Nuts
Five large epidemiological studies in the USA all seem to indicate that the risk of myocardial infarction is lower for people who eat nuts on a regular basis than for those who do not. In the three studies where nut consumption was reported in the same manner, and after adjusting for other (known) confounding dietary factors, there was a roughly 40% reduced risk by consuming nuts at least 5 times/week, compared to less than once a week. One of the studies was conducted on just over 31 000 Seventh Day Adventists in California, who were interviewed about their dietary habits and tracked for 12 years. The lower risk with higher consumption was statistically significant both among overweight and lean individuals, and was not dependent on the level of other cardiovascular risk factors. In the Physicians’ Health Study, the risk of sudden cardiac-related death was lower among persons who ate nuts more than 2 times/week (relative risk 0.53; 95% 95% CI 0.30–0.92) compared with those who seldom or never ate nuts (P for trend = 0.01). However, there was no apparent relation to deaths from other causes or non-fatal myocardial infarctions.
Randomised intervention studies of increased nut intake are lacking. Therefore, the presence of confounders in the observational studies cannot be ruled out. Certainly, when most of these studies were conducted, there was a general belief among
US health-conscious people that nuts are beneficial, an opinion strongly promoted by John H. Kellogg in the early twentieth century.
The suggested nutritional benefits of nuts include being rich in minerals, vita- mins and soluble fibre. Similar to olive oil, the fat in most nuts is dominated by the monounsaturated fatty acid oleic acid (18:1n-9). The potential disadvantages include a high content of omega-6 fatty acids and phytic acid, as well as a high energy density. A handful of nuts, particularly hazel nuts, drastically increases the omega-6/omega-3 ratio. The phytate content of nuts impedes the body’s ability to absorb minerals such as iron, calcium, zinc and magnesium, where a lack of the last two minerals has been suggested as a contributing factor for ischaemic heart disease481,1639. The low water content means that an extra handful of nuts goes down easily, which is expected to contribute to weight gain. The fact that nuts are rich in plant sterols may help to maintain beneficial levels of serum cholesterol, but the net effect on the heart and blood vessels is unclear (see under ‘Plant sterols’).
Peanuts are not nuts in the botanical sense, and should not be considered so in the context of healthy foods. They belong to the family of legumes (Leguminosae), which also includes beans, peas and lentils. Peanuts are thought to actually increase the risk of atherosclerosis via mechanisms that are partly independent of their fat (see Section 4.3).
Coconut is considerably different from other nuts in that coconut fat is dominated by saturated fatty acids (see also Section 3.2). This probably partly explains why serum cholesterol levels among some Pacific Islanders are not entirely favourable. Even if these populations apparently manage to consume large amounts of coconuts without any problems, people in the Western world are advised to be careful, particularly with coconut fat, which in contrast to coconut meat provides only calories without any vitamins, minerals or fibre.
Dietary fibre
There is insufficient high-quality research on the importance of dietary fibre for ischaemic heart disease. If people who prefer whole grains have less heart disease, it may be because of other elements of their healthy lifestyle. The only published controlled trial of dietary fibre on the risk of developing ischaemic heart disease, the DART study mentioned above, showed a tendency towards increased mortality from cardiovascular diseases in people eating more dietary cereal fibre. The study took roughly 2000 British men, who had had a previous myocardial infarction, and randomly assigned them either a high intake of dietary fibre (≥18 g/day) or an ordinary diet. After 2 years, a total of 123/1017 patients in the fibre group had died, compared with 101/1016 (P = 0.16) from the other group. The number of subjects who developed cardiovascular disease were, respectively, 109 and 85 (P = 0.10). After adjustment for possible confounding factors such as health state and medication, the increased risk of developing ischaemic heart disease was statistically significant (hazard ratio 1.35; 95% CI 1.02–1.80), although it subsided after more than 10 years (hazard ratio 1.11; 95% CI 0.96–1.29). Since cereals made up the main source of fibre, it should not be ruled out that the tendency towards increased mortality was a detrimental effect of the increased grain consumption.
The widely accepted hypothesis that dietary fibre, including cereal fibre, prevents ischaemic heart disease is based on epidemiological studies. However, one of the largest, and most often cited, prospective observational studies points to the appearance of confounders that could possibly explain a large part of the relationship. The study recorded the dietary habits of close to 40000 healthy women, primarily nurses, by means of frequency formulas. During the 6-year follow-up period, 570 new cases of disease were noted, including myocardial infarction (177), stroke, percutaneous transluminal coronary angioplasty, coronary bypass operations or cardiovascular death. Women in the quintile with the highest fibre intake (median 26.3 g/day) had, compared with the lowest quintile (median 12.5 fibre/day), a relative risk for developing any type of cardiovascular disease of 0.65 (95% CI 0.51–0.84), and 0.46 (0.30–0.72) for undergoing a myocardial infarction. After correcting for differences in cardiovascular risk factors, however, the relative risk dropped to 0.79 (95% CI 0.58–1.09, not significant) and 0.68 (95% CI 0.36–1.22, not significant) respectively.
