The grand opening of the first KFC restaurant in the Kenyan capital, Nairobi, last year drew a serious crowd. It was never on a par with McDonald's 1990 debut in Moscow but there was palpable excitement. The queues outside the fried chicken restaurant in one of the city's mushrooming shopping malls were taken as further evidence of the rise of the country's middle class.

Most of the diners admitted they had been drawn by the “hype” but a year on KFC is still full and has opened several more outlets. The opening was broadly seen as Kenya catching up with the rest of the world, another signpost on the way to middle-income status rather than any cause for health concerns.

People looking for answers to why Africa has gone from hunger crises to mounting obesity without ever passing through a healthy nutrition phase tend to focus on why the continent is different. The same tired “African exceptionalism” that is used to explain poverty levels or civil wars is wheeled out. Some point to cultural differences, wondering whether the heft is associated with wealth and prosperity rather than the Western stereotype of idleness and sloth.

Unusual practices like the forcefeeding of girls in Mauritania or breast ironing in Mali are seized on to draw sweeping conclusions about “traditional beliefs” over body shapes and sizes. A lot of time has been spent by researchers asking men in countries like South Africa and Kenya whether they like big bums or skinny women. The answers, as you would expect on a continent of a billion people, are mixed.

In fact, what the rising obesity rates reveal are consumer habits that broadly follow what's been seen elsewhere in the world. The same factors are creating the fat. Urbanisation is on the increase throughout sub-Saharan Africa and the move from the village to the city is adding weight to the argument.

Africa now has more cities of over one million people than China and a larger collective middle class than India. Convenience foods full of fat and starch are waiting for the new arrivals that can afford them in the cities.

The highest obesity rates broadly follow the GDP per capita, with sub-Saharan Africa's biggest economy South Africa having by far the biggest problem. Other emerging economies like Kenya are following suit, often with exactly the same companies and products. It's no coincidence that South African investors brought KFC back to Kenya (it was there previously in the 1970s briefly).

Nairobi's shiny new plazas as well as its petrol station drive-ins are full of South African fast food outlets, most of them aping US counterparts.

Already creaking public health systems are in no position to deal with an obesity epidemic. Social attitudes to weight will have to shift without much help from governments. If there is a pan-African middle class emerging then it is most visible on satellite television where adverts imploring Mums to feed up their kids have started to give way to those telling people to watch their weight. The message may not be getting through.