Oxalate (Oxalic Acid) and Its Relation with Nutrition and Health

From the World’s Healthiest Foods . . . . . . . .


While many people think about oxalates as some rare and undesirable component of food, oxalates are naturally-occurring substances found in a wide variety of foods and they play a supportive role in the metabolism of many plants and animals and in our human metabolism as well. So in terms of our overall health and diet, oxalates are neither rare nor undesirable. (For persons interested in the chemical nature of oxalates, these substances are strong acids constructed out of two carboxylic acids, usually abbreviated in biochemistry as COOH groups.) It is also worth noting here that in a practical and non-technical sense, “oxalate” and “oxalic acid” are two different terms for the same substance.

Oxalates can sometimes become problematic, however, if they overaccumulate inside our body. The key site for problems with overaccumulation is our kidneys. If the concentration of oxalates in our urine becomes too high, simultaneous with an overly high concentration of calcium, our kidneys are at risk of calcium oxalate kidney stone formation due to supersaturation of our urine with calcium oxalate salts. Worldwide, 5-15% of all persons are estimated to develop some form of kidney stones, with calcium oxalate stones accounting for about 80% of all stones formed.

Non-food sources of oxalates

Even if we did not eat oxalate-containing foods, we would still have oxalates in our body since we are able to make them in a variety of ways. (In fact, only 20-40% of the oxalates in our blood come from the foods we eat.) Our “internal” ways of making oxalates include:

(1) creating them from amino acids like hydroxyproline in our liver;
(2) taking vitamin C and transforming it into oxalate; and
(3) having our red blood cells synthesize oxalates from glyoxylate.

Because oxalates can be created from amino acids in our liver, and because proteins are constructed out of amino acids, the total amount of protein that we eat may sometimes be related to the amount of oxalates that are formed using this amino acid pathway.

However, in research studies on healthy persons not at special risk of kidney stone formation, high levels of protein intake nearing 150 grams per day have failed to consistently show increased levels of urinary oxalates or increased risk of kidney stone formation. It is persons already known to have problems with kidney stone formation who have been shown to be affected by high protein intake, with about one-third of “stone formers” getting unwanted increases in their urinary oxalate levels in conjunction with a high protein diet. We mention this protein issue not to try and provide treatment recommendations for persons with kidney stone problems—that step is one that should be taken with a healthcare provider—but to give an example of the way that oxalates can be made inside of our body and why dietary sources of pre-formed oxalate only tell one part of the story here.

It’s worth noting that studies on vitamin C supplementation have shown mixed results in terms of their impact on risk of kidney stone formation. Several studies show increased oxalate excretion following vitamin C supplementation. However, some of these same studies show decreased urinary calcium oxalate supersaturation and decreased risk of stone formation. So, the jury is still out on the exact set of relationships here.

Food sources of oxalates

As mentioned earlier, about 20-40% of the oxalates in our bloodstream come from preformed oxalates in our food. While oxalates are found in both plant and animal foods, plant foods have long been the research focus here since some plants have especially high concentrations. Among foods that we do not profile on our website, rhubarb is the most concentrated source of preformed oxalates and contains between 450-650 milligrams in about 3-1/2 ounces. Chocolate can also be a concentrated source, with the oxalate content increasing along with the percentage of cocoa contained in the chocolate. An average for 76% cocoa chocolate bars is approximately 250 milligrams per 3-1/2 ounces. But this amount can nearly double in a chocolate bar that is 100% cocoa.

Among foods that we profile on our website, the most concentrated oxalate sources (all listed in terms of milligrams per 3-1/2 ounces) include spinach (750-800 mg), beet greens (600-950 mg), almonds (380-470 mg), Swiss chard (200-640 mg), cashews (230-260 mg), and peanuts (140-184 mg). It is important to note that you will often find very different results in plant oxalate content due to differences in varieties, planting conditions, harvesting conditions, and measurement technique. It is also worth pointing out that the leaves of plants almost always contain higher oxalate levels than the roots, stems, and stalks.

