Benefits Of Kale For Weight Loss

Overview

Regarded as a ‘’superfood’’ and deemed every famous celebrity’s best friend, kale is a food most of us associate with health, wellness, weight loss, "and detoxification" and despise the taste of. 

Is kale overhyped and just another glamorised ‘’fad’’ food, or is broccoli and cauliflower’s famous sibling the true secret to a lean, "hourglass" figure and a long, healthy life? Could kale, with its delicate leaves really stop us from getting cancer, or is this just another overexaggerated claim to get our hopes up and woo us into buying? One way to find out: munch your way right in! 

What is kale

Brassica oleracea L. var. Acephala, or kale, as most of us are familiar with, is a cruciferous leafy green vegetable from the Brassicaceae family, the very same family that broccoli, cauliflower, cabbage, Brussels sprouts, arugula (or rocket) and collard greens are members of.¹ Kale is thought to originate from eastern Turkey and is considered one of the ‘’healthiest foods’’¹ on the planet, ranking as the 15th most nutrient-dense food by the United States Center for Disease Control.² Known for their strong and bitter taste, kale leaves come in a variety of shapes and sizes, and with differing characteristics.³ Examples include green kale, dwarf kale, marrow-stem kale, tronchuda kale, scotch kale, tree kale, bore kale and curly leaf kale, with the latter being the most popular type. Kale leaves are often consumed either raw in salads or cooked, steamed, boiled, juiced, blended in smoothies, and even oven-roasted for a healthier, lower calorie alternative to chips.

Does kale help you lose weight

Unfortunately, besides one inconclusive rat study, no scientific evidence investigating the role of kale consumption in weight loss exists to date. The study, which was published in 2021, showed that rats that were put on a high-fat diet supplemented with kale over a 12-week period experienced a very small loss in weight, one that was not significantly different from those fed a high-fat diet alone.⁴ However, compared to the rats who did not consume kale, the kale-consuming rats gained less adipose tissue-specific inflammation from the high-fat diet. 

Benefits of kale for weight loss

Despite the discouraging findings from the above study and the lack of studies available to support the role of kale consumption in human weight loss, kale has some excellent properties that, at least in a logical sense, make it a potentially beneficial food for weight loss. Firstly, kale is very low in calories, with 1 cup (21g) of raw kale containing approximately 7.35 kilocalories (kcal).⁵ 

Secondly, kale possesses both a high water (~81.38%) and dietary fibre (~3%) content,⁶ making it a low energy dense food.⁷  In humans, several studies have consistently shown that diets containing foods with low energy density aid weight loss and could even prevent weight regain.^7,8  

The high dietary fibre content of kale makes it an especially beneficial food for weight loss as it is suggested to promote satiety and thus decrease overall calorie intake via the following: 1) delaying gastric emptying and 2) decreasing the rate of nutrient absorption, particularly from fats and proteins.⁹

Other health benefits of kale

Perhaps the most interesting and widely investigated health benefits kale consumption offers is its potential to reduce the risk of cancer in humans.¹⁰ 

The anti-cancer, anti-mutagenic, and anti-inflammatory activities of kale are attributed to a specific class of secondary metabolites called glucosinolates. Glucosinolates are hydrolysed or broken down by the enzyme myrosinase when kale leaves are damaged either via insect and pathogen attack, chewing, chopping, or cooking. This action releases multiple bioactive compound products including cyanides, thiocyanates and isothiocyanates. 

According to research, sulforaphane (4-methylsulfonyl butyl isothiocyanate) and indole-3-carbinol, in particular, are the two isothiocyanates in kale that offer the most protection against cancer.¹⁰ Sulforaphane, for example, has been recently shown to impede the growth of multiple types of cancer in humans including liver (hepatoblastoma),¹¹ pancreatic,¹² prostate,¹³ and melanoma (skin).¹⁴ 

The growth rate of other types of human cancer including breast¹⁵ and, colon¹⁶ was also reported to decrease via indole-3-carbinol. 