It should be noted that there are two kinds of dietary fibre. The first is insoluble fibre, which is mainly found in cereals, and which does not appear to affect cardiovascular risk factors very much. The other kind is soluble fibre, which dominates in fruits, root vegetables and nuts, and has been shown to improve blood lipids and glucose metabolism, particularly when ingested in high amounts1780. Insoluble dietary fibre has two main benefits, it is satiating due to its filling capacity in the stomach, and it prevents constipation. Soluble fibre
has the same effects, but it is thought to improve blood lipids and carbohydrate metabolism more than insoluble fibre225,516,1780, hence it probably has a greater ability to prevent cardiovascular disease and diabetes. A case–control study in Italy found fruit fibre to be protective against myocardial infarction, while no such effect was found for cereal fibre. In contrast, a pooled analysis of cohort studies found both kinds of fibre to be inversely related to ischaemic heart disease mortality.
Food choice
From all we know, fruits, vegetables, nuts, fish and lean meat can be eaten without
restrictions. The beneficial effects of a Palaeolithic diet in our study mentioned
above are probably applicable to most people with type 2 diabetes. Fruit has a high
water content and therefore facilitates caloric restriction. It is also low in fat and
salt, while high in soluble fibre, vitamins and minerals. Furthermore, most fruits
have a low GI (melons are relatively high GI). Even ripe bananas are acceptable
with a mean GI of 50 (glucose = 100). However, fruits are considerably
less protein-rich (per unit of energy) than vegetables, which therefore results in a
higher glycaemic load. When fruits replace cereal grains, as will be the case for
most Westerners with diabetes, the total glycaemic load decreases. Nevertheless,
blood sugar should be carefully monitored, particularly when fruit consumption
exceeds 500 g/day and in case of high intakes of grapes or ripe bananas. Serum
triglyceride levels can rise in sensitive patients since fruit contains relatively high
levels of fructose1843. The fructose content in fruit is apparently too low to pose a
significant problem for most people with type 2 diabetes. The amount of fructose
in 500 g of fruit is approximately 25 g, while available evidence suggests that a
fructose intake below 60 g is safe.
Nuts are a complicated issue. Many of them are not actually nuts in a botanical
sense (e.g. peanuts). Nuts are energy-dense often provide a relatively high amount
of energy for the amount of work involved, and may have made up an essential part
of the diet during certain time periods. However, they are obviously critical for the
mother plant to be able to spread its genes. Therefore, it is logical to find that nuts
that are protected by very hard shells have a lower content of phytochemicals than
other nuts. Nevertheless, many stone fruits and seeds may have tasted worse –
and been more poisonous – during the Palaeolithic era than today. Almonds at
that time are thought to have been more bitter than sweet. At the beginning of
agriculture, our ancestors supposedly prioritised the sweetest types of almonds, an
early example of plant breeding.
Phytochemicals
As previously mentioned, the plant kingdom contains thousands of bioactive substances
and other natural chemicals, many of which are thought to be part of their
defence systems against herbivores. The highest concentrations are generally found
in young plants, also in the most vital parts (sprout, seeds, beans and roots). Such
phytochemicals can often make up 5–10% of the plant’s dry weight. Prehistoric
foragers were able to limit the negative health effects by having access to a
large number of various plant species, by consciously avoiding the most poisonous
ones, and by the use of cooking.
Plant lectins are carbohydrate-binding proteins from plants. Strictly, they are
defined as ‘plant proteins with at least one non-catalytic domain that is bound
reversibly to a specific mono or oligosaccharide’. They bind to certain sugars
because they contain carbohydrate residues themselves, i.e. they are glycated
(=glycosylated). The most important function of plant lectins is thought to be
protection against attacks by plant-eating animals. The highest concentration
is found in seeds, beans, potatoes and peanuts. Unrefined grain products, on the
whole, have a higher lectin content than refined seed products. Lectins in wheat,
rye, rice and potatoes bind to GlcNAc-domains (GlcNAc = N-acetylglucosamine)
on receptors in the ‘host organism’ (Table 3.19). Lectins are often not destroyed
during normal cooking. Cooking beans in a pressure cooker deactivates their lectin,
which has been shown for Turkish beans and which is expected to be true for
most plant lectins.
Plant lectins are, compared to other dietary proteins, unusually resistant to enzymatic
breakdown in the intestines and can penetrate the intestinal mucous membrane,
finally being deposited in the internal organs. The ability to avoid destruction
and be absorbed in the intestines makes plant lectins excellent carriers for pharmaceutical
substances. The deposition in internal organs is useful for histochemical
identification of specific cell systems in the microscope. A well-studied effect
of a number of plant lectins is agglutination of red blood cells. The long-term,
potentially negative effects of lectins on humans are addressed in the sections on
atherosclerosis (Section 4.3), insulin resistance (Section 4.6), cancer (Section 4.11)
and autoimmunity (Section 4.15).
