Food and diet

News on fry-ups overcooked

“A bacon fry-up at breakfast could be the healthiest start to the day,” according to the Daily Express. Several other papers also optimistically declared that a fried breakfast is a healthy option, based on a study in mice.

In the study, various groups of mice were fed different sequences of high- and low-fat feeds but had the same total calorie intake over 24 hours. The research, which is reportedly one of the few studies to have looked at the effect of meal times, found that eating a fatty meal just after waking was not as bad for the mice as eating a fatty meal before sleeping. The researchers suggest that, for mice in this study, the first meal of the day appeared to dictate the workings of their metabolism for the rest of the day. It is important to note that the mice ate high-fat mouse food, rather than the “full English breakfast” mentioned by newspapers.

Although caution is needed in drawing firm conclusions for human healthy eating from animal studies, this sort of study supports the plausible theory that fat may be metabolised in mammals in different ways depending on the time of day that it is eaten. Before we can claim that a fatty, calorific breakfast is good for the body, the theory needs to be tested in humans.

Where did the story come from?

This research was carried out by Professor Molly Bray from the University of Alabama and colleagues from other institutions in the US. This work was supported by Kraft Foods, the US Department of Agriculture, the Agricultural Research Service Association and the US National Heart, Lung, and Blood Institute. The study was published in the peer-reviewed International Journal of Obesity.

The research has been covered by several newspapers, many of which reported the optimistic news that “chocolate is good for your heart” earlier this week. Despite the fact that the study gave feed mixture to mice, most newspapers reported that a fried breakfast is a healthy option for humans. This notion appears to be based on quotes from the researchers, which suggest that their results may have direct implications for human health.

What kind of research was this?

This research tested the theory that the time of day at which dietary fat is consumed affects aspects of metabolism in mice. Mice are usually active during hours of darkness, so the researchers created a controlled environment with a cycle of 12 hours of light and 12 hours of darkness. The researchers fed the mice various combinations of different high-fat or low-fat feeds across each 24-hour cycle. At the end of 12 weeks, numerous biological factors were measured in the mice.

This animal research was designed to provide control groups and comparisons for several eating patterns, but how these relate to human patterns of eating is not discussed in the research. It is likely that an English breakfast has a different composition to the high-fat diet (45% energy from fat) fed to these mice, and suggestions that the two are comparable is questionable.

What did the research involve?

The researchers designed four pairs of experiments in which they fed groups of mice either low-fat feed (10% energy from fat) or high-fat feed (45% energy from fat) in a structured manner during 24-hour cycles of sleeping and eating. Due to their nocturnal nature, the mice slept during the first 12 hours (under light) and were active in the second 12 hours (darkness).The researchers also had a comparison "control phase", during which the mice received no food for short periods.

The four experiment pairs were:

  • Experiment one: a high-fat diet during light followed by low-fat diet during darkness (the mice’s awake/active period) versus a low-fat diet during light followed by a high-fat diet during darkness.
  • Experiment two: no food during light followed by a high-fat diet during darkness versus no food during light followed by a low-fat diet during darkness.
  • Experiment three: no food during light followed by a high-fat diet for the first four hours of darkness (the early awake/active period) and a low-fat diet for the next eight hours versus no food during light, a low-fat diet for eight hours and a high-fat diet for the last four hours of darkness (the late awake/active period).
  • Experiment four: no food during light followed by a high-fat diet for the first four hours of darkness, a gap of four hours without food, and then a low-fat diet for four hours versus no food during light, a low-fat diet for four hours, four hours without food, and then a high-fat diet for the last four hours of darkness. This last experiment was designed to simulate an early fatty meal compared to a late fatty meal.

The researchers measured a range of metabolic factors in the mice, including energy intake, body weight, percent body fat and glucose tolerance, as well as energy expenditure, breathing and physical activity. The body’s resistance to insulin and the levels of the “hunger hormone” leptin, fats and glucose in the blood were also measured.

What were the basic results?

The researchers reported that during experiment one, both sets of mice adjusted their food intake and energy expenditure so that metabolic measures remained within normal ranges.

They say that in the other three experiments, the variations in diet during the active period (darkness) had an influence on metabolism. Mice fed a high-fat meal at the beginning of the active period were able to retain “metabolic flexibility” in response to dietary challenges, meaning that they adapted their subsequent eating and metabolism to achieve a lower intake of calories.

Mice fed a high-fat meal at the end of the active period had higher total calorie intakes. Consumption of a high-fat meal at the end of the active phase led to increased weight and fat gain, glucose intolerance, and high levels of insulin, fats (triglycerides) and leptin, a hormone linked to obesity.

Importantly, these variations in energy and metabolism were independent of total daily energy intake or fat-derived energy intake.

How did the researchers interpret the results?

The researchers say that the time of day at which carbohydrate or fat is consumed “markedly” influences many metabolic (cardiometabolic syndrome) measures.


This study provides some useful indicators about metabolism that may have some relevance to human diet. However, it should not be taken as an endorsement that a fry-up is healthy or better for you than a breakfast of cereal or fruit, as several newspapers have suggested. There are a number of points to note when considering this research:

  • The diets fed to these mice may not equate to the sorts of diet eaten by humans.
  • Using studies in mice to develop theories about human health and biology is an accepted part of early research, but such research needs to be followed by studies in humans, where possible.
  • There are known long-term health implications, such as cardiovascular problems, associated with eating diets high in fat and saturated fat. While some of the mice on high-fat diets did not gain weight, it does not mean that fry-ups could be considered a healthy option.
  • The researchers suggest that an early low-fat meal switches the body to prefer carbohydrate as an energy source. However, the low-fat feed given to the mice was very high in simple sugars rather than complex carbohydrates, which may have played a role in altering the mice’s metabolism.

The timing of meals and their nutrient content may be an important part of what makes humans feel full and how they metabolise calories. This aspect of obesity research will need further study.

NHS Attribution