Eating like an emperor in the morning and like a beggar in the evening – according to popular belief, it matters how much we eat and when. A research team has now examined in more detail what lies behind it. To do this, test subjects either ate a lot of fat in the morning and more carbohydrates in the evening, or vice versa. Samples from the adipose tissue showed that the activity of more than a thousand genes changed depending on the time of fat and carbohydrate intake: A high-fat meal in the evening activated pro-inflammatory genes, whereas if fats were consumed primarily in the morning and carbohydrates in the evening, this promoted insulin sensitivity in the adipose tissue examined. This suggests that the popular saying is at least partly right: a meal that is too high in fat in the evening has a rather unfavorable effect on the metabolism.
Our biology and our metabolism are shaped by our daily rhythm and internal clock. Changes in gene activity, hormone release and cellular reactions influence almost all processes in our body – including digestion. Studies show that, for example, fat burning and blood sugar regulation fluctuate over the course of 24 hours and have characteristic rhythms. But what does this mean for a healthy diet or losing weight? According to the common assumption, it should be cheaper to consume certain nutrients such as fat, proteins or carbohydrates when the body can process them well – so that blood sugar does not rise for too long and excess energy does not end up in fat deposits.
Fats for breakfast or dinner?
In fact, studies show that the timing of food intake plays a role in human metabolism. “Numerous studies show that late eating is associated with weight gain, disturbances in energy expenditure and changes in the circadian rhythms of appetite, stress and sleep hormones,” report Jorge Soliz-Rueda from the German Institute for Nutritional Research Potsdam-Rehbrücke (DIfE) and his colleagues. They have now examined whether and how the composition of morning and evening meals – particularly in terms of fat and carbohydrates – affects metabolism. The focus was on time- and diet-dependent gene activity in the subcutaneous fat tissue of 29 overweight men without diabetes or metabolic diseases.
During the study, subjects followed one of two different eating plans for four weeks each. These were identical in terms of the calories consumed, but differed in the temporal distribution of fat and carbohydrates: In one trial, the test subjects received a carbohydrate-rich breakfast with a low fat content and in the evening a meal with 50 percent fat content but few carbohydrates. In the second four-week test run it was the other way around. The four-week interventions were followed by 12-hour assessment days of high-carbohydrate and high-fat meal tolerance tests. Three times during the study days – morning, afternoon and evening – the researchers took samples from the subcutaneous fatty tissue, which they analyzed for various biochemical parameters and their gene activity.
Influence on insulin sensitivity and inflammatory markers
The analyzes showed that gene activity in fatty tissue changes over the course of the day – and that the type and extent of these changes are also influenced by the distribution of nutrients during the day. “When the composition of the morning and evening meals changes, this causes a profound change in the transcriptome in adipose tissue,” write Soliz-Rueda and his colleagues. Depending on the diet plan, the groups differed in a third of the 1,386 genes that varied over the course of the day. Some of these genes had altered circadian rhythms, others stopped their circadian fluctuations altogether or went from non-oscillating genes to circadian-variable ones. Among these genes were those that control sugar and fat metabolism as well as inflammatory processes.
For example, a high-fat diet in the morning and a high-carbohydrate diet in the evening promoted genetic markers that increase the insulin sensitivity of fat tissue – this could have a positive effect on sugar and fat metabolism. “The second important result was the major effect of nutrition on inflammatory processes in the tissue,” reports the team. If the test subjects ate a high-fat meal in the evening, this increased the activity of genes that promote various immune reactions and inflammation. An unfavorable nutrient distribution throughout the day could therefore promote inflammatory processes in fatty tissue, which are considered a risk factor for metabolic diseases such as obesity and type 2 diabetes.
Better to eat high fat in the morning than in the evening
“Our results confirm initial findings that the shift in macronutrient distribution throughout the day measurably influences the metabolic processes in adipose tissue,” says senior author Olga Ramich from the German Institute for Nutritional Research. “Late, high-fat meals could therefore trigger unfavorable molecular processes that may promote long-term inflammation and metabolic problems.” These findings could offer new starting points for the prevention of obesity and type 2 diabetes. “In addition to the amount of calories and nutrient composition, meal timing could also be an important part of personalized nutritional recommendations in the future,” explains Ramich. However, she also emphasizes that longer studies with more test subjects are necessary in order to derive concrete nutritional recommendations from these results.
Source: Jorge Soliz-Rueda (German Institute for Nutritional Research Potsdam-Rehbrücke) et al., Food Research International, doi: 10.1016/j.foodres.2026.118685