense flavor, its consumption is not very common. Nonetheless, an alternative tastes better and can donate even more natural retinol.

100 grams of cooked calf’s liver already provide the following vast amounts of retinol (*):

• Vitamin A: 1411%
• Vitamin D: –
• Fat: 6,3 g
• Protein: 28,4 g
• Carbohydrates: 3,8 g
• Calories: 192

Since our body can store nutrients, calf or beef liver consumption, even in minimal amounts, can ensure an adequate supply of vitamin A.

2. Goose Liver

The liver tends to be the go-to food for vitamin A. Accordingly, poultry liver cannot be missing from the best vitamin A foods list.

In particular, goose liver can impress with a considerable amount of retinol (*):

• Vitamin A: 620%
• Vitamin D: –
• Fat: 4,3 g
• Protein: 16,4 g
• Carbohydrates: 6,3 g
• Calories: 133

In addition to goose liver, which we know as a delicacy, chicken liver can be a less expensive alternative that offers an equally desirable nutritional profile.

3. Cod Liver Oil

We know cod liver oil as a time-honored home remedy for childhood illnesses or malnutrition made primarily from cod.

Although cod liver oil had raison d’être, many still can’t stand its taste. However, most high-quality cod liver oils today advanced to a mild fishy taste instead.

Accordingly, 100 grams of cod liver oil provide a concentrated load of A and D vitamins (*):

• Vitamin A: 2000%
• Vitamin D: 2500%
• Fat: 100 g
• Protein: –
• Carbohydrates: –
• Calories: 902

With this in mind, we must note that it is a dietary supplement, although it is not synthetic.

However, cod liver oil could not be missing from this list due to the ideal combination of fat, vitamin D, and vitamin A.

As we will see shortly, this combination of nutrients ensures the best possible absorption of vitamin A.

Besides, cod liver is also available as a preserve in whole food form, which is also rich in retinol.

4. Caviar

Caviar is an exotic entry on our list of the best foods high in vitamin A. The following look at the nutrient profile tells us why it should by no means be missing here (*):

• Vitamin A: 18%
• Vitamin D: 58
• Fat: 17.9 g
• Protein: 24.6 g
• Carbohydrates: 4,0 g
• Calories: 252

Its purchase and the composition of macro and micronutrients represent a luxury. Due to the high vitamin D and fat content, caviar ensures the best possible retinol absorption.

Also, the fat is predominantly omega-3 fatty acids, which on the one hand, bring enormous health benefits and, on the other hand, are rarely available in such a potent form.

5. Grass-fed Butter

It is often still wrongly associated with weight gain and heart disease. Nevertheless, butter is a true superfood from next door as long as it comes from grass-fed cows.

Its saturated fatty acids are stable at high temperatures and are more likely to protect against cardiovascular disease than to promote it (Mozaffarian et al. 2004³⁴).

Besides vitamin K2 and beta-carotene, 100 grams of pasture butter also provide a considerable amount of retinol (*):

• Vitamin A: 71%
• Vitamin D: –
• Fat: 81.4 g
• Protein: –
• Carbohydrates: –
• Calories: 714

In summary, grass-fed butter is a healthy fat that is affordable and a top-notch source of vitamin A. Hence, these are good reasons to always keep a packet in your fridge.

6. Tuna

Tuna steak represents a particularly delicate food for vitamin A intake. Just two 100-gram pieces of bluefin tuna can provide almost the daily requirement of retinol (*):

• Vitamin A: 44%
• Vitamin D: –
• Fat: 4,9 g
• Protein: 23,2 g
• Carbohydrates: –
• Calories: 144

Also, with tuna, the fatter the piece, the better the nutrient absorption.

7. Goat Cheese

Among the many types of cheese, goat cheese is my favorite as a vitamin A food.

One of the main reasons is that goat cheese cannot contain casein-A2, which the body converts into a lectin-like protein called beta-casomorphin.

Therefore, sheep and goat cheeses are always safe choices for a sensitive gut.

Here are the nutritional facts about this vitamin A food (*):

• Vitamin A: 21%
• Vitamin D: –
• Fat: 21,1 g
• Protein: 18.5 g
• Carbohydrates: 0.9 g
• Calories: 268

Moreover, keto diet enthusiasts may rejoice, as goat cheese contains more fat than protein with a low caloric value.

8. Pasture-Raised Eggs

Are you surprised that life comes from an egg?

