In times gone by, Navajo Indians ate whatever mother nature was generous enough to bestow, their existence intimately and spiritually bound up with the land on which they lived.
Their food would have been the envy of any modern dietitian as they foraged for pinon nuts and wild potato, and nibbled on sumac berries, yucca fruit, prickly pears and bee-weed greens.
Today, life is very different in the Navajo Nation, the largest American Indian reservation in America. It covers an area in Arizona, New Mexico and Utah that is the size of Scotland and has a tribal population of 300,000.
The modern Navajo, who prefer to be called the Dine, are facing an intensifying health crisis caused by a complete transition to a diet based largely on fried potatoes, tortillas, cookies, crisps, sugary drinks and Spam.
According to the Navajo Area Indian Health Service, around 25,000 people in the region have type 2 diabetes and 75,000 are pre-diabetic.
The obesity rate for some age groups is up to 60 per cent, and high blood pressure and heart disease are proliferating. Tribal politicians have been forced into action and this week the reservation became the first place in the United States to begin taxing junk food, despite opposition from some cash-strapped Navajo themselves.
The Healthy Dine Nation Act covers food and drink with "minimal-to-no nutritional value" and makes it subject to a 2?per cent tax.
As well as predictable items like ice cream and sweets, it includes fruit juice, sugar-free Jell-O, diet sodas and energy drinks. The drinks industry, desperate to prevent a precedent that could be repeated elsewhere in America, sent lobbyists to the reservation, but to no avail.
A survey found that up to 90?per cent of the food sold in Navajo grocery shops qualified for the tax. Meanwhile, an existing tax of 5?per cent on fruit and vegetables has been scrapped.
The new tax will bring in an estimated $1?million (pounds 670,000) a year which will pay for projects including farmers' markets, community vegetable gardens, greenhouses, and exercise equipment.
The Navajo have had the right to make and enforce laws and to raise taxes since the 1830s, thanks to the US Supreme Court enshrining the right of tribes to self-government by defining them as semi-autonomous "domestic dependent nations" within America. The Navajo Nation has its own police department and courts and elects its own president and tribal council, which sits in its capital of Window Rock, Arizona.
Speaking to The Daily Telegraph, Gloria Begay, of the Dine Community Advocacy Alliance, which campaigned for the food tax, said: "Navajo used to farm the land in corn fields and look after cattle and sheep. A lot of lifestyle changes have happened and a lot of unhealthy food has come on to the reservation.
"There are very few jobs and most of them are sedentary. People don't do the physical exercise they used to. In the past we had very little meat in our diet, it was just herbs and plants and so on.
"But if you go into a Navajo grocery store now it's aisles and aisles of potato chips and soda pops and huge amounts of bad canned food.
"We need to get fresh goods back into stores, but the stores can't afford refrigerators for fresh fruits and vegetables."
Turning the Navajo diet around will also be hampered by logistics. The reservation covers 27,000 sq miles but has only 10 big supermarkets.
Traditional hogans - tree bark and mud structures in which Navajo used to live - have been replaced by trailers and houses, but many of those do not have electricity and refrigerators.
Unemployment is at around 50?per cent, rising to 90?per cent in some of the 110 tribal districts, and 42 per cent of the population live below the poverty line.
Some Navajo have to make 100-mile round trips, often on dirt roads, to reach the nearest shop, which may well be a rudimentary affair located at a petrol station.
The vast majority of the reservation has been officially declared a "food desert" by the US Department of Agriculture, meaning residents live more than 10 miles from the nearest supermarket.
Many Navajo also aren't happy at the prospect of improving their diet, mainly because of the extra cost. At a local store the cost of a dozen apples is $7 (pounds 4.70), the equivalent of seven frozen ready meals, packed with fat and salt.
Another contributory factor to the poor diet of the Navajo is rooted in a cruel episode of their history. In the 1860s, Kit Carson, the American frontiersman, rounded up members of the tribe and burned down their hogans, and the US Army force-marched them 300 miles - a journey known as the "Long Walk" - to Fort Sumner where they were interned. It was then that they began eating "frybread" - bread deep-fried in lard - which continues to be popular to this day.
Now every Navajo has a relation who is diabetic, and diabetes is a problem in one third of pregnancies. By some estimates parts of the reservations will have a 90?per cent diabetes rate by 2020.
Ben Shelly, the president of the Navajo Nation, initially rejected the "junk food tax", partly based on concerns that small businesses would suffer. But he later changed his mind, saying: "Diabetes is an enemy that we will conquer by fighting this war together."
A visible example of how attitudes to food are changing is Jonathan Nez, the chairman of the Navajo Nation's budget committee, who co-sponsored the bill that led to the junk food tax.
Four years ago, he weighed 300lbs but trimmed down by changing his diet. He described the tax as a way of moving "back to the way we used to live".