Dr Ryde's Casebook

Dr David Ryde is a retired GP and holistic health practitioner who lived in Oxford for some years around the millennium. During his time in Oxford he contributed several talks and articles for the members' newsletter, including those below which followed a question and answer format. They deal with sources of calcium for vegans, essential fatty acids, and the absorption of iron from plant foods.

Q. Concern has been expressed over the adequacy of calcium in the vegan diet. What are the best sources of calcium for vegans, is it necessary to include high-calcium or calcium-fortified foods in the diet, and what is the best diet for ensuring adequate bone strength in order to minimise the risk of osteoporosis in later life?

A. Vegans obtain their calcium from vegetation, just like herbivorous animals such as cattle and horses. Good sources include vegetables such as broccoli, Brussels sprouts, collards and kale, dried figs and raisins, all types of beans and peas (especially tofu prepared with calcium sulphate), whole grains, nuts and seeds. The element boron, found almost exclusively in plant foods, also helps the body to retain calcium, which is particularly well absorbed from low-oxalate vegetables such as kale (only 20-30% of the calcium in the average diet is absorbed by the body). Dairy products are rich in calcium, but also contain unhealthy amounts of fat and dietary cholesterol, and can cause allergies, lactose intolerance and juvenile diabetes. They contain no fibre and little iron. Plant foods do not have these disadvantages. Thus, calcium-fortified foods are not essential for vegans.

Research has shown that a high-calcium diet is no guarantee against osteoporosis. Indeed, Eskimos have the highest incidence of osteoporosis in the world despite consuming a massive 2000 mg of calcium per day. The typical Westerner consumes 1000-1500 mg of calcium daily, compared to the 500 mg per day consumed by the inhabitants of developing countries in Africa and Asia where the intake of dairy products is much lower and the incidence of osteoporosis is less than 20 per cent of that in the West. The reason for this is that excess animal protein leaches calcium from the bones. Salt, coffee and alcohol also limit calcium absorption. An ecological study found a strong positive association between animal protein intake and hip fracture rates in women. Thus diets which are low in animal protein, such as those of rural Africa and Asia, may be in calcium balance despite a low calcium intake, whereas high protein diets may lead to a net loss of calcium from the body despite a high dietary intake. Osteoporosis is, therefore, a disease of calcium loss rather than one of calcium lack. Studies of vegetarians suggest that their loss of bone in later life is lower than that of age-matched non-vegetarians. Nutrition aside, weight-bearing exercise is important in promoting bone strength.

Q. Nutritionists advise us to reduce the amount of fat in our diet. However, I've heard that some fats are "essential". Which are they, what foods do we obtain them from, and how much do we need?

A. In the western diet, approximately 40% of calories derive from fat, 40% from carbohydrate and 20% from protein. In rural areas of developing countries the corresponding figures are 10%, 80% and 10% respectively. This massive contrast in dietary patterns, together with smoking and lack of exercise, plays a significant part in the development of degenerative diseases. Much of the fat consumed in the western diet is saturated fat of animal origin, compared with the smaller quantities of mainly unsaturated fats from the plant foods which make up the bulk of the diet in poorer countries.

The meat of domesticated animals is generally rich in cholesterol and saturated fat which further stimulates cholesterol production. In contrast, plant foods contain little or no cholesterol and are generally low in saturated fat. Thus, although Chinese peasants typically consume 25% more calories per kilogram of body weight than their western counterparts, they are very rarely obese. However, within a generation of emigrating to an affluent western nation, such people are often overweight and beginning to suffer from degenerative conditions such as heart disease, diabetes, stroke, hypertension and certain cancers. Mediterranean peoples consume healthier fats than North Europeans, but they still consume too much fat, leading to obesity in later life. However, their largely plant-based diet, rather than their fat intake, may explain their lower incidence of heart disease.

Our bodies convert excess calories into fat, but there are two unsaturated fats which are called "essential fats" because they must be present in the diet. These fats, or oils as they are liquid at room temperature, are the omega 3 and omega 6 fatty acids, and they are found mainly in green leafy vegetables, nuts and seeds, and fish. The essential fatty acids (EFAs) are important for growth, fertility, kidney function and immunity, and for a healthy heart, arteries, joints and skin. Only modest amounts of extra virgin plant oils should be used in the kitchen, the remaining EFAs being obtained from eating nuts, seeds and fresh green leafy vegetables.

Though modest amounts of essential oils are beneficial an excess may be harmful, giving rise to diabetes and blood clotting defects for example. They are best obtained from a variety of plant foods rather than from fish which is rich in cholesterol and an increasing source of chemical residues, especially farmed fish. Ground raw linseed (flaxseed) is an excellent source of omega 3 oil (use an electric coffee grinder or mortar and pestle to crush the seeds). Two tablespoons a day can reduce blood cholesterol levels and makes an excellent laxative. Sunflower seeds are a good source of omega 6 oils.

Q. I've heard that the iron in plant foods is less well absorbed than the iron in meat. Are vegetarians at greater risk of iron deficiency?

A. Iron is present in food in two forms: haem iron, which is found in meat; and non-haem iron, which is found in plant foods such as cereals, nuts, pulses and vegetables. Only a small proportion of dietary iron is absorbed by the body, the amount depending on the individual. However, it is generally agreed that 20 to 30 per cent of haem iron is absorbed, whereas only about 5 per cent of non-haem iron is absorbed, but this may be increased or decreased by other dietary and physiological factors.

The absorption of non-haem iron is inhibited by phytates, oxalates and phenols, which are present in plant foods, and by the tannin in tea. However, dietary fibre does not affect iron absorption, which is just as well because wholegrain cereals are a major source of iron in the vegetarian diet. The good news for vegetarians is that vitamin C (present in most fruits and vegetables) can significantly enhance the absorption of non-haem iron. For example, 75 mg vitamin C (equivalent to that found in 200 ml pure orange juice) can increase the absorption of non-haem iron by three to four times if consumed at the same meal as the iron-bearing foods. There is also evidence to suggest that the body is able to absorb more iron when body stores are depleted and, conversely, to reduce iron absorption when there is a danger of overload (like many nutrients, iron is toxic in excess).

Studies of vegetarians have generally found that their iron intake is similar to or even higher than that of omnivores. Although haemoglobin concentrations in vegetarians are generally within the normal range, some studies have found significantly lower serum ferritin levels (a measure of the amount of iron stored in the body) in vegetarians compared to omnivores, probably because of the lower absorption of non-haem iron noted above. This may actually be an advantage since recent studies have linked high iron reserves with an increased risk of heart disease and non-insulin dependent diabetes.

In summary, a well-planned vegetarian or vegan diet (dairy products are a poor source of iron) which contains plenty of fruits and vegetables, along with wholegrain cereals, nuts, pulses and seeds is likely to provide adequate iron. Rich sources of iron for vegetarians include bran cereals, fortified breakfast cereals, lentils, figs, cashew nuts, and sesame and sunflower seeds. Other good sources include wholemeal bread and low-oxalate green leafy vegetables (sorry Popeye, but the high oxalate content of spinach limits its value as a source of iron). The use of cast iron cooking utensils can also boost iron intake. Therefore, there is no reason why a well-nourished vegetarian or vegan should be at greater risk of iron deficiency. Indeed, in twenty years of meeting or consulting hundreds of vegetarians I cannot recall a single case of iron-deficiency anaemia.

Dr David Ryde, April 1998, July 1998 and January 1999

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This page created 23 June 1998 by Kate L Pugh and updated 31 December 2001 by Paul Appleby.