In the end, a pasture-raised egg has everything in it that a living thing needs to thrive (*):

• Vitamin A: 16%
• Vitamin D: 8%
• Fat: 10,0 g
• Protein: 12,0 g
• Carbohydrates: –
• Calories: 140

The balance of vitamins A, D, fat, and protein is unique to eggs. Thus, they are the ideal way to absorb retinol.

And please do not remove the egg yolk, as that is where most of the nutrients are hiding.

9. Blue Cheese

Because of its excellent ratio of fat to protein, blue cheese also made it onto my list of the best foods with vitamin A (*):

• Vitamin A: 15%
• Vitamin D: –
• Fat: 28,7 g
• Protein: 21,4 g
• Carbohydrates: 2,3 g
• Calories: 353

Besides, enhancing a wide variety of dishes with its distinctive flavor is always an option.

10. Mackerel

Finally, another fatty fish is on the list of the best foods high in vitamin A.

Although claims can vary widely, 100 grams of mackerel contains approximately the following nutrients (*):

• Vitamin A: 4%
• Vitamin D: 101
• Fat: 15.6 g
• Protein: 20.8 g
• Carbohydrates: –
• Calories: 230

To obtain the highest possible content of omega-3 fatty acids, it is essential that the mackerel does not come from a fish farm but was caught wild. Besides, mackerel caught in the north Atlantic sea is usually low in mercury.

Although this fish may not have an impressively high retinol content, it provides excellent vitamin A absorption due to the high amounts of vitamin D and healthy fat.

Now let’s take a detailed look at why this is so.

How to Absorb Vitamin A from Foods

Many people wonder whether vitamin A is fat-soluble or water-soluble. While B and C vitamins are water-soluble, A, D, E, and K are fat-soluble.

For this reason, you should eat vitamin A foods and healthy fats. Although this may sound cumbersome, it’s easier than you think.

Eat Healthy Fats

Vitamin A belongs to the group of fat-soluble vitamins. Therefore, we can’t absorb vitamin A properly without a low-fat diet.

Besides, fat can increase the conversion of carotenoids into usable vitamin A.

Fortunately, however, nature leaves nothing to chance. That’s why natural foods with vitamin A, such as fish, eggs, and dairy products, are equally rich in fat.

In particular, healthy animal fats like grass-fed butter or organic lard can help with retinol absorption.

Our grandmothers knew what they were doing when they used lard for cooking. Unfortunately, decades of advertising campaigns have turned even our grandparents into consumers of industrially refined oils instead.

In addition to improved nutrient absorption, animal fats have the advantage of being low in polyunsaturated fatty acids. In contrast, omega-6-heavy vegetable oils oxidize when heated, introducing free radicals into our bodies.

Nevertheless, not every vegetable oil is unsuitable. In contrast, virgin coconut oil has the highest amount of saturated fatty acids and is best suited for high temperatures. Moreover, olive and avocado oils are options for lower temperatures.

Combine Vitamin A with Vitamin D Foods

Like retinol, vitamin D is fat-soluble. Since efficiency is a significant factor in vitamin A uptake, this fact plays into our hands.

On the one hand, vitamin D helps the body utilize vitamin A; conversely, it prevents possible vitamin A toxicity.

Vitamin D consumption is necessary to strengthen bone mineralization and prevent fractures through consuming vitamin A (Joo et al. 2015³⁵).

For example, cod liver oil enjoys a reputation as a natural remedy because it contains a unique balance of vitamin A and vitamin D in a bioavailable form.

It seems reasonable that cod liver oil was once prescribed to babies, adults, to the elderly.

Besides, you may well allow yourself 15 minutes of sunshine daily to boost your body’s vitamin D synthesis, weather permitting.

The Bottom Line

Animal foods are not as bad as their reputation, especially regarding vitamin A.

Since most people can only convert minimal amounts of beta-carotene into retinol, you should reach for liver, fatty fish, eggs, or dairy products if you’re deficient in vitamin A.

The same nutrient deficiencies that are a global problem are often limited to animal sources. In addition to vitamin A, people lack various B vitamins today.

Perhaps you, too, have resorted to taking a vitamin A or B complex supplement without thinking about natural whole-food alternatives.

Frequently Asked Questions (FAQ)

What foods are rich in vitamin A?

