Cardiovascular disease

Summary

Heart disease and stroke are caused by blockages in the arteries that prevent blood flow to the heart or brain. High cholesterol increases the risk and LDL ‘bad’ cholesterol is the main cause. The WHO say 56 per cent of heart disease and 18 per cent of strokes are caused by cholesterol levels above 3.2mmol/l. UK guidelines recommend aiming for a level below 5.0mmol/l but levels in the UK are among the highest in the world averaging around 5.5mmol/l. 

Geographical variation and migrant studies show that heart disease is caused by lifestyle and environmental factors. Many studies have looked at the effects of diet and meat specifically. Researchers from the Harvard School of Public Health found a much higher risk among people eating high levels of processed meat. The NIH-AARP Diet and Health Study found that red and processed meats were linked to early death in men and women, from cancer as well as CVD.

Results from the Nurses’ Health Study showed that red meat is linked to a higher risk of heart disease and replacing one daily serving of meat with nuts lowered the risk by 30 per cent. The Nurses’ Health Study and the US Health Professionals Follow-up Study together found that replacing meat with healthier protein also lowered the risk of stroke. A third study, combining both groups again, found that each daily serving of red and processed meat increased the risk of death from CVD by 18 per cent and 21 per cent, respectively. 

The EPIC study found that people eating more than 160g of processed meat a day were 28 per cent more likely to have CVD than those eating less than 20g. They did not find the substantive evidence against red meat that other studies have found but EPIC participants eat less meat and are healthier than the general population, this may have masked the harmful effects of red meat. The facts remain that foods that are high in saturated fat (meat and dairy) raise cholesterol which is a risk factor for CVD. 

A review from the Cochrane Library found that reducing saturated animal fat (but not total fat), reduced the risk of CVD by 14 per cent. They say lifestyle advice should continue to include reducing saturated fat and replacing some of it with healthier unsaturated fats.

Other substances in meat have been linked to CVD; haem iron increases the risk of heart disease, which is why lower rates are seen in women who menstruate and people who donate blood regularly.

Vegans have lower cholesterol, lower blood pressure and a lower risk of CVD. British vegetarians have a whopping 32 per cent lower risk of hospitalisation or death from heart disease than meat-eaters.

The risk of heart disease and stroke can be significantly reduced by a diet that provides more plant-based sources of protein compared with the typical Western diet. Put simply, ditching meat lowers your risk of suffering a heart attack or stroke.

Cardiovascular disease (CVD) is a general term that describes diseases of the heart and blood vessels such as heart disease and stroke. Blood flow to the heart, brain or body can be reduced as the result of a blood clot (thrombosis), or by a build-up of fatty deposits or plaques inside an artery that cause the artery to harden and narrow (atherosclerosis). Heart attacks and strokes are mainly caused by blockages that prevent blood from flowing to the heart or brain. High cholesterol levels increase the risk and LDL ‘bad’ cholesterol is the main source of artery-clogging plaques.  

Professor T. Colin Campbell’s China Study observed very low rates of heart disease in the southwest Chinese provinces of Sichuan and Guizhou between 1973-1975 during which time not one single person died of heart disease before the age of 64 among 246,000 men and 181,000 women (Campbell and Campbell, 2005). Campbell suggested these figures reflect the important protective role of low blood cholesterol levels observed in rural China.  

According to the WHO, almost a fifth (18 per cent) of global stroke events and over half (56 per cent) of global heart disease is attributable to cholesterol levels above 3.2mmol/l (WHO, 2003), millimole per litre is the standard unit for measuring cholesterol. Current UK guidelines state that we should aim for a cholesterol level below 5.0mmol/l. A joint report between the Medical Research Council and the British Heart Foundation found that the average cholesterol level for people aged 16 and above in the UK is about 5.5mmol/l and about one-fifth of people have levels above 6.5mmol/l. In China (where there is much less heart disease), the average cholesterol level in the cities is about 4.5mmol/l for men and women aged 35-64 and levels in the countryside are even lower (MRC/BHF, 2006).  

