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

Behind the Headlines

Wednesday June 27 2012

A 'steak' through the heart: could low-carb Atkins diet be a risk?

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.

 

Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on twitter.

Links to the headlines

Heart attack risk in dieting. Daily Express, June 27 2012

Atkins-style diet warning: Low-carb, high-protein diet linked to greater risk of heart disease and strokes. The Mirror, June 27 2012

Breakfast egg 'can raise heart disease risk'. The Daily Telegraph, June 27 2012

Links to the science

Lagiou P, Sandin S, Lof M, et al. Low carbohydrate-high protein diet and incidence of cardiovascular diseases in Swedish women: prospective cohort study. Published online June 27 2012

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Comments

The 6 comments posted are personal views. Any information they give has not been checked and may not be accurate.

simplexity said on 09 July 2012

Welldone Entri88, your math is correct.....fair and valid points also...discussion, reflection, refutation...all good science....

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Entri88 said on 02 July 2012

I don't agree with the last post that Zoe Harcombe makes "fair and valid" points. On her blog, Zoe claims that "we independent bloggers have better things to do than correct ‘researchers’ who should know better", yet she has posted a rebuttal to the research on this website, on her own blog (a very lengthy one) and on the BMJ website (very lengthy too) where she is listed as having no conflict of interest. Really? What about the fact that this research contradicts the nutritional theories put forward by Zoe Harcombe in her diet books? I'd say that's a pretty major conflict of interest.
Also, Harcombe says she has to "correct 'researchers'", yet she can't tell the difference between incidence and cumulative incidence. She writes: "The claimed association, not causation, is tiny 1,270 incidents of all CVD among 43,396 women over 680,745 study years = an incident rate of 0.19%. i.e a 99.81% chance of NOT suffering any CVD during the 15-16 yr study."
No, the 0.19% is the incidence rate of getting CVD per one study year. The cumulative incidence is the incidence rate multiplied by the duration of the study, which comes out at 2.9%.

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simplexity said on 02 July 2012

I find this site useful, informative and on the whole balanced and critical. Occasionally the detailed science is missing but then it is a resource meant for non specialists rather than experts. That’s the point!

Zoe makes some fair and valid points that add to the discussion around this research and the illumination as to its quality and applicability.

It s a shame the following two posts add nothing but an insight into the contributors respective limbic system’s and their tendency toward vitriol.

Keep up the good work NHS choices

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Zoe Harcombe said on 29 June 2012

1) The claimed association, not causation, is tiny
1,270 incidents of all CVD among 43,396 women over 680,745 study years = an incident rate of 0.19%. i.e a 99.81% chance of NOT suffering any CVD during the 15-16 yr study. The 'highest risk' group had a 99.77% chance of NOT suffering any CVD. The 'lowest risk' group had a 99.86% chance of NOT suffering any CVD.

The 'highest risk' group might expect a 0.04% lower chance of remaining CVD free than the average woman in the study. This does not justify the BMJ headline of "Experts warn of significant cardiovascular risk with Atkins-style diets."

2) The scoring system is absurd
Low protein = 1, high protein = 10, low carb = 10, high carb = 1. A score of 11 can be low protein (1) and low carb (10) or high protein (10) and high carb (1) or any sum between: 1+10, 2+9, 3+8, 4+7, 5+6, 6+5,7+4, 8+3, 9+2 or 10+1. All give 11 as score.” What conclusion would this lead to?

3) Atkins is not low carb high protein
Atkins is low carb, whatever protein naturally occurs in food and high fat. This brings us on to:

4) The 'researchers’ do not seem to know the nutritional composition of food
There are 3 macro nutrients: carbs, fat and protein. All food has at least one macro nutrient, most have 2 and some have 3.

The only foods on this planet with no protein are sucrose and oils. Every other food contains protein. Sucrose is 100% carb – no fat or protein and oils are 100% fat – no carb or protein.

This article assumes that the higher the carb intake and the lower the protein intake, the lower risk of CVD the women shall have. Hence our Swedish women should have eaten nothing but sucrose for 15.7 years and they would have had 100% carb intake and 0% protein intake (and 0% fat intake – presumably also ideal). They would have had no vitamins or minerals, no protein, let alone complete protein and no fat, let alone essential fats, but they would have met the recommendations emanating from this article.

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thdlover said on 29 June 2012

atkins is not low card high protein its high fat - get it right! ANd as the NHS you should know that high fat is not bad!

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lotharmat said on 29 June 2012

With this level of research and the inability to admit you made a mistake; I'm embarrassed to work for the NHS

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Analysis by Bazian

Edited by NHS Choices

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