The food highest in vitamin A is liver, whether beef, chicken, pork, or cod. Other options with bioavailable retinol for vitamin A deficiency include eggs, dairy products, and fatty fish.

How can I increase my vitamin A?

You can naturally increase your vitamin A levels by eating liver, fish, eggs, or dairy.

Which fruit is rich in vitamin A?

True, pre-formed vitamin A (retinol) can only be obtained from animal sources. Plant sources, such as sweet potatoes, carrots, or broccoli, contain only pro-vitamin A (beta-carotene), which the body can only convert to vitamin A in tiny amounts.

Can you eat too much vitamin A?

Yes, you can overeat vitamin A. There’s a condition called vitamin A toxicity, but it will rarely be reached by consuming foods rich in vitamin A. However, you should be very careful when consuming vitamin A supplements.

Studies

#1-7

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²Lin Y, Dueker SR, Burri BJ, Neidlinger TR, Clifford AJ. Variability of the conversion of beta-carotene to vitamin A in women measured by using a double-tracer study design. Am J Clin Nutr. 2000 Jun;71(6):1545-54. doi: 10.1093/ajcn/71.6.1545. PubMed PMID: 10837297.

³Hickenbottom SJ, Follett JR, Lin Y, Dueker SR, Burri BJ, Neidlinger TR, Clifford AJ. Variability in conversion of beta-carotene to vitamin A in men as measured by using a double-tracer study design. Am J Clin Nutr. 2002 May;75(5):900-7. doi: 10.1093/ajcn/75.5.900. PubMed PMID: 11976165.

⁴Hickenbottom SJ, Follett JR, Lin Y, Dueker SR, Burri BJ, Neidlinger TR, Clifford AJ. Variability in conversion of beta-carotene to vitamin A in men as measured by using a double-tracer study design. Am J Clin Nutr. 2002 May;75(5):900-7. doi: 10.1093/ajcn/75.5.900. PubMed PMID: 11976165.

⁵Lin Y, Dueker SR, Burri BJ, Neidlinger TR, Clifford AJ. Variability of the conversion of beta-carotene to vitamin A in women measured by using a double-tracer study design. Am J Clin Nutr. 2000 Jun;71(6):1545-54. doi: 10.1093/ajcn/71.6.1545. PubMed PMID: 10837297.

⁶Hickenbottom SJ, Follett JR, Lin Y, Dueker SR, Burri BJ, Neidlinger TR, Clifford AJ. Variability in conversion of beta-carotene to vitamin A in men as measured by using a double-tracer study design. Am J Clin Nutr. 2002 May;75(5):900-7. doi: 10.1093/ajcn/75.5.900. PubMed PMID: 11976165.

⁷Wiseman EM, Bar-El Dadon S, Reifen R. The vicious cycle of vitamin a deficiency: A review. Crit Rev Food Sci Nutr. 2017 Nov 22;57(17):3703-3714. doi: 10.1080/10408398.2016.1160362. Review. PubMed PMID: 27128154.

#8-15

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¹¹Huang Z, Liu Y, Qi G, Brand D, Zheng SG. Role of Vitamin A in the Immune System. J Clin Med. 2018 Sep 6;7(9). doi: 10.3390/jcm7090258. Review. PubMed PMID: 30200565; PubMed Central PMCID: PMC6162863.

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¹³Park K. Role of micronutrients in skin health and function. Biomol Ther (Seoul). 2015 May;23(3):207-17. doi: 10.4062/biomolther.2015.003. Epub 2015 May 1. Review. PubMed PMID: 25995818; PubMed Central PMCID: PMC4428712.

¹⁴Ozuguz P, Dogruk Kacar S, Ekiz O, Takci Z, Balta I, Kalkan G. Evaluation of serum vitamins A and E and zinc levels according to the severity of acne vulgaris. Cutan Ocul Toxicol. 2014 Jun;33(2):99-102. doi: 10.3109/15569527.2013.808656. Epub 2013 Jul 5. PubMed PMID: 23826827.

¹⁵Beatty S, Koh H, Phil M, Henson D, Boulton M. The role of oxidative stress in the pathogenesis of age-related macular degeneration. Surv Ophthalmol. 2000 Sep-Oct;45(2):115-34. doi: 10.1016/s0039-6257(00)00140-5. Review. PubMed PMID: 11033038.

