What happens when you eat fat?
November 5, 2019
8 min read
In our first post in this mini-series on nutritional responses, we looked at what happens as our bodies break down and use the food we eat.
This time, we’re going to take a deeper dive into what happens when you eat fat, and why measuring blood fat changes over time during our PREDICT studies is so important for understanding your personal nutritional responses.
What happens to the fat in our food when we eat?
As we covered in our previous post, nearly all the fat in our diets is present in the form of triglycerides.
Triglycerides contain three chain-like fatty acid molecules (shaded blue in the diagram below) stuck together with a bridging molecule called glycerol (shaded pink).
When you eat or drink anything containing fat, these triglycerides make their way to your intestines, where they are absorbed and packaged into specialized delivery structures called chylomicrons. The chylomicrons carry the triglycerides around the blood, delivering them wherever they are needed.
We can study responses to fat when you eat by collecting a sample of your blood and measuring its triglyceride content. In PREDICT 2, we did this by asking participants to collect small spots of blood on special pieces of paper on two days of the 11-day at-home study.
Fasting fat levels don’t tell us about how your body responds to food
If you have your blood triglyceride levels measured in your doctor’s office, you will usually give a blood sample before you have your first meal of the day, so you are in a fasted state.
Your doctor will let you know your results and if your triglyceride levels are below 150 mg/dL (milligrams per 100 millilitres of blood), which is generally considered ‘normal.’
However, your fasting triglycerides are not representative of how your body spends most of its time, because your body spends up to 18 hours a day digesting and breaking down the meals that you take in throughout the day.
Your blood sugar level spikes up and down during the day in response to meals and snacks, while your blood fat level steadily rises then falls.
This means that your triglyceride levels actually rise with everything you eat or drink, typically peaking in the afternoon or evening.
How do we measure blood triglycerides in our PREDICT studies?
In our PREDICT 1 study, participants came into our clinic first thing in the morning after fasting for 10 hours and avoiding alcohol and not exercising the day before. It sounds strict, but it helps to make sure they are all into the same metabolic state to start with.
As a scientific breakfast treat, they were given one of our special muffin meals containing 50g of fat and 85g of carbohydrate.
‘Fifty grams of fat may sound like a lot, but it’s quite typical of a large main meal, and it is important that our muffins contain enough fat to give a good triglyceride response in the blood,’ says our scientific collaborator and fat response expert Dr Sarah Berry from King’s College London.
‘After they finished their muffins, participants relaxed for the next 6 hours while we took hourly blood samples. We sent the blood off to a lab, who measured the levels of triglycerides,’ she explains.
‘We drew some graphs using our data and, as you can see, all the people in the study had very different blood fat responses after eating exactly the same muffin meal.’
Blood fat measurements in 1,000 participants in our PREDICT 1 study, tracked for 6 hours after eating an identical meal.
Unfortunately, we can’t invite everyone to our lab as it would get pretty crowded. Instead, our PREDICT 2 study uses a novel technology that lets people monitor their blood fat levels from the comfort of their own home.
‘Before and after eating certain muffin meals they will prick their finger and put a spot of blood onto a specially prepared piece of paper,’ says Sarah.
‘At the end of the study, they stick their bits of paper in the mail, and we do all the hard work of measuring the triglyceride levels and figuring out how they responded to their muffins.’
What is the problem with a ‘bad’ response to fat anyway?
We have used our muffin recipes in many studies, so we have a pretty good idea of what a ‘good’ or ‘bad’ response to them looks like.
When we talk about a ‘bad’ response to fat, what we mean is that the levels of triglycerides in your blood remain high for a long time after eating.
If the levels of triglycerides remain high in your body for a long time, this can trigger harmful stress responses and inflammation. In the long term, raised triglycerides have been linked to conditions such as type 2 diabetes and heart disease.
What affects our response to fat? And can you ‘fix’ a bad response?
There’s a mountain of research out there about what affects our responses to fat.
There are some factors that affect how we respond to fat that we can’t change. For example, your responses to fat usually get worse with age, and several genes influence how you metabolize fat after eating.
Intriguingly, men tend to have higher peaks in blood lipids after eating compared with women, but women who have been through the menopause are likely to have worse responses to fat that those who haven’t.
The good news is that much of the available research shows that many of the things that affect how we respond to fat are things that we can change and improve.
Unsurprisingly, what you eat has a big impact on your fat response:
- The more fat in your meal, the worse your response
- The type of fat you have eaten changes your response too
- Eating a meal high in protein means that it takes longer for your stomach to empty, which makes your fat response slower
- High-fiber meals also slow down digestion, making your response to fat better
- The structure of your food changes how quickly you can absorb fat and, therefore, your overall response
But it’s not just about what you eat at each individual meal. Your day as a whole also affects your responses to fat:
- Exercising 11-20 hours before a meal improves your fat response
- Stress makes your response to fat worse
- You are likely to have a better response to fat if you split your food into three meals over the day compared with six
- The contents of your previous meal can also influence your fat response (the so-called ‘second meal effect’)
Even how you live your life can also impact your fat responses:
- Being very overweight makes you more likely to respond badly to fat
- Your long-term diet can change your responses, diets low in saturated fats, and low carbohydrate diets generally improve fat responses
- Smokers may have poorer responses to fat
- Insulin resistance and type 2 diabetes are associated with poor responses to fat
Your responses to fat are as unique as you are
Looking at this long list of things that change how you respond to fat, it’s not surprising that even identical twins can have very different responses to the same foods. And, of course, you’re unlikely to eat a meal that consist purely of fat and nothing else.
We eat foods, not single nutrients, so we need to understand how everything combines together to affect our health and metabolism.
Our PREDICT studies are the first studies that combine lots of different factors together – including responses to fat and sugar, genetics, sleep, activity and the microbiome – to understand personal nutritional responses.
We've used all of this data to develop a unique at-home test kit that can help anyone discover the best foods for their biology. Ready to take the first step? Get our at-home test kit today and discover your personal responses to foods.