THE GRANNY TYRE

Healthy ageing wasn’t always a subject that interested me – it seemed to be a long way off and yet here I am in the thick of it. Like most people, I’ve gained unwanted weight during adulthood – since my 20s, about 10% of my body weight. My Body Mass Index (BMI) is still within the ideal range (it’s gone from 19 to 21 kg/m2) but I’ve often wondered if this is healthy or not. I’ve always eaten a healthy diet and exercised regularly. While some women have thickening waistlines, my fat is focused under the belly button. I call it my granny tyre. I’ve noticed it in other women my age, particularly those that are slim, perhaps because it’s just more obvious.

Men also gain weight as they age, often much more than women. But they put it inside the abdomen (i.e., visceral fat) rather than hips and thighs – and that spells trouble metabolically. Ageing also corresponds to more sedentariness and losing muscle mass yet our basal or resting metabolic rate remains stable throughout most of adulthood. This is true even when we account for the loss of muscle. Using the gold standard technique (doubly labelled water), Professor Herman Pontzer showed that between the ages of 20 to 60 years, basal energy remained the same (1). There was a small decline between 60 and 80 years.

Studies have shown that women’s weight in late menopause increases by an average of 2 kg (or 4 lbs). Body fat around the organs (visceral fat) increases from 5-10% to 15-20% of all fat (2). Unfortunately, this is associated with a heightened risk of cardiovascular disease and type 2 diabetes. Estrogen therapy during and after menopause has been shown to improve quality of life as well as lower the risk of chronic disease. Reassuringly, new hormonal therapies do this without increasing the risk of breast cancer (3).

Given my interest in human evolution, I began to wonder if my granny tyre was related in any way to the ‘Grandma hypothesis’. Twenty years ago, anthropologist Kristen Hawkes at the University of Utah speculated that human females uniquely evolved to live long after menopause for a good reason – it served to increase the survival of their offspring – children and grandchildren (4). Mothers with new babies are vulnerable. It’s difficult to forage for food with one arm carrying a helpless baby. It’s even harder if there’s a toddler or another child who must also be fed. Of course, Dad might have played a role here, but chances are it was… Grandma!

The Grandma hypothesis has support from observational studies in Finland, where collection of population data goes back a long way. The researchers found that mothers who could call upon their own mother for assistance did indeed have more offspring – and at shorter intervals (5). In the evolution of a primate with an enormous brain, survival of the fittest called for a physically fit, post-menopausal woman to help ‘mother the mother’.

Now, coming back to the granny tyre – is it associated with a longer life? Yes (!) – according to the AGES-Reykjavik Study (6). Researchers Annemarie Koster, Tamara Harris and others were able to study body fat depots in over 6000 men and women, aged 66 to 96 years in Iceland. They used a precise method called CT (computed tomography) scanning that enabled them to determine the actual volume of fat in different regions. Their findings were so surprising, I’ve never forgotten them.

In the men, increases in visceral and thigh fat predicted greater mortality (higher death rate) in the 11 years of follow-up. This relationship remained significant even after accounting for differences in weight and height, education, smoking, physical activity, alcohol and presence of diabetes or coronary heart disease. In women too, increments in visceral fat were associated with higher all-cause mortality but increases in subcutaneous (i.e., fat under the skin) abdominal fat was linked to significantly lower risk of death. Furthermore, the effect was even stronger in women who were normal weight or overweight, but weaker in women with obesity.

Very few studies directly measure regional fat depots as they did in the AGES-Reykjavik Study. I used Google Scholar to search for more information. Reassuringly, higher abdominal subcutaneous adipose tissue, or fat, appears to have a protective metabolic effect, perhaps as a metabolic sink for excess lipids, reducing the deposition of fat around organs. It could also be indicative of better energy reserves if there is trauma or a crisis. For example, it could be helpful in people with lower muscle mass (sarcopenia) and patients with cancer or those undergoing cardiovascular surgery, predicting better survival (7). Most of us tend to lose muscle as we age.

So, my granny tyre may indeed be helpful if my goal is to live a long life. However, I can’t be sure if my granny tyre is a result of my healthy eating habits, efforts to take regular exercise or my family history (genes) or all three. My dad is thriving at the ripe old age of 101 years, one grandmother lived to 106, the other to 98, my mum to 96. I recall she had a granny tyre too. My husband says I have all the risk factors for living too long. My only wish is that I can bring him along for the ride.

Read more:

1. Pontzer and colleagues. Daily energy expenditure through the human life course. Science, 2021.
2. Toth, and colleagues, Menopause-Related Changes in Body Fat Distribution. Annals of the New York Academy of Sciences, 2000.
3. Davis and colleagues. Understanding weight gain at menopause. Climacteric, 2012.
4. Hawkes, The grandmother effect. Nature, 2004.
5. Lahdenperä, and colleagues. Fitness benefits of prolonged post-reproductive lifespan in women. Nature, 2004.
6. Koster and colleagues. Fat distribution and mortality: the AGES-Reykjavik Study. Obesity (Silver Spring), 2015.
7. Ebadi and colleagues. Subcutaneous adiposity is an independent predictor of mortality in cancer patients. British Journal of Cancer, 2017.

Emeritus professor Jennie Brand-Miller held a Personal Chair in Human Nutrition in the Charles Perkins Centre and the School of Life and Environmental Sciences, at the University of Sydney until she retired in December 2022. She is recognised around the world for her work on carbohydrates and the glycemic index (or GI) of foods, with over 300 scientific publications. Her books about the glycemic index have been bestsellers and made the GI a household word.