Other oxalate-containing foods (listed in milligrams of oxalate per 3-1/2 ounces) include:

  • other green leafy vegetables not found in the high-oxalate examples above (5-150 mg)
  • berries, which typically contain between 10-50 mg (with the important exception of gooseberries which can contain 60-90 mg)
  • lemon and lime peel (80-110 mg)
  • nuts besides the high-oxalate nuts listed earlier (40-350 mg)
  • legumes (10-75 mg): with legumes, it is also worth noting that lentils, split peas, black-eyed peas, and garbanzo beans tend to fall at the lower end of this already-low spectrum with 10 mg or sometimes even less, while black beans, navy beans and soybeans tend to fall at the upper end of this low spectrum with 50 mg or more)
  • grain flours (40-250 mg): with grains and grain products, it is worth noting that brown rice flour and brown rice pastas are among the lowest in oxalate content
  • pasta noodles (made from grains) (20-30 mg)

In addition to this highlighted list above, it is worth nothing that most fruits and vegetables contain measurable amounts of oxalates in the small-to-moderate range. We’ve seen studies on grapes, for example, showing 3-5 mg; pineapple 5 mg; plums 10 mg; collards 5-75 mg; celery 11-20 mg; and green beans 15 mg. Okra is a vegetable that usually shows up higher on the oxalate scale at 140-150 mg. Parsley is also worth mentioning here at about 100 mg.

One final note about the oxalate content of lemons and limes: as indicated above, the peels of these fruits have been analyzed as high in oxalate content. However, the juice of these fruits (e.g., lemon and lime juice) is not only low in oxalates, but also high in other organic acids called citrates. Research suggests that the high citrate content in lemon and lime juice might actually help lower risk of calcium oxalate kidney stone formation. By binding together with calcium in place of oxalates, citrates can help reduce risk of urine supersaturation with calcium oxalate.

Oxalates and health

Two aspects of oxalates have been extensively studied from a health perspective: their relationship to kidney stone formation and their relationship to calcium absorption and metabolism.

Kidney Stone Formation

In research studies, some individuals have been shown to be “hyperabsorbers” of oxalate from the intestinal tract. In other words, their bodies uptake more oxalate than would normally be expected. In principle, the greater the amount of oxalate that gets absorbed into the body, the greater the amount that will reach the kidneys and raise the level of urinary oxalates. When combined with high levels of urinary calcium, there can be increased risk of calcium oxalate kidney stone formation.

Unfortunately, this general description oversimplifies what turns out to be a fascinating and more complicated set of bodily circumstances. First, oxalate only gets absorbed from our digestive tract when it is in soluble form. Sodium oxalate and potassium oxalate are the predominant soluble forms. By contract, calcium oxalate is insoluble, and magnesium oxalate is poorly soluble. So the form of the oxalate is important in the absorption process.

Second, our gut bacteria turn out to play a critical role in the amount of oxalate available for absorption since numerous species of gut bacteria are able to break down oxalate. These species include Oxalobacter formigenes, numerous species of Lactobacillus, and several species of Bifidobacteria. In fact, a good number of studies are underway to investigate the role of oral probiotic supplements and their impact on oxalate absorption.

Third, research has shown that the overall combination of foods that we eat during a meal (including both oxalate-containing and non-oxalate-containing foods) can significantly impact the amount of soluble oxalates available for absorption from our digestive tract. We’ve seen a study on Indian cuisine, for example, in which multiple-ingredient dishes like spinach (palak) also containing Indian cottage cheese (paneer) lowered the amount of soluble oxalates available for absorption by about 15-20%. So, as you can see, the relationship between dietary intake of oxalates and oxalate absorption is complicated. In general, since only 20-40% of blood oxalates originate from food, and since 85-95% of individual show no tendency to form calcium oxalate kidney stones, we don’t expect most people to have kidney stone-related problems from routine enjoyment of the foods that we profile at WHFoods.

Calcium Metabolism

An ongoing controversy in oxalate research involves the degree to which food oxalates interfere with calcium absorption from those foods. In general, calcium can be a somewhat difficult mineral to absorb from food. Even at very low levels of dietary intake—in which case you might expect the absorption rate to increase—calcium only tends to be absorbed at a rate of about 35%. But this generalized rate of absorption can vary dramatically from food to food, and the presence of oxalates in food is definitely a dietary factor that lowers calcium absorption (through the formation of insoluble calcium oxalate salts).