During the Palaeolithic period, plant lectins were consumed on a daily basis, but
probably not in the same, high concentrations of today, and not exclusively from
one or a few plant species.
Protease inhibitors are substances in beans and seeds, which inhibit proteindegrading
enzymes in the digestive tract such as trypsin, chymotrypsin and amylase.
This very ancient defence mechanism of plants allows their seeds to pass through
the entire gastrointestinal system (mouth–oesophagus–stomach–gut) undamaged,
thereby surviving excretion and later to grow in the ground. The concentration
of protease inhibitors in beans and cereals is so high that the digestion of dietary
proteins (including other than those in the seed) can be substantially reduced.
Endocrine disruptors such as phytoestrogens constitute another kind of defence
mechanism in the plant’s struggle for survival. These hormone-like substances
can interfere with the consumer’s reproductive system, in which case the defence
strategy may be to cause infertility rather than death. It has been suggested that
isoflavones (also called flavonoids), the phytoestrogens that appear in large amounts
in cereals exert an antioxidative effect, but good evidence is still lacking.
Other hormone-like plant sterols have the effect of reducing blood lipids by hampering
cholesterol absorption in the intestines.
Cyanogenic glycosides are produced by more than 2500 plant species. Their
production and conversion to cyanide constitutes a well-studied defence mechanism
against herbivores. Cyanide can be acutely toxic when consumed in high
amounts, but more relevant in this context is when it is ingested regularly in low
concentrations. Cyanide is then converted to thiocyanate, a potentially goitrogenic
substance which can increase the need for dietary iodine and may cause goitre, even
when iodine intake is high. These are well-documented consequences of certain staple
foods in the Third World such as cassava, bamboo shoots, sweet potatoes, lima
beans and millet. Out of the foods listed, only the edible parts of the cassava plant
contain large amounts of thiocyanate, otherwise the problem is by most authors
considered negligible and is not thought to be a contributing cause of goitre in Europe.
Even linseeds contain cyanogenic glycosides. The National Swedish Food
Administration has issued a warning against consuming more than 1–2 tablespoons
of linseeds on a daily basis, since 10 tablespoons daily caused mild neurological
symptoms in one human, most likely through cyanide exposure (
www.slv.se, in
Swedish). The long-term effects of a low intake are unknown.
Pyrrolizidine alkaloids are a large group of chemicals that are produced by over
6000 plant species, some of which are used for food or herbal teas. Low-level
exposure to pyrrolizidine alkaloids is suspected as the underlying cause of endemic
cirrhosis in certain cultural groups in Asia and Africa. The high rate of primary
liver cancer in South Africa may be due to chronic low-level pyrrolizidine alkaloids.
Other organs that are thought to be affected are kidneys, stomach, brain, lungs,
heart and the reproductive system.
Phytic acid is found in seeds (including beans) with the aim of binding phosphorus
and other minerals. The case of phytic acid may be an exception to a common
rule in plant–animal interactions, that bioactive plant constituents are designed to
damage the consuming ‘host’. Apparently, in the case of phytate, the damage is
only coincidental. When phytic acid is consumed, it binds to dietary minerals and
trace metals such as iron, zinc, calcium and magnesium, to form phytate salts with
these ions. As a consequence, these nutrients largely pass unabsorbed through the
intestines and are excreted.
Since phytic acid is mainly found in the germ part of the seed, whole grain
flour is unhealthier than refined flour in this respect. However, the seed also
contains phytases that can break down the phytic acid under beneficial circumstances.
In the best case, if the seeds are tossed in the right way,
soaked and allowed to sit at proper temperature and pH for a sufficient number
of hours (or days), the phytic acid can almost be eliminated. The various types of
grains distinguish themselves in that, out of our four most popular cereals, it is
most difficult to reduce phytic acid in oats. In today’s industrially produced, rolled
oats, the phytases are completely destroyed, which means that it is then impossible
to reduce the phytic acid content. In contrast with humans, rats have a high
capacity to degrade phytates in their intestines, an apparent adaptation to regular
consumption of seeds.
Acrylamide is formed during baking or high-temperature cooking (frying,
grilling, deep-fat frying) of starchy foods, i.e. heating the surface of the food. This
type of food treatment was probably uncommon during the Palaeolithic (apart
from roasting carbohydrate-free foodstuffs like meat and fish). Heating meat and
fish does not form acrylamide. In terms of Western food, crisps and chips have the
highest amount of acrylamide, while bread, breakfast cereals and hash browns are
the dominant sources of acrylamide for the average European. Acrylamide has been
proposed to cause cancer, based on experiments on animals, but the connection is
uncertain.
Knowledge of the health effects of bioactive substances is fragmentary, and in
many cases, it lacks the perspective of evolutionary biology. Research about alkylresorcinols
and biogenic amines has concentrated on the detrimental aspects, while
research on hormone-like substances and antioxidants most often has highlighted
the alleged beneficial effects.