It could be argued that genetic differences between races may affect the risk factors for CVD and other diseases. However, Campbell’s observations that Japanese men in Hawaii and California have much higher levels of blood cholesterol and incidence of heart disease than Japanese men in Japan confirms that some risk factors are environmental rather than genetic. In other words, the choices we make about the food we eat and how we live can have a significant impact on heart health.  

Campbell also observed that the intake of animal protein correlates directly with heart disease incidence, which he attributes to the cholesterol-raising effect of animal protein. Conversely, Campbell notes that eating plant protein lowers cholesterol (Campbell and Campbell, 2005). 

About 610,000 people die of heart disease in the US every year, that’s one in every four deaths (CDC, 2015). Around 800,000 people in the US have a stroke each year, where stroke is the fifth leading cause of death and a major cause of adult disability (CDC, 2016). Quantifying the role of meat consumption on these events is of great scientific and public health importance. To address this important question, researchers from the Harvard School of Public Health performed a systematic review of the evidence looking for relationships between red meat, processed meat and both types of meat combined (referred to as total meat) with the risk of heart disease and stroke. They examined 20 studies from 10 different countries (including 1,218,380 individuals, 23,889 cases of heart disease and 2,280 strokes) and found a significantly higher risk among those consuming the most processed meat (Micha et al., 2010). They suggest that the high sodium and nitrate preservative levels used in processed meats could be responsible.   

The Nurses’ Health Study is one of the largest investigations into risk factors for major chronic diseases ever conducted. It began in 1976 when 121,700 female registered nurses aged 30-55, residing in 11 US states, provided detailed information on their medical history and lifestyle. Every two years, follow-up questionnaires have been sent to update information on potential risk factors and to identify newly diagnosed cases of heart disease and other illnesses. The large number of participants and high follow-up rate with updated dietary information provides an excellent way of identifying the links between certain foods and disease. 

In 2010, the Nurses’ Health Study reported on the relationship between major dietary protein sources and heart disease in women (Bernstein et al., 2010). Higher intakes of red meat and whole-fat dairy products were associated with a higher risk of heart disease, while higher intakes of nuts, fish and poultry were associated with a lower risk. Again, this doesn’t mean that fish and chicken are good for you; it may just show that they are not quite as bad as red and processed meat. Also, it should be remembered that this study dates back to 1980 when the composition of chicken meat was different; now the average broiler hen contains more fat than protein (Wang et al., 2010). The Nurses’ Health Study found that replacing a serving of meat with one of nuts had the greatest effect, lowering the risk of heart disease by 30 per cent. The usual suspects linking meat and disease were discussed; saturated fat, cholesterol, high sodium, HCAs and haem iron.  

Two years later, the same research group looked at the risk of stroke in women from the Nurses’ Health Study and a large cohort of men from the US Health Professionals Follow-up Study (43,150 men followed for 22 years and 1,397 strokes). Results also showed that replacing meat with other sources of protein lowered the risk of stroke (Bernstein et al., 2012). Replacing a serving of meat with nuts lowered the risk of stroke by 17 per cent.

The same year, the association between red meat and CVD was also investigated in these two large cohorts (Pan et al., 2012). They also found that high intakes of both red and processed meat were associated with a higher risk. Most processed meat contains pork or beef, but it may also be made using other red or white meats including chicken and turkey. With each serving per day of red meat and processed meat, the risk of cardiovascular death increased by 18 per cent and 21 per cent respectively. Substituting other sources of protein lowered the risk.  

In Europe, a large EPIC study including 493,179 individuals followed over 12.7 years and 5,556 deaths from CVD, revealed an increased risk in cardiovascular death in those consuming high levels of processed meat. A 28 per cent increased risk was seen in those consuming more than 160g of processed meat per day compared to those eating less than 20g (Rohrmann et al., 2013). They concluded that those with a high consumption of processed meat are at increased risk of early death, in particular due to CVD but also to cancer and that health promotion activities should include specific advice on lowering processed meat consumption.

People typically think of processed meat as only referring to pork and beef, but this category can also include chicken and turkey. Processed meat is treated by salting, curing or smoking to improve the shelf life, colour and taste of the food. These processes lead to an increased intake of carcinogens or their precursors: NOCs, HCAs and PAHs (Rohrmann et al., 2013). So chemicals used in processed meats may play a role in CVD by damaging blood vessels. In particular, sodium and nitrites might explain the additional harm of processed meats (Pan et al., 2012).