In addition to offering protection against cancer, the anti-inflammatory activity of kale is suggested to support heart health and reduce the risk of cardiovascular and inflammatory diseases.¹⁷ High levels of the ‘’bad’’ cholesterol, low-density lipoprotein (LDL), is considered to be one of the biggest risk factors of cardiovascular disease,¹⁸  the levels of which have been shown to decrease following kale consumption in both rats ⁴ and humans.¹⁹ 

The liver metabolises cholesterol into bile acids, small molecules that are reabsorbed and released into the circulation when a high-fat meal is consumed in order to aid fat digestion.¹⁸ Interestingly, kale contains compounds termed bile acid sequestrants that bind to bile acids and prevent their reabsorption and recirculation,²⁰ causing the liver to increase cholesterol metabolism in order to make up for the loss of excreted bile acids, further leading to the reduction of LDL.^18,20 

As kale is rich in vitamins including vitamin K and minerals such as calcium, potassium, and magnesium,^3,21 all of which have been consistently shown to improve bone mineral density in humans,²² kale consumption may also promote and maintain bone health. Vitamin K, in particular, has been recently reported to moderately decrease the risk of fractures²³ and thus, potentially delay or even prevent the development of osteoporosis, a chronic disease that causes weakening and eventually, loss of bone tissue.²⁴ 

Nutritional facts

In addition to glucosinolates, dietary fibre, minerals (potassium, calcium and magnesium) and vitamin K, kale is a rich source of other vitamins including vitamins A, C and folate as well as beneficial prebiotic carbohydrates (lactulose (disaccharide), inulin (polysaccharide), fructo-oligosaccharides, gluco-oligosaccharides) and antioxidants including carotenoids (β-carotene, α-carotene, and lutein) and polyphenols (quercetin, kaempferol and hydroxycinnamic acids).^3,21 

Out of all vegetables, kale is demonstrated to have the highest potassium concentration and a calcium bioavailability that exceeds that of cow’s milk by almost 27%! (58.8% vs 32%).³ 

It is worth noting that the nutrition content of kale varies depending on whether it is raw or cooked, with cooked kale²⁵  having a higher nutrient bioavailability than raw kale.⁵  To retain most of the antioxidants, nutrients and minerals, steaming kale is preferable to boiling and other cooking methods.²⁶  

Side effects and other concerns

Even kale, with its ‘’powerful’’ chemoprotective and innumerable health-promoting effects, is not without its downsides. For one, conventionnally grown kale (in contrast to organic kale) is filled with pesticides, recently ranking as the third food with the highest pesticide content by the Environmental Working Group.²⁷  

Kale also contains compounds termed antinutrients which, as their name suggests, hamper the absorption and digestion of essential nutrients, thus increasing the risk of nutrient deficiencies in those consuming kale in excess.³ Kale is especially rich in the antinutrients oxalate and phytate, both of which decrease the bioavailability of minerals including calcium as well as iron and zinc, respectively. 

Oxalates, in particular, can increase the risk of developing kidney diseases by promoting the growth of kidney stones,²⁸ making kale an unfavourable and even harmful food option for those with kidney issues. Kale can also be dangerous for patients on  blood thinners (anticoagulants) due to its high vitamin K content.²⁹  Vitamin K, besides its above-mentioned bone-protective effects,^22,23,24 acts as a coagulant and binds to blood thinners such as Warfarin, making them less efficient at preventing blood clot formation.²⁹

Less serious and harmful than these health risks, although uncomfortable and potentially embarrassing,  kale consumption can also give rise to digestive side effects including constipation, gastrointestinal irritation, flatulence and bloating.²¹ 

Summary

Kale is a cruciferous leafy green vegetable with a nutrition profile that is superior to that of most fruit and vegetables, making it the 15th most ‘’healthiest’’ and nutrient-dense food on the planet.  Despite being delicate, kale leaves are packed with antioxidants, vitamins, minerals, dietary fibre, prebiotics, carotenoids, polyphenols and glucosinolates (to name a few), all of which have been demonstrated to promote heart health, bone health and even protect against cancer in humans. 

In contrast to the protective health benefits provided by kale consumption to date, no scientific evidence is available to support the role of kale consumption in weight loss, at least not in humans. Regardless, kale could still aid weight loss due to its low energy density (high dietary fibre and water content), especially when supplemented with a healthy low-calorie diet and regular physical activity. 

Whether aiming to lose weight or improve general health, kale should not be consumed in excess as this could not only lower the efficiency of certain medications such as blood thinners, but also predispose individuals to kidney diseases, nutrient deficiencies, and uncomfortable digestive issues.