system
Given the following post, respond as if you're commenting on Reddit:
human
Can better nutrition in a person's 20s make up for poor nutrition in childhood and adolescence ?

During childhood years (8-12) i never had good nutrition. I often ate junk food especially like chips etc. even though i was hungry. In adolescence unfortunately money was tight so i did not eat well either. Now that im my early adult hood, i am making better choices.

Can this improved nutrition, make up for what my body had missed back then ?
gpt
Disclaimer: the topic is complicated, and does not lend itself to simple, compact answers. Nor are the citations straightforward, e.g not single topic studies, but lots of fundamental biology & biochemistry, so apologies in advance for lack of citations and nuance.

Two scenarios:

1. Malnutrition resulting in growth consequences. Typically chronically low or insufficient protein intake, exacerbated by insufficient calorie intake. This may result in stunting/failure to reach height potential, delay to or incomplete sexual maturation, fragile bones, and more (i.e. neurological deficits in the extremity, from micronutrient deficiencies)

2. Overnutrition of poor quality. This may result in excess weight, and attendant health/hormonal consequences. 

The first type (1) is unfortunately permanent if the malnutrition extended over the majority of the growth phase, I.e. past puberty. This rarely happens in developed countries, where the problem is overwhelmingly of the second (2) sort.

Since heart disease and other diseases of excess take decades to develop, the evidence available suggests ample opportunity to improve outcomes - mainly by losing weight and adopting a diet of medium protein, low saturated fat, high in polyphenol and fiber, e.g. a traditional Mediterranean style diet. There isn't full consensus on whether this "repairs" (reverses) whatever damage has already been done, but there is clear evidence that even with symptomatic heart disease, lifestyle improvement (diet, weight, exercise) is a very effective tool for reducing cardiac events.

In the second (2) case it is also typical for the diet to be rich in macronutrients but poor in micronutrients, the consequences of which may or may not be permanent either. For example, it is now clear that fiber and various phytonutrients like anthocyanins are essential for immune function, and to make a long story shorter, probably are involved in many types of cancer, but are conclusively known to affect bowel cancer. There is evidence that beneficial lifestyle changes can re-normalise abnormal pre-cancerous tissue in the bowels. So that can probably be thought of as limited reversal (but would not be the case once a precancerous polyp has become a malignancy, at which point lifestyle changes will surely improve overall health outcomes and survival, but only medical intervention addresses the actual problem).