However, two further considerations cause us not to be worried in a broad sense about interference with calcium absorption from oxalates. First is the nature of the public health recommendations for calcium. These recommendations—like all nutrient recommendations—take the realities of absorption into account. At WHFoods, for example, our recommended daily intake level for calcium is 1,000 milligrams. This recommended level factors in the amount of calcium absorption from different foods, including foods like spinach that contain high levels of oxalates.

Second is the research on different populations or population subgroups that eat different mixtures of plant and animal foods. Studies show individuals who eat largely plant-based diets (i.e., vegetarians) do not have greater calcium deficiency or increased risk of osteoporosis, which you might predict if substances like oxalates were impairing calcium absorption in a way that would create a health risk. Calcium is definitely not absorbed as well from oxalate-containing versus non-oxalate-containing foods, but from our perspective this difference does not make intake of oxalate-containing foods either irrelevant or counter-productive in terms of their impact on calcium status. We therefore continue to recommend enjoyment of all WHFoods fruits and vegetables as worthwhile contributors to calcium intake, including those with higher oxalate concentrations.

Uncommon conditions that require strict oxalate restriction

There are some relatively rare health conditions that do require strict oxalate restriction. These conditions include absorptive hypercalciuria type II, enteric hyperoxaluria, and primary hyperoxaluria. Dietary oxalates are usually restricted to 50 milligrams per day under these circumstances.

The effect of cooking on oxalates

Cooking has a relatively small impact on the oxalate content of foods. In fact, we’ve seen one recent study examining oxalate changes in 20 different green leafy vegetables in cooked versus raw form which found no significant changes for any of the 20 vegetables. We’ve also seen studies that have focused on the blanching or boiling of green leafy vegetables and these studies show little to no decrease in oxalate content. At the very most, you should not expect more than a 5-15% decrease in oxalate content from the cooking of a high-oxalate food. For all of the above reasons, it does not make sense to us for you to consider overcooking an oxalate-rich food for the purpose of reducing its oxalate content. Research studies have made it clear that overcooking results in the loss of many different vitamins and minerals, and so the end result of overcooking is very likely to be a much less nutritious diet that is only minimally lower in oxalates.

Practical take-away

For the vast majority of individuals who are not at special risk of calcium oxalate kidney stone formation—or do not have any of the rare health conditions that require strict oxalate restriction—oxalate-containing foods should not be a health concern. Under most circumstances, high oxalate foods like spinach (including both baby and larger leafed mature spinach) can be enjoyed either raw or cooked and incorporated into a weekly or daily meal plan. For persons with health histories that make kidney stones a health concern, we recommend consultation with a healthcare provider to develop a diet plan and take other steps that can lower individual health risks.

Source: The World’s Healthiest Foods

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Is Duck Fat Actually Healthy for You?

Some people swear duck fat is healthy, and even save it from the roasting pan to use as a spread. Here’s the surprising truth about this fat

“All in all, duck fat is a good fat,” says Alejandro G. Marangoni, a professor in the department of food science at the University of Guelph. Good fat includes monounsaturated and polyunsaturated fats, and bad fats include saturated. “Duck fat is pretty high in monounsaturated fat, and thus not too high in saturates,” he says. “I actually have a jar of it in my fridge.” So consider ordering that duck confit next time you go out for a fancy dinner. As you can see from this breakdown of one tablespoon (15 mL), duck fat ranks between butter and olive oil.

Source: Best Health

Opinion: Nutrition Literacy for the Health Literate

Stefania Velardo wrote . . . . .

A recent discussion with a family member brought to light a new diet plan that had endorsed by her personal trainer. As we engaged in discussion, she excitedly recounted the list of food inclusions within the new diet; for example, she could eat white table sugar and saturated fat aplenty, amidst other appealing food choices. Yes to baked potatoes with lavish lashings of butter. Yes to coffee with sugar and cream.

As the conversation progressed I became more intrigued as she out-lined an extensive list of food exclusions, including whole grains and green leafy vegetables.

This provided a great contrast to her previous diet plan, which emphasized lean protein, limited fat and dairy, and an abundance of raw vegetables.