Processed meats such as sausages, salami and bacon also have a higher content of saturated fats and cholesterol than red meat which is often consumed after removing visible fat, whereas the proportion of fat in sausages often reaches 50 per cent of the weight or even more (Rohrmann et al., 2013). Both high saturated fat and cholesterol intake are related to the risk of heart disease.  

In contrast to the US studies discussed, this EPIC study found no statistically significant association between red meat and cancer or cardiovascular death. The authors suggested this may be explained by the higher meat consumption in the US than in Europe (Rohrmann et al., 2013). EPIC participants tend to be healthier than the general population. It seems likely that the lower meat consumption, and the removal of fat from red meat among the health conscious, coupled to a healthier lifestyle, masked the harmful effect red meat has in relation to the risk of cardiovascular disease that numerous other studies have revealed.   

Beef, pork, poultry and lamb all contain high levels of fat, especially saturated fat and eating a lot of saturated fat can raise cholesterol levels in the blood which increases your risk of heart disease. Cholesterol is a type of fat that travels in the bloodstream. Our bodies need some, but high levels in the blood (particularly LDL or “bad” cholesterol) can build up and block the arteries increasing the risk of CVD. If a clot blocks an artery to the heart, a heart attack may follow. If a clot blocks an artery to the brain, a stroke may occur. Foods that are high in saturated fat (meat and dairy) raise cholesterol levels in the blood by promoting production of cholesterol in the liver. In fact, saturated fat is recognised as the single dietary factor that has the greatest negative effect on LDL ‘bad’ cholesterol (Hu et al., 2001).

Cholesterol levels in the UK are among the highest in the world. Studies show that vegetarians and vegans have lower cholesterol levels, lower blood pressure and a lower risk of diabetes than meat-eaters. As a result, the risk of developing CVD in vegetarians and vegans is significantly lower. A study from the University of Oxford found that British vegetarians have a whopping 32 per cent lower risk of hospitalisation or death from heart disease than meat-eaters (Crowe et al., 2013). 

Cochrane Reviews are often referred to as the gold standard in science. A 2012 review from The Cochrane Library analysed 48 studies including over 65,000 participants. It was found that reducing saturated (animal) fat, but not total fat intake, reduced the risk of heart attack and stroke by 14 per cent (Hooper et al., 2012). The authors concluded that lifestyle advice to those at high risk of CVD (and probably also to those with a lower risk), should continue to include the permanent reduction of saturated fat and partial replacement by healthier unsaturated fats. There is much evidence that replacing saturated fat with polyunsaturated fat or monounsaturated fat in the form of olive oil, nuts, seeds and other plant oils can reduce the risk of CVD (Willett et al., 2014). 

The idea that high iron stores can increase the risk of CVD was first proposed in the Lancet in 1981 (Sullivan, 1981). Sullivan proposed that menstrual blood loss could be responsible for the lower risk of CVD seen among premenopausal women compared to men and postmenopausal women. Since then, other studies have confirmed that high iron stores are a risk factor for CVD (Salonen et al., 1992). 

Research suggests that haem iron from red meat, but not non-haem iron from plant foods, is associated with heart disease. In the Netherlands, among a group of 16,136 women aged 49-70, a high haem iron intake from meat was associated with a 65 per cent increase in heart disease risk (van der A et al., 2004). These women had relatively high haem intake intakes; an average of 1.8mg per day, much higher than the 0.5mg per day observed in the UK 2003 NDNS (Henderson et al.¸ 2003). Interestingly because women of reproductive age lose iron by menstruation, the authors hypothesised that these women may gain some protection from iron overload and therefore from heart disease too. If this were true, the relationship between high iron intake and heart disease would be stronger in non-menstruating women. To test their theory, they divided the women into those who had periods and those who didn’t and found that menstruating women had a significantly lower risk of heart disease (10 per cent compared to 58 per cent). So it would seem losing blood every month protects against iron overload and therefore heart disease. An earlier report from the Nurses’ Health Study, this time looking at women with type 2 diabetes, also found that haem iron increases the risk of heart disease (Qi et al., 2007). Again, the authors suggested that losing blood every month conferred protection against heart disease as the association between haem iron and heart disease was stronger in postmenopausal women.  

