A big bottom 'is good for the heart'

“Having a big bottom is good for you,” according to The Sun. Several newspapers have reported similar stories based on a research article that suggests that it is better for body fat to be stored around the hips, thighs and bottom than around the waist.

The research article draws on various other studies to support its argument that the risks of cardiovascular and metabolic diseases are lower in people who store fat in their lower bodies. However, the purpose of the review was to use selected research to justify further research into this relatively unknown association, and not to look at all the research on the subject. As such, the piece may have omitted research contradicting the authors’ theory. Much further research is needed to understand how the chemicals and hormones involved in fat storage have an effect on cardiovascular diseases.

Where did the story come from?

Dr Konstantinos Manolopoulous and colleagues from Oxford University wrote this review, which was published in the peer-reviewed International Journal of Obesity.

Several newspapers were keen to promote the health benefits of “big bottoms”, with some suggesting that the protective effect of hip and thigh fat was due to the way it responds to hormones or a reduction in signalling chemicals in the body.

What kind of research was this?

This was a non-systematic review that summarised selected research on gluteofemoral fat (fat stored around the hip, thighs and bottom), its role in protecting against conditions such as cardiovascular disease and the way in which its storage is regulated by the body.

In this type of non-systematic review several pieces of evidence are selected and presented in order to argue the case for a particular theory or line of research. However, as this review has not been conducted systematically it may omit research or data that does not support the authors' theory and may not give a complete picture of all the research on the subject.

The studies looking at possible protective effects of gluteofemoral fat are likely to be based on cohort and cross-sectional studies. These types of study can only show associations between factors but cannot prove that one factor necessarily causes another. This means it is only possible to conclude that there is an association between gluteofemoral fat and lowered heart risk, but not that this type of fat provides any protection.

This review also used physiological and biological studies to provide clues to the potential mechanisms behind any possible protective effect.

What did the research involve?

In their review the researchers looked at:

• The studies that had shown that gluteofemoral fat was protective. They presented information on associations between the fat and cholesterol levels, the health of the arteries, coronary heart disease risk factors, insulin levels and diabetes.
• The biological mechanisms of the storage and release of fat in the gluteofemoral area.
• The roles of hormones and inflammatory cytokines (signalling molecules released by cells during inflammation).
• What happens in the body following loss of fat from the hip, thighs and bottom.

The review is extensively referenced but the methods used to conduct the review are not described. For example, it is not clear how the studies were selected for the review or how any conflicting evidence was assessed.

What were the basic results?

The researchers presented a selection of studies that suggest that thigh circumference, hip circumference or leg fat tissue mass are associated with lower total cholesterol, less stiffness of arteries, a lower prevalence of undiagnosed diabetes and a reduction in heart-disease risk factors such as blood pressure.

The authors also cite studies that suggest that the relative ease of losing weight from the abdomen compared to the lower body may be due to the fact that the fat cells in these areas respond to insulin in different ways. They also suggest that most daily fatty acid storage and breakdown occurs in the abdominal fat, whereas the fat around the buttocks, hips and thighs is more involved in long-term storage.

Another theory suggested by the authors is that stomach fat and gluteofemoral fat may release different types and amounts of hormones. They speculate that the different ways hormones are released in these areas may affect their protective abilities, but they provide only limited evidence directly to support this.

Newspaper reports have said that the inflammatory cytokine chemicals in the body can contribute to cardiovascular disease, insulin resistance and diabetes. The review article said that inflammatory cytokines can be released by fat cells, but did not present strong evidence for differences in the way cytokines are released in different fat storage areas, and it remains unclear what role cytokines released from fats might play in disease.

The researchers discuss some rare hormonal syndromes and genetically linked fat storage disorders that can affect cardiovascular disease and diabetes. However, they do not use the research on these conditions to support the effect of normal gluteofemoral fat metabolism in more common conditions.

How did the researchers interpret the results?

The researchers concluded that “body fat distribution is a major determinant of metabolic health”, and that thigh and buttock tissue exerts specific functional properties that are associated with an improved metabolic and cardiovascular risk profile. They also say that studies suggest that these properties are due to the way fatty acids are released in the lower body.

Conclusion

This was a non-systematic, narrative review that highlighted selected research that supports the authors' theory that the way in which different fat stores are regulated may have implications for disease.

This type of review is useful for arguing that a particular theory may warrant further research by presenting areas where further study may prove beneficial. However, while this review suggests an association between gluteofemoral fat stores and decreased disease, this area would benefit from a systematic review assessing the full range of data available. While the theory presented is of interest, extensive research is needed to determine the mechanisms underlying this proposed effect.