When I questioned her about this apparent contradiction, and the fact that this plan undermined well established government dietary guidelines that promote a wide variety of foods, she assured me that this diet was based on concrete scientific evidence and further legitimized through a worldwide movement of online posts, blogs, and forums. This is not the first occasion on which I have engaged in conversations that elicit differing views of scientific nutrition recommendations.

One woman recently testified, “I read that I should be eating rice malt syrup instead of honey.” whereas another person pondered, “Should I be eating a Paleo Diet?” Such conversations reflect a healthism discourse that emphasizes the need to know about our food and take responsibility for making the right choices. But what happens when the right choices are confusing?

A key tenet of nutrition literacy is one’s ability to access, understand, and use nutrition information in ways that promote health. Whereas low nutrition literacy presents a barrier to healthy eating, less is known about the challenges faced by supposedly health-literate individuals who are health conscious and possess a basic level of nutrition knowledge.

In revisiting the opening example, it is important to note that the family member in question is an enthusiastic, fit woman in her thirties who boasts high levels of literacy and numerous university degrees. On paper she demonstrates a strong ability to access, understand, and use nutrition information and resources in a health-promoting way. She cares about nutrition; yet her ideas clearly contradict government advice. She is arguably health literate, but to what extent?

This is not a new problem by any means. Health and dieting are profitable markets that continue to expand globally. Over the years we have witnessed countless pervasive weight-control myths and fads along with various products and lifestyles promoted within a health discourse. Yet the proliferation of the Internet and a surge of short courses, inflated credentials, and pseudo-expert blogs arguably make it more difficult to educate consumers about what constitutes professional nutrition advice. The existence of multiple stakeholders with competing interests means that consumers are confronted with increasing amounts of contradictory nutrition information through the media. It is not surprising, then, that nutrition messages are often a key source of confusion and uncertainty for individuals.

Nutrition practitioner and health educators have a clear role in promoting and disseminating credible, reliable nutrition information and resources to the public. More than ever, our roles must extend to challenging art increasing wave of pseudo-science while advocating for sound nutritional guidance so that people can develop positive relationships with a wide range of foods. High levels of critical nutrition literacy are clearly required to discern credible information based on rigorous scientific evidence from personal testimonials endorsed by so-called professionals. Developing these skills among different population groups, including the supposedly health literate, is essential at a time when many consumers feel confused, skeptical, and helpless.

Source: Journal of Nutrition Education and Behaviour

Challenges in Meeting Nutritional Requirements

Anna M. Rangan wrote . . . . .

A brief history

Did you know that the early development of dietary standards and recommendations was largely a result of wars and economic depression? In order to determine how much food a person needed to prevent starvation, the first dietary standards for energy and protein were proposed in 1862 by Dr Edward Smith in the UK—2800 kcal and 80 g protein for a working man. It was not until the beginning of the twentieth century that it was generally accepted that foods contained other, as yet unidentified, essential nutrients.

In response to the Great Depression of the 1930s, the League of Nations (now known as the United Nations) committee on nutrition recommended people to include ‘protective foods’ in their diets, with an emphasis on milk, leafy vegetables, eggs, fish and organ meats. As more information about nutrient requirements was accumulated, dietary standards for energy, protein, fat and a few vitamins and minerals were proposed. Canada and the USA were the first countries to publish dietary standards and allowances to evaluate diets, and now, many countries and health authorities have their own sets of recommended nutrient intakes.

Nutrient Reference Values

In Australia, the nutritional requirements are provided in the form of Nutrient Reference Values (NRVs). These include requirements for individuals (recommended dietary intakes) and for groups (estimated average intakes or EARs) based on the traditional concept of adequate physiological or metabolic function and/or avoidance of deficiency states. Chronic disease prevention is dealt with separately, using acceptable macronutrient distribution ranges and suggested dietary targets. These NRVs are set by the National Health and Medical Research Council and the New Zealand Ministry of Health and are based on the latest available scientific knowledge. NRVs apply to a healthy population and vary according to age, gender and life stage. They are set on the basis of long-term (i.e. usual) nutrient requirements.

The Eat for Health website has a number of calculators that can be used to estimate dietary energy and nutrient requirements based on age, gender, weight and physical activity level, as well as to determine the recommended number of servings of core (and discretionary) foods to be consumed each day. These estimates are developed for people of all ages and backgrounds in the general healthy population, including those with common diet-related risk factors such as being overweight.