#16-24

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¹⁷Sommer A. Preventing blindness and saving lives: the centenary of vitamin A. JAMA Ophthalmol. 2014 Jan;132(1):115-7. doi: 10.1001/jamaophthalmol.2013.5309. PubMed PMID: 24407830.

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²⁰Joo NS, Yang SW, Song BC, Yeum KJ. Vitamin A intake, serum vitamin D and bone mineral density: analysis of the Korea National Health and Nutrition Examination Survey (KNHANES, 2008-2011). Nutrients. 2015 Mar 10;7(3):1716-27. doi: 10.3390/nu7031716. PubMed PMID: 25763530; PubMed Central PMCID: PMC4377877.

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²²Zulet MA, Puchau B, Hermsdorff HH, Navarro C, Martínez JA. Vitamin A intake is inversely related with adiposity in healthy young adults. J Nutr Sci Vitaminol (Tokyo). 2008 Oct;54(5):347-52. doi: 10.3177/jnsv.54.347. PubMed PMID: 19001764.

²³McGrane MM. Vitamin A regulation of gene expression: molecular mechanism of a prototype gene. J Nutr Biochem. 2007 Aug;18(8):497-508. doi: 10.1016/j.jnutbio.2006.10.006. Epub 2007 Feb 22. Review. PubMed PMID: 17320364.

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²⁷Khillan JS. Vitamin A/retinol and maintenance of pluripotency of stem cells. Nutrients. 2014 Mar 21;6(3):1209-22. doi: 10.3390/nu6031209. Review. PubMed PMID: 24662164; PubMed Central PMCID: PMC3967188.

²⁸Purton LE. Roles of retinoids and retinoic Acid receptors in the regulation of hematopoietic stem cell self-renewal and differentiation. PPAR Res. 2007;2007:87934. doi: 10.1155/2007/87934. PubMed PMID: 17846663; PubMed Central PMCID: PMC1950592.

²⁹Jafari SM, Heidari G, Nabipour I, Amirinejad R, Assadi M, Bargahi A, Akbarzadeh S, Tahmasebi R, Sanjdideh Z. Serum retinol levels are positively correlated with hemoglobin concentrations, independent of iron homeostasis: a population-based study. Nutr Res. 2013 Apr;33(4):279-85. doi: 10.1016/j.nutres.2013.02.004. Epub 2013 Mar 11. PubMed PMID: 23602245.

³⁰ODS. Vitamin A Fact Sheet for Health Professionals. Bethesda, MD: Office of Dietary Supplements, National Institutes of Health, 2020. Retrieved 2021 March 19, from https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/.

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³¹Dibley MJ, Sadjimin T, Kjolhede CL, Moulton LH. Vitamin A supplementation fails to reduce incidence of acute respiratory illness and diarrhea in preschool-age Indonesian children. J Nutr. 1996 Feb;126(2):434-42. doi: 10.1093/jn/126.2.434. PubMed PMID: 8632216.

³²Murphy S, West KP Jr, Greenough WB 3rd, Cherot E, Katz J, Clement L. Impact of vitamin A supplementation on the incidence of infection in elderly nursing-home residents: a randomized controlled trial. Age Ageing. 1992 Nov;21(6):435-9. doi: 10.1093/ageing/21.6.435. PubMed PMID: 1471582.

³³van Stuijvenberg ME, Schoeman SE, Nel J, le Roux M, Dhansay MA. Liver is widely eaten by preschool children in the Northern Cape province of South Africa: Implications for routine vitamin A supplementation. Matern Child Nutr. 2020 Jul;16(3):e12931. doi: 10.1111/mcn.12931. Epub 2019 Dec 17. PubMed PMID: 31845541; PubMed Central PMCID: PMC7296811.

³⁴Mozaffarian D, Rimm EB, Herrington DM. Dietary fats, carbohydrate, and progression of coronary atherosclerosis in postmenopausal women. Am J Clin Nutr. 2004 Nov;80(5):1175-84. doi: 10.1093/ajcn/80.5.1175. PubMed PMID: 15531663; PubMed Central PMCID: PMC1270002.

³⁵Joo NS, Yang SW, Song BC, Yeum KJ. Vitamin A intake, serum vitamin D and bone mineral density: analysis of the Korea National Health and Nutrition Examination Survey (KNHANES, 2008-2011). Nutrients. 2015 Mar 10;7(3):1716-27. doi: 10.3390/nu7031716. PubMed PMID: 25763530; PubMed Central PMCID: PMC4377877.