As well as menstruation in women, voluntary blood donation is an important cause of blood loss that can affect iron stores. Numerous studies have shown that regular blood donation can reduce the risk of CVD (Meyers et al., 2002; Tuomainen et al., 1998). The loss of iron associated with giving blood could be the reason for the observed reduction in risk. More recently another study showed that high-frequency blood donation was associated with lower iron stores, improved vascular function and reduced   stress in blood donors (Zheng et al., 2005).   

Figure 3.0 Oxidative stress caused by high iron stores in the body. 

Figure 3.0 illustrates the proposed mechanism underlying the harmful role of haem iron in CVD. It involves iron’s role as a catalyst in the formation of free radicals and the subsequent oxidation of biological molecules such as DNA and LDL cholesterol (Hunnicutt et al., 2014). This mechanism is thought to underlie the basis of neurodegenerative disorders (such as Alzheimer’s disease), certain cancers and CVD (Muñoz-Bravo et al., 2013). The oxidation of LDL cholesterol may be a key initial event in the progression of atherosclerosis and CVD (Niki, 2011). It has also been suggested that low body iron stores may protect against CVD through limiting the oxidation of LDL cholesterol (Meyers et al., 2002).  

The idea that iron is linked to CVD has been challenged in a small number of studies. In a letter to the British Medical Journal, researchers from Finland countered this argument by asserting that all of the negative studies used unreliable measurements of iron status (such as serum iron concentration, transferrin iron saturation) or had other design problem (Hemilä and Paunio, 1997). They said:

There exists a widespread conviction that the more iron in the diet the better. This is what is still written in medical textbooks. This conviction is, however, the enemy of the truth.”

  • Tomi-Pekka Tuomainen, Research fellow
  • Riitta Salonen, Senior clinical research fellow
  • Kristiina Nyyssönen, Clinical biochemist
  • Jukka T Salonen, Academy professor

Research Institute of Public Health, University of Kuopio, 70211 Kuopio, Finland

So convinced of the link are some researchers that they have suggested reducing our iron intake to reduce the risk of CVD. Some say the evidence may even be strong enough to recommend ending iron fortification and supplementation and to start advising people to donate blood regularly. Of course you could reduce the risk of CVD just by avoiding red meat, processed meat, chicken and seafood which are the main sources of haem iron (Qi et al., 2007). Others caution that given the extent of iron deficiency, any decision to reverse iron fortification and supplementation policy should be based on extremely sound science (Sempos, 2002). The obvious solution is to obtain non-haem iron from plant foods as the body only absorbs as much of this type of iron as it needs. The absorption of haem iron from meat is unregulated; it is absorbed in the gut whether it is needed or not so excessive consumption can lead to iron overload and an increased risk of CVD.

The NIH-AARP Diet and Health Study found that red and processed meats were linked to early death in both men and women including that from cancer and CVD (Sinha et al., 2009). White meat (chicken and turkey) did not have the same links but the authors of the study did point out that processed meat included luncheon meats and cold cuts made of red and white meat and low-fat hotdogs made from chicken or turkey. They said that red and white processed meat products can overlap as both can include bacon, sausage, ham, chicken and turkey.  

The sensible approach is to eat a well-balanced vegan diet containing plenty of iron-rich plant-based foods such as pulses (peas, beans, lentils and soya bean products such as soya milk and tofu), dark green leafy vegetables (such as broccoli, bok choy and watercress), fortified breakfast cereals, wholegrains (such as wholemeal bread, brown rice and wholemeal pasta), dried fruits (such as raisins, prunes, apricots and figs) black treacle and, in moderation, plain dark chocolate.

Taken together, these scientific findings support the links between meat and CVD, one of the UK’s biggest killers. This evidence supports the idea that CVD risk can be reduced by a diet that provides more plant-based sources of protein compared with the typical Western diet (Richter et al., 2015). Put simply, ditching meat lowers your risk of suffering a heart attack or stroke.