Australian Health Survey (2011–2013)

The nutrient intakes of the Australian population were evaluated as part of the Australian Health Survey of 2011–2013 conducted by the Australian Bureau of Statistics (ABS). Detailed information was collected on dietary intake from over 12 000 participants aged two years and above using a structured 24-hour recall method. By collecting two days of recall data, the researchers were able to apply a statistical model, the National Cancer Insitute (NCI) method, to estimate the ‘usual intake’ of nutrient distributions for each age and gender group. This method is superior to using only one day or the mean of two days in the estimation of population distributions. This ‘usual intake’ can then be used to assess the nutritional adequacy of the population’s dietary intake. The prevalence of nutrient inadequacy can be evaluated from the proportion of people who do not meet their nutritional requirements or the ‘EAR cut-point method’.

The ABS report on Usual Nutrient Intakes of 2011–2012 provides a summary of the population’s usual intake of selected nutrients and estimates the proportion with inadequate or excessive intakes. Any vitamins or minerals obtained from supplements were not included. This report showed that almost all Australians met their nutritional needs for protein, niacin, vitamin C, vitamin B12, phosphorus and selenium. In addition, most men met their requirements for folate, iodine and iron.

Folate and iodine intakes were adequate for most of the population, with the exception of women aged 19–50 years. In this age group, 10% did not meet their folate or iodine requirements, both important nutrients for babies’ development. Since the introduction of mandatory fortification of bread with folate, thiamin and iodine in 2009, the prevalence of inadequate intakes has greatly improved. Biochemical tests undertaken as part of the Australian Health Survey showed that the vast majority of women of child-bearing age had sufficient red blood cell (RBC) folate levels to prevent neural tube defects. More alarming were the iodine levels, with two thirds of women of child-bearing age considered as being below the recommended levels. Iodine deficiency was particularly high in Tasmania and lowest in West Australia. The report suggests that mandatory fortification may not be enough to meet the additional iodine requirements of these women. Iodine is an essential nutrient required for growth and deficiency and can result in impaired mental and physical development.

Other nutrients of particular concern were calcium, vitamin D, sodium, iron and zinc. Over half of the Australian population surveyed had inadequate intakes of calcium. This reached 90% for adolescent girls and women aged 50 years and above. Low calcium intake can result in lower peak bone mass attainment and increase the risk of osteoporosis, a condition affecting one in four women over the age of 50. In addition, vitamin D deficiency was common in the Australian population. Although not strictly a dietary deficiency, as sunlight exposure is the major source of vitamin D, the Australian Health Survey found that 23% of the population had insufficient levels (<50 nmol/L). Severe vitamin D deficiency can lead to rickets in children and osteoporosis in older people.

In contrast, sodium intakes were excessively high, with more than half of the population exceeding the upper level of intake (UL). As this did not include sodium added at the table or during cooking, these numbers are likely an underestimate. High sodium intakes have been linked to high blood pressure and a range of other health conditions, such as heart failure, kidney problems, stomach cancer and osteoporosis.

Iron intakes were inadequate among adolescent girls and pre-menopausal women, with 40% not meeting their requirements through diet alone. Iron deficiency is common in this age group and can lead to fatigue, reduced performance and cognitive functioning. Zinc intakes, on the contrary, were particularly low among adolescent boys and adult men. Requirements for zinc are higher in men than women, and the greatest prevalence of inadequacy was among men aged 71 years and above (66%). Although overt zinc deficiency is relatively uncommon in Australia, the symptoms of mild deficiency may include growth impairment and reduced immune function.

Perhaps surprisingly, large sections of the population did not meet the required intakes for magnesium and vitamin B6 and, to a lesser extent, vitamin A and thiamin as these nutrients are not usually considered to be at risk. Magnesium and vitamin B6 intakes were inadequate for one in three people. Severe deficiencies of magnesium and vitamin B6 deficiencies are rare, and signs and symptoms are usually not present in mild deficiency. Vitamin A intakes were low for approximately 20% of adolescents and adults and thiamin for 20% of adult women. Vitamin A deficiency is rare in Australia, and the incidence of thiamin deficiency Wernicke–Korsakoff syndrome, which was the stimulant for introducing mandatory thiamine fortification to wheat flour for bread making in 1991, has now reduced significantly.

