'Heart risk from low-carb, high-protein diets'

There’s a “heart attack risk in dieting”, according to the Daily Express, while The Daily Telegraph reported that a “breakfast egg can raise heart disease risk”. Neither of these over-the-top headlines clearly represented the research on which they were based.

The news stems from a large, long-term Swedish study of women aged 30 to 49, looking at their diets and whether they developed cardiovascular disease. Researchers wanted to understand the long-term effects of low-carbohydrate, high-protein diets. They found that proportional decreases in carbohydrate intake and increases in protein intake were associated with a small increase in the risk of cardiovascular disease such as heart attack and stroke.

This sort of study can provide useful estimates about a link, but there are some important limitations, such as the need to account for other lifestyle choices and the fact that eating habits were only established once, at the start of the study.

While the media coverage mostly focused on the Atkins diet, it is important to note that this study was not assessing women who followed any particular diet. Despite some drawbacks, this research supports existing advice to follow a balanced diet in order to stay healthy.

Where did the story come from?

The study was carried out by researchers from the University of Athens Medical School and other institutions from the US, Scandinavia and Europe. It was funded by grants from the Swedish Cancer Society and the Swedish Research Council. The study was published in the peer-reviewed medical journal BMJ and is an open access article.

This study was picked up by a variety of papers, which mostly ran attention-grabbing headlines about the ill-effects of the well-known Atkins diet. Although the study did not focus specifically on the Atkins diet, low-carbohydrate and high-protein intake (characteristic of the Atkins diet) was studied. The Telegraph’s headline of a “breakfast egg can raise heart disease risk” is misleading because an occasional eggy breakfast alone is unlikely to lead to poor heart health, and is not what this study examined. The Telegraph’s headline is particularly baffling, because the paper's news story goes on to state that “this latest study is not principally about eggs”.

What kind of research was this?

This was a prospective cohort study looking at the long-term consequences for cardiovascular health of low-carbohydrate diets, generally involving a high-protein intake.

Results from prospective studies are usually considered to be more robust than retrospective studies, which either use data that was collected in the past for another purpose, or ask participants to remember what has happened to them in the past. In this study, the researchers used data collected as part of the Swedish Women’s Lifestyle and Health Cohort. The original aim of this particular cohort is not reported but is likely to have been designed to look at how various lifestyle factors affect health outcomes.

What did the research involve?

Between 1991 and 1992 the Swedish Women’s Lifestyle and Health Cohort recruited 43,396 women aged 30 to 49 years old, with no history of cardiovascular disease, from the Uppsala region of Sweden. They completed an extensive health and lifestyle questionnaire that included questions about dietary intake. The women recorded their dietary intake (how often they ate and the amount they consumed) of about 80 food items and drinks for the six-month period before entering the study. The food item groups were vegetables, legumes, fruits and nuts, dairy products, cereals, meat and meat products, fish and seafood, potatoes, eggs, sugars and sweets. The researchers translated the women’s self-reported food consumption into nutrient and energy intake.

The women were followed for an average of 15.7 years, and the researchers used national Swedish registries to identify hospital discharge information and reported deaths to look for first diagnoses of cardiovascular diseases (as recorded by recognised disease classification codes). These included reports of:

heart disease
stroke (due to a blood clot or bleed
subarachnoid haemorrhage (bleeding in the outer layers of the brain)
peripheral artery disease (narrowing of the arteries in the legs)

The researchers estimated the intakes of protein and carbohydrates for each women and assigned her a score from 1 (very low protein intake) to 10 (very high protein intake). Conversely, carbohydrate intake was scored as 1 (very high intake) to 10 (very low intake). Scores were analysed separately by adding them together to give a low-carbohydrate, high-protein score ranging from 2 to 20. They looked at how this related to new diagnoses of cardiovascular disease, adjusting for various other cardiovascular risk factors that could confound the analyses, such as smoking, blood pressure and fat intake.

What were the basic results?

A total of 1,270 cardiovascular events were identified as happening to the 43,396 women over about 15 years, and these broke down as:

heart disease (703 events)
stroke (any type, 364 events)
subarachnoid haemorrhage (121 events)
peripheral artery disease (82 events)

The researchers’ analysis found that both high-protein and low-carbohydrate scores were significantly associated with an increased rate of these cardiovascular events. A one-tenth (one point) increase in protein intake was associated with a 4% increase in the risk of any new cardiovascular events (rate ratio 1.04, 95% confidence interval 1.02 to 1.06). A one-tenth decrease in carbohydrate intake was associated with a (borderline significant) 4% increase in the risk of any new cardiovascular event (rate ratio 1.04, 95% confidence interval 1.0 to 1.08). A two-unit increase in a composite low-carbohydrate, high-protein score was associated with a 5% increase in the risk of any new cardiovascular event (rate ratio 1.05, 95% confidence interval 1.2 to 1.08).

Researchers also found a reduced risk of cardiovascular disease with increasing levels of education and physical activity. Risk of cardiovascular disease was increased with tobacco smoking and a history of high blood pressure.

How did the researchers interpret the results?

The researchers concluded that “low-carbohydrate, high-protein diets, used on a regular basis and without consideration of the nature of carbohydrates or the source of proteins, are associated with increased risk of cardiovascular disease”. They estimated that a 20g decrease in daily carbohydrate intake and a 5g increase in daily protein intake would lead to a 5% increased risk of cardiovascular disease.

Conclusion

This large study provides some evidence of a link between low-carbohydrate, high-protein diets and increased risk of cardiovascular events such as strokes. However, there are some limitations to this study:

The women’s diet was only assessed once, at the beginning of the study. This assessment included the women’s diet in the previous six months, but may not reflect the average intake of different food groups over longer periods.
The women reported their own dietary intake. Self-reporting like this can make the results less reliable and could have led to incorrect categorisation of women according to their protein and carbohydrate intake.
Although the researchers attempted to adjust their results for other factors that may have contributed to cardiovascular events, this type of study may not have taken into account all contributing factors. For example, the women’s cholesterol levels were not assessed.
Limited information is given on how the researchers determined which women had existing cardiovascular disease at the start of the study. The research may therefore have included women with existing disease.
Because this was a study that only included women, its findings can’t be applied to men. However, this doesn’t mean that men shouldn’t be concerned about their diet.

The focus of the media coverage upon the Atkins diet is because the dietary pattern in the study mimicked some of the Atkins diet’s recommendations of a low-carbohydrate, high-protein diet. However, it is important to note that this study was not assessing women who followed any particular diet. Consequently, some of the headlines are misleading. The Telegraph’s warning that a “breakfast egg can raise heart disease risk” is simply inaccurate as it implies that those eating a healthy diet including eggs may be at risk of poor heart health. This is simply not the case.

Despite its limitations, this study supports existing advice to follow a healthy balanced diet for optimal health.