Based on these shortcomings in meeting nutritional requirements, it is evident that many people are not following the dietary guidelines and consuming the recommended number of core foods. Fruit, vegetable, dairy products and meat and alternatives were all consumed in insufficient amounts. Less than 5% of the population met the recommended vegetable intake, and only 10% met the guidelines for dairy products. Compliance was the highest for meeting servings of fruit and grains, although refined cereals made up the bulk of grain servings. Excessive amounts of discretionary foods and beverages—high in saturated fat, sugar, salt or alcohol—were consumed and made up 35% of the total energy intake. Alcohol intakes were particularly high in middle-aged Australian adults.

A few limitations of this survey need to be mentioned. The underestimation or underreporting of food and beverage intake was highly likely and appeared to have increased since the previous national nutrition survey in 1995, particularly for males. Energy underreporting was estimated to be approximately 21% for women and 17% for men. Underreporting is unlikely to affect all foods and nutrients equally and must be considered when interpreting results from this survey. In order to translate food into nutrients, an up-to-date food composition and measures database is essential, and the AUSNUT (2011–2013) database was created specifically for this survey by Food Standards Australia New Zealand (FSANZ). This is a comprehensive database containing over 5500 foods and beverages, with a complete nutrient profile of 53 nutrients. Nutrient data were derived from a variety of methods, such as laboratory analysis, imputed from similar foods, calculated using a recipe approach or borrowed from other databases. Although many foods were directly analysed, the nutrient content is an average of a particular sample of foods and ingredients and should be considered an approximation only.

Even though Australia is one of the wealthiest countries in the world, with a plentiful and safe food supply able to meet the population's nutritional requirements, most people are not choosing their foods wisely. Too often, healthy food choices are compromised because of a lack of time or skills to prepare balanced meals and the abundant availability of cheap energy-dense convenience foods. Australians are eating out more and spending less time preparing foods at home. Discretionary foods, such as biscuits and chocolate, are becoming everyday foods, and many Australians believe that their diets are healthy.

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Heart Group Advises Personalized Nutrition Counseling

Health care providers need to consider people’s ethnic, cultural and personal preferences when offering healthy eating advice, a new American Heart Association scientific statement says.

Health care providers also need to use easy-to-understand language and food examples.

“It’s one thing for professionals to summarize the data and develop guidelines. An entirely different strategy is required to translate those guidelines into daily behaviors embraced in real life, by real people,” said Linda Van Horn, the statement writing group chair. She is a professor of preventive medicine (nutrition) at Northwestern University Feinberg School of Medicine in Chicago.

“To consumers, it’s an apple, an orange, a pizza. It’s not saturated fat, refined carbohydrates and potassium. That’s nutrition jargon familiar among professionals, but not normal conversation for the average American,” Van Horn said in an AHA news release.

Advice about healthy eating can be adapted to different cultural and personal preferences, as well as affordability.

Van Horn noted that some ethnic groups have higher rates of heart disease and stroke, so addressing certain eating habits in those groups may help lower those risks.

Some examples: Blacks report consuming more sugar-sweetened beverages and fewer fruits and vegetables than whites; and people of Chinese, Korean and Japanese heritage tend to have diets lower in saturated fats, but higher in salt content, the AHA said.

“Identifying optimal nutrition for preserving good health is an ongoing, dynamic science. But there are fundamental principles we know now about how some foods decrease risk of heart disease, while other foods increase risk. For example, I’ve yet to see a study reporting that fruits and vegetables are bad for you,” Van Horn said.

The statement, published in the journal Circulation, provides health care providers with practical ways to help patients make healthier food choices. For example, there are portion sizes for different types of vegetables, whole grains, meat, poultry, eggs and many other products.

“We translate the recommendations into real-life, buy-it-at-the-grocery store, order-it-on-the-menu type choices,” Van Horn said.

“Counseling people about healthy eating is a process, but when someone becomes motivated to actually make dietary changes such as eating more fruits and vegetables, amazing things happen. In as little as three weeks, they develop a preference for those foods and even begin to miss them if they’re not on the plate,” she said.

Source: U.S. Department